State of the Universe

Friday seminar series of the Cosmology and Astroparticle Physics group at TIFR

Upcoming Seminars

Previous Seminars


April 2024

April 17, 2024
11:00 am (IST)

What Gravitational Waves can tell us about Fundamental Physics

Debarati Chatterjee (IUCAA)


Abstract: Neutron stars are effectively cosmic laboratories to study the behaviour of matter under extreme conditions, far beyond the reach of present terrestrial nuclear or high energy experiments. It is conjectured that exotic particles such as strange baryons or deconfined quark matter may appear in the ultra-high density environment of the neutron star interior, and this could in turn affect their observable properties. With the recent detection of Gravitational Waves from binary mergers of neutron stars, a new window has opened up, complementary to conventional multi-wavelength electromagnetic observations. Gravitational waves contain information about the interior composition of neutron stars and their detection can provide important constraints on the theories of dense matter. In this talk I will discuss some of our recent investigations on the implications of the state-of-the-art multi-messenger observations on our understanding of nuclear and particle physics.

April 3, 2024
11:00 am (IST)

Star formation exists in all early-type galaxies \(-\) evidence from ubiquitous structure in UV images

Divya Pandey (Aries)


Abstract: Recent surveys have demonstrated the widespread presence of UV emission in early-type (elliptical/S0) galaxies, suggesting the presence of star formation in many of these systems. However, potential UV contributions from old and young stars, together with model uncertainties, makes it challenging to confirm the presence of young stars using integrated photometry alone. This is particularly true in ETGs that are fainter in the UV and have red UV-optical colours. An unambiguous way of disentangling the source of the UV is to look for structure in UV images. Optical images of ETGs, which are dominated by old stars, are smooth and devoid of structure. If the UV is also produced by these old stars, then the UV images will share this smoothness, while, if driven by young stars, they will exhibit significant structure. In this talk, I will compare the UV and optical morphologies of 32 ETGs (93 percent of which are at z<0.03) using quantitative parameters (concentration, asymmetry, clumpiness and the Sersic index), calculated via deep UV and optical images with similar resolution. Regardless of stellar mass, UV-optical colour or the presence of interactions, the asymmetry and clumpiness of ETGs is significantly (often several orders of magnitudes) larger in the UV than in the optical, while the UV Sersic indices are typically lower than their optical counterparts. The ubiquitous presence of structure demonstrates that the UV flux across our entire ETG sample is dominated by young stars and indicates that star formation exists in all ETGs in the nearby Universe.


March 2024

March 20, 2024
11:00 am (IST)

Cosmic rays from star clusters

Biman Nath (RRI)


Abstract: The conventional wisdom is that Galactic cosmic rays are accelerated in supernova shocks of massive stars. However, before they explode, massive stars also produce strong stellar winds. They also appear in clusters, and the collective effect of stellar winds over time can be quite strong, and comparable to the blast wave energy of a supernova explosion. I will argue that stellar winds from massive star clusters offer an alternative site for cosmic ray acceleration, producing up to a quarter of all Galactic cosmic rays, especially in the range of \(10^7-10^9\) GeV, and can also explain a number of phenomena that cannot be accommodated within the supernova paradigm.

March 13, 2024
11:00 am (IST)

Primordial Black Hole Archaeology with Gravitational Waves from cosmic strings and induced tensor perturbations

Anish Ghoshal (University of Warsaw)


Abstract: Ultra-low mass primordial black holes (PBH) which may briefly dominate the energy density of the universe but completely evaporate before the big bang nucleosynthesis (BBN), may lead to interesting observable signatures. We propose a novel test of this scenario by detecting its characteristic doubly peaked gravitational wave (GW) spectrum in future GW observatories. Here the first-order adiabatic perturbation from inflation and from the isocurvature perturbations due to PBH distribution, source tensor perturbations in second-order and lead to two peaks in the induced GW background. These resonant peaks are generated at the beginning of standard radiation domination in the presence of a prior PBH-dominated era. We explore the possibility of probing a class of baryogenesis models wherein the emission of massive unstable particles from the PBH evaporation and their subsequent decay contributes to the matter-antimatter asymmetry. We then include spinning PBHs and consider the emission of light relativistic dark sector particles, which contribute to the dark radiation (DR) and massive stable dark sector particles, thereby accounting for the dark matter (DM) component of the universe. The ISGWB can be used to probe the non-thermal production of these heavy DM particles, which cannot be accessible in any laboratory searches. For the case of DR, we find novel complementarity measurements of \(\Delta N_{\rm{eff}}\) from these emitted particles and the ISGWB from PBH domination. Our results indicate that the ISGWB has a weak dependence on the initial PBH spin. However, for gravitons as the DR particles, the initial PBH spin plays a significant role, and between the only above a critical value of the initial spin parameter \(a^{*}\), which depends only on initial PBH mass, the graviton emission can be probed in the CMB experiment. In the second part of the talk we will discuss how such a PBH-dominated era can be probed successfully using gravitational waves (GW) emitted by local and global cosmic strings. In addition to the step-like suppression of the GW spectrum, we propose a novel feature - a knee in the step - which provides information on the duration of the PBH-dominated era. Detecting GW from cosmic strings by detectors like LISA, ET, or BBO would set constraints on PBHs with masses between \(10^6\) and \(10^9\)g for local strings with tension G\(\mu = 10^{-11}\), and PBHs masses between \(10^4\) and \(10^9\)g for global strings with symmetry-breaking scale \(\eta = 10^{15}\)GeV.

March 6, 2024
10:00 am (IST)

Cosmology and galaxy evolution in the outskirts of galaxy clusters

Roan Haggar (University of Waterloo)


Abstract: Galaxy clusters are some of the most extreme objects in the Universe: huge haloes of dark matter, filled with hot intracluster gas, that are able to shape the lives of galaxies that reside within them. While the central regions of clusters are the most extreme, the quieter outskirts of clusters also hold a huge amount of information, much of which can be extracted with the aid of cosmological simulations. In this talk, I will show how simulations can be combined with observable properties of galaxy clusters to make inferences in two different areas of cosmology and astrophysics. Firstly, I will show how we can predict the frequency of backsplash galaxies -- galaxies that have previously passed through a cluster but now reside in its outskirts \(-\) to better understand the dependence of galaxy evolution on cosmic environment. I will then show how we can use measurements of the density profiles of clusters to constrain cosmological parameters, and break existing degeneracies between these parameters.

March 1, 2024
2:30 pm (IST)

A status review of Fast Radio Bursts

Shriharsh Tendulkar (TIFR)


Abstract: Fast Radio Bursts (FRBs) are a new class of millisecond timescale, extragalactic radio transients. With estimated luminosities between \(10^{42}-10^{44}\) ergs/s, they are a trillion times more luminous than the pulsar pulses they resemble. Due to their prodigious volumetric rate (\(\sim10^4-10^5\) FRBs \(\rm{Gpc}^{-3}\,\rm{yr}^{-1}\)), and large reach (\(\sim\)Gpc), FRBs promise to be excellent cosmological probes\(-\)their dispersion and rotation measures can help probe baryon and magnetic field distributions in the Universe. However, before we use FRBs as cosmological probes, we need to understand their intrinsic properties and biases. We still do not know what the origins of FRBs are, whether repeating and non-repeating FRBs originate from the same astrophysical channel, and whether they are associated with other transients. In this talk, I will review the status of the field, and discuss some recent and new results from the CHIME/FRB, ASKAP, and GMRT telescopes.


February 2024

February 21, 2024
11:00 am (IST)

Accuracy of the small-scale structure of the Lyman-alpha forest in cosmological hydrodynamical simulations

Nabendu Kumar Khan (TIFR)


Abstract: Confronting measurements of the high-redshift Lyman-alpha forest with cosmological hydrodynamical simulations has proved to be a source of stringent constraints on the thermal state of the intergalactic medium (IGM) as well as the small-scale matter power spectrum. In recent times, however, such investigations have led to inconsistent results. This problem will worsen with the arrival of new measurements of the forest that are expected to be accurate to within 5%. In this talk, I will discuss the robustness of current theoretical models of the Lyman-alpha forest that use hydrodynamical cosmological simulations. I will focus on the dependence of the Lyman-alpha forest statistics in these models on the assumed initial conditions. I will discuss the role of glass and grid-based initial conditions, the numerical gravitational coupling between gas and dark matter particles, and cosmological radiation density on the Lyman-alpha forest. By comparing results from five of the most commonly used initial conditions codes, I will show that models that produce the correct linear theory evolution of the power difference between dark matter and baryons predict a Lyman-alpha flux power spectrum that differs from conventional models by up to 50% at k = 0.1 s/km, at redshifts z = 2--5. The difference rapidly worsens towards smaller scales and higher redshifts. While this difference is far larger than the measurement uncertainties expected in upcoming datasets, the finding also suggests a path forward towards more accurate models of the Lyman-alpha forest. I will end my talk by discussing implications for forest-based inferences of the mass of the dark matter particle and the thermal state of the IGM.

February 6, 2024
2:30 pm (IST)

Observational imprints of primordial non-Gaussianity through loop-level contributions

Raghavendra (RRI)


Abstract: In this talk, I intend to discuss the observational signatures of scalar non-Gaussianity through their loop-level contributions to the power spectra, using two specific approaches. The first is a classical approach, by which I shall illustrate the imprints of non-Gaussian contributions from cubic-order action on observables namely, the cosmic microwave background and the scalar-induced secondary gravitational waves (GWs) [1,2]. The second is a quantum field theoretic approach, through which I shall present the computation of loop-level contribution due to the quartic-order action [3]. In these computations, I shall mainly focus on a specific class of inflationary models called ultra slow roll models, which is a promising candidate for the production of primordial black holes (PBHs) and secondary GWs. Through these results using different approaches at different orders and on different observables, I shall emphasize that the non-Gaussian contributions can seriously affect the structure and amplitude of the power spectra and hence warrant careful consideration in calculations while comparing models against data. Lastly, I shall discuss the outlook of these results, particularly in the context of observational quantities that have non-linear dependence on the scalar power, such as the computation of PBH population.


January 2024

January 29, 2024
1:30 pm (IST)

Reconstructing the Early and Late-time Universe

Kushal Lodha (KASI)


Abstract: Despite the success of the standard LCDM model in explaining current observations, the imminent wave of future CMB and LSS data motivates a search for even more robust cosmological frameworks. I will present some bottom-up approaches for reconstructing the early and late universe. In the first half of the talk, I will discuss using genetic algorithms to find features in the primordial power spectrum. In the second half, I will cover our ongoing work to reconstruct dark energy and modified gravity using Gaussian processes and Crossing statistics. If time permits, I will also discuss how these techniques could also be used for consistency checks between cosmological datasets.

January 24, 2024
11:00 am (IST)

A pan-chromatic study of cosmic evolution of AGN using JWST

Devang Liya (Newcastle university)


Abstract: There is evidence of a significant evolution in the galaxy properties around the time of cosmic noon. Since galaxies and their central supermassive black hole (SMBH) co-evolve, understanding of the evolution of SMBH over these redshift could give crucial insights into galaxy evolution. However, past multi-wavelength AGN studies were unsuccessful in disentangling the nuclear emission from the stellar emission at these redshifts. JWST's high resolution infrared imaging has now made it possible to separate these components and study the nuclear emission from AGN which is dominated by the emission from the "torus" structure made of dust and gas, predominantly emitting in the mid-infrared. In this talk, I will give an overview of the instruments onboard JWST along with early public programs. I will then outline our methods to study the evolution of AGN properties as a function of redshift using a large sample of AGN formed by combining data from multi-wavelength surveys and high resolution IR imaging. Finally, I will present the preliminary results from our study of 92 X-ray AGN sources in the EGS field, and conclude with future direction of such studies.

January 19, 2024
2:30 pm (IST)

The observer-dependence of the Hubble parameter: a covariant perspective

Jessica Santiago (National Taiwan University)


Abstract: Given the exponential growth on the upcoming supernovae data available, the possibilities of rigorously testing the cosmological principle becomes ever more real. One of the ways to do so is by measuring the multipole decomposition of the Hubble and deceleration parameters.
In this presentation, I will discuss the observational-theoretical approach, initially introduced by Kristian & Sachs, which allows for the interpretation of data in non-homogeneous and anisotropic universes. I will also explore the effects introduced by the relative motion between the observer and the reference frame defined by galaxies (the matter frame), demonstrating that the luminosity distance should be corrected in such cases.

January 9, 2024
2:30 pm (IST)

In shape with minimal energy. A variational principle for protohaloes

Marcello Musso (Universidad de Salamanca)


Abstract: Analytical models of structure formation routinely assume that dark matter halos form from peaks of the initial density field, smoothed with some spherically symmetric filter. This works reasonably well for identifying the center of mass of protohaloes, but gives no information about their shapes. To provide a more realistic description of protohalo boundaries, one must go beyond the spherical picture. I suggest that this can be done with a simple variational principle, solving for shapes of fixed given mass that minimize the enclosed potential energy. Such regions are delimited by isosurfaces of (a slightly modified version of) the gravitational potential. I will show that these equipotential surfaces provide an excellent description of protohalo shapes, orientations, and associated torques.


December 2023

December 19, 2023
2:30 pm (IST)

Strongly lensed supernovae: New cosmological and astrophysical probes in the time-domain era

Suhail Dhawan (University of Cambridge)


Abstract: Strongly lensed supernovae are excellent, independent probes to measure the Hubble constant and weigh in on the Hubble tension. In my talk I will discuss the time delay measurements for the first resolved strongly lensed Type Ia supernova, iPTF16geu and the discovery of the first lensed SN with the Zwicky Transient Facility, SN Zwicky. I will review ongoing spectroscopic analysis of SN Zwicky, to test - with high-fidelity data - whether high-redshift SNe Ia are similar to their near universe counterparts, an important systematic for dark energy inference. In the LSST era, we expect tens of lensed SNe per year. In this talk, I will summarise ongoing work on the impact of the survey cadence and systematics on the lensed SN discovery rate.

December 1, 2023
9:30 am (IST)

Chronicling the reionization history with \(6 < z < 7\) quasars

Dominika Durovcikova (MIT, USA)


Abstract: The Epoch of Reionization (EoR) represents the last major phase transition of our universe, where the intergalactic medium (IGM) transitions from a completely neutral state following recombination to the ionized state we observe it in today. At redshifts around \(z < 6\) the observed Lyman-series forest in quasar spectra show that the IGM is already highly ionized with volume averaged neutral gas fractions of \(\sim10^{-4}\), while at \(z > 7\) the observed damping wings in the quasar spectra indicate very high neutral gas fractions of > 30%. However, the bulk of the evolution of the EoR remains largely unconstrained to date.
In this work, we use a sample of 19 quasar spectra at redshifts \(6 < z < 7\) to close this gap in our understanding of the time evolution of the EoR. By applying a neural network based method to reconstruct the quasar emission around the partially absorbed Lyman-alpha line, we study the emergence of the Lyman-alpha damping wings with cosmic time. We further compare these predictions to two different simulation-based reionization models, which allows us to present robust constraints of the neutral gas fraction at these redshifts. Concurrently, our method also allows us to constrain the lifetimes of the quasars in our sample, which is crucial for our understanding of the formation of supermassive black holes in the early Universe.


November 2023

November 24, 2023
2:30 pm (IST)

Blown away: relative velocities and the first galaxies

Luke Conaboy (University of Nottingham, UK)


Abstract: Supersonic relative velocities between baryons and dark matter in the nascent Universe affect the formation and growth of the first galaxies, by impeding the accretion of gas into the potential wells of the dark matter haloes. In turn, this can delay the onset of the formation of the first stars and as such is an important effect to consider in the context of the first galaxies. I will talk about our work self-consistently sampling the relative velocity in a cosmological zoom simulation and discuss some future avenues of exploration.

November 17, 2023
2:30 pm (IST)

Understanding cosmic reionization: the production and escape of Lyman-continuum photons in high-redshift galaxies unveiled with VANDELS and JWST

Ryan Begley (Institute for Astronomy, University of Edinburgh)


Abstract: The quest to gain a full understanding of the Epoch of Reionization (EOR) is hampered by our incomplete knowledge of the ionizing properties of the young star-forming galaxies (SFGs) that drive this phase transition of the Universe. A key limitation is the uncertainty in the fraction of hydrogen ionizing (Lyman continuum) photons that escape galaxies (fesc,LyC) into the intergalactic medium (IGM). In this talk, I will discuss recent measurements of fesc,LyC wielding ultra-deep spectroscopy from the VANDELS survey, and examine its connection with other key galaxy properties such as the escape fraction of Ly\(\alpha\) photons, UV luminosity and stellar mass. Another crucial component required in piecing together the timeline of reionization is the ionizing photon production rate of high-redshift galaxies. Complimentary to constraints on LyC escape, I therefore combined VANDELS with the latest data from JWST to explore the redshift evolution of the average ionizing photon production rate using the [O iii]+H\(\beta\) nebular emission of galaxies over the redshift range \(2 \leq z \leq 8\).

November 3, 2023
2:30 pm (IST)

The 21-cm forest as a simultaneous probe of dark matter and cosmic heating history

Yue Shao (Northeastern University, Shenyang, China)


Abstract: The hyperfine structure absorption lines of hydrogen atoms against high-redshift radio point sources, known as the 21 cm forest. They provide a unique probe of small-scale structures during the epoch of reionization, and can be used to constrain the properties of the dark matter thought to govern small-scale structure formation. However, the signals are easily suppressed by heating processes that are degenerate with a warm dark matter model. We propose a probe of both the dark matter particle mass and the heating history of the Universe, using the one-dimensional power spectrum of the 21-cm forest. This approach not only breaks the dark matter model degeneracy but also increases the sensitivity. Using the 21-cm forest observation with the upcoming Square Kilometre Array, it is promising to simultaneously determine both the dark matter particle mass and the heating level of the early Universe, shedding light on the nature of dark matter and the first galaxies.


October 2023

October 27, 2023
2:30 pm (IST)

Simulating the epoch of Helium reionization in photon conserving semi numerical code SCRIPT

Akanksha Kapahthia (NCRA, Pune)


Abstract: Helium reionization refers to the cosmic epoch when the second electron of helium (HeII) gets ionized, predominantly due to quasars and ends at \(z\sim\)3. The impact of helium reionization on the thermal and ionization state of the IGM has important implications for using the lyman alpha forest for precision cosmology at these redshifts. In this talk I will present efficient semi numerical simulations to model the thermal and ionization state of the IGM during helium reionization. For this purpose we modified SCRIPT, which was originally developed for modelling hydrogen reionization.The resolution convergence of SCRIPT allows for running coarse grid simulations, leading to an improved computational efficiency. We make appropriate modifications to SCRIPT for modeling inhomogenous helium reionization and the corresponding thermal history of the IGM is modelled via a subgrid prescription. Our model has three main free parameters i.e. the global clumping factor (C), the temperature increase due to photoheating (\(T_{\mathrm{re}}\)) and the quasar spectral energy distribution (SED) index, (\(\alpha_{\mathrm{UV}}\)) . Our fiducial model reproduces the average empirical measurements of the ionization and thermal history of the IGM during this epoch. The efficiency of our semi numerical model shows promising prospects for parameter estimation during this epoch in future, using measurements of the HeII Lyman alpha forest. I will also briefly touch upon how we intend to model the Lyman alpha forest measurements in our coarse grid simulation box.

October 20, 2023
2:30 pm (IST)

Primordial black holes and stochastic inflation beyond slow roll

Swagat Mishra (University of Nottingham, UK)


Abstract: Primordial Black Holes (PBHs) may form in the early universe, from the gravitational collapse of large density perturbations, generated by large quantum fluctuations during inflation. Since PBHs form from rare over-densities, their abundance is sensitive to the tail of the primordial probability distribution function (PDF) of the perturbations. It is therefore important to calculate the full PDF of the perturbations, which can be carried out non-perturbatively using the 'stochastic inflation' framework. In single field inflationary models, generating large enough perturbations to produce an interesting abundance of PBHs requires violation of slow roll. It is therefore necessary to extend the stochastic inflation formalism beyond slow roll, and consequently there has been a surge in the research interest in this direction in the recent years. A crucial ingredient for this is the stochastic noise matrix corresponding to the small wavelength fluctuations. In this talk, after providing an introduction to PBHs and ultra slow-roll inflation, the speaker will discuss analytical and numerical calculations of these matrix elements for an inflaton potential with a feature which violates slow roll and produces large, potentially PBH generating, perturbations. The seminar will be based on the following work carried out at the Particle Cosmology Group, University of Nottingham, in collaboration with Prof. Edmund J. Copeland and Prof. Anne M. Green: https://arxiv.org/abs/2303.17375

October 13, 2023
11:00 am (IST)

Probing cosmic dawn from the ground to the lunar orbit

Yidong Xu (Chinese Academy of Sciences, Beijing, China)


Abstract: Various 21 cm signals from neutral hydrogen have been proposed to probe the cosmic dawn, including the global 21 cm spectrum, the 21 cm tomography, and the 21 cm forest. They will provide valuable insights into the early structure formation during the cosmic dawn, and have been identified as the key scientific goals for a number of ground-based and space-borne radio experiments, including the international Square Kilometre Array and the Hongmeng project. In this talk, I'll talk about some of the recent research progresses on the theories and 21 cm probes for the cosmic dawn. I will also introduce the Hongmeng project, which is going to measure the global 21 cm signal on lunar orbit.


September 2023

September 15, 2023
2:30 pm (IST)

The pairwise and cross-pairwise y-type polarised kinetic Sunyaev

Aritra Kumar Gon (TIFR)


Abstract: We develop a new theoretical framework for studying the pairwise and cross-pairwise polarised kinetic Sunyaev Zeldovich (pkSZ) effect arising from the transverse peculiar velocity of galaxy clusters. The pkSZ effect is second order in peculiar velocities and has a spectrum that can be decomposed into y-type and blackbody components. We consider pairing of clusters with other clusters as well as cross-pairing of clusters with galaxies from spectroscopic galaxy surveys. We develop and compare estimators of the pairwise pkSZ effect and study the detectability of the pairwise signal with cluster catalogs consisting of a few hundred thousand clusters expected from surveys such as eROSITA and CMB-S4. We find that cross-pairing clusters with galaxies from a large overlapping spectroscopic survey having a few billion galaxies will enable us to detect the pairwise pkSZ effect with CMB-S4. The pairwise pkSZ effect will thus open up a new window into the large-scale structure of the Universe in the coming decades.

September 8, 2023
2:30 pm (IST)

Zeldovich effect from the transverse velocity of galaxy clusters

Digvijay Wadekar (IAS, Princeton)


Abstract: Nearly all of the previous gravitational wave (GW) searches in the LIGO-Virgo data include GW waveforms with only the dominant quadrupole mode \((l,m)=(2,2)\), i.e., omitting higher-order modes such as (l,m)=(3,3),(4,4). I will present detections of new black hole mergers in the LIGO-Virgo O3 data from a search pipeline that includes the higher-order modes. Some of the new detections are astrophysically interesting as the black holes occupy the upper mass gap and/or are in high-redshift range. Towards the end, I will change gears and present results on GW searches for exotic objects with large tidal deformabilities (e.g., boson stars and black holes with axion clouds).

September 1, 2023
9:00 am (IST)

Muon spin force

Yohei Ema (University of Minnesota)


Abstract: Current discrepancy between the measurement and the prediction of the muon anomalous magnetic moment can be resolved in the presence of a long-range force created by ordinary atoms acting on the muon spin via axial-vector and/or pseudoscalar coupling, and requiring a tiny, \(\mathcal{O}(10^{-13}\,{\rm eV})\) spin energy splitting between muon state polarized in the vertical direction. We suggest that an extension of the muon spin resonance (\(\mu\)SR) experiments can provide a definitive test of this class of models. We also derive indirect constraints on the strength of the muon spin force, by considering the muon-loop-induced interactions between nuclear spin and external directions. The limits on the muon spin force extracted from the comparison of \(^{199}\)Hg/\(^{201}\)Hg and \(^{129}\)Xe/\(^{131}\)Xe spin precession are strong for the pseudoscalar coupling, but are significantly relaxed for the axial-vector one. These limits suffer from significant model uncertainties, poorly known proton/neutron spin content of these nuclei, and therefore do not exclude the possibility of a muon spin force relevant for the muon \(g-2\).


August 2023

August 22, 2023
2:30 pm (IST)

Condensed dark matter with a Yukawa interaction

Raghuveer Garani (INFN, Florence)


Abstract: In this talk, we explore possible phases of a condensed dark matter (DM) candidate taken to be in the form of a fermion with a Yukawa coupling to a scalar particle, at zero temperature but at finite density. This theory essentially depends on only four parameters, the Yukawa coupling, the fermion mass, the scalar mediator mass, and the DM density. At low fermion densities, we delimit the Bardeen-Cooper-Schrieffer (BCS), Bose-Einstein Condensate (BEC), and crossover phases as a function of model parameters using the notion of scattering length. We will proceed to discuss the BCS phase in detail by consistently including emergent effects such as the scalar density condensate and superfluid gaps. Within the mean field approximation, we derive the consistent set of gap equations valid in both the non-relativistic and relativistic regimes. Numerical solutions to the set of gap equations are presented, in particular when the mediator mass is smaller and larger than the DM mass, at instances where a phase transition from BEC-crossover-BCS can be observed. We shall conclude with a few applications of the above formalism. Specifically, description of DM equation of state and their astrophysical implications for asymmetric DM halos, and gravo-thermal catastrophe.


July 2023

July 28, 2023
2:30 pm (IST)

High accuracy waveform models for high precision gravitational wave astronomy

Tousif Islam (Dartmouth)


Abstract: Characterization of gravitational wave (GW) signals from the merger of binary black holes (BBHs) heavily rely on accurate gravitational waveform models. The most accurate way to compute waveforms is by solving the Einstein equation directly. This can only be done numerically (e.g. numerical relativity) which is computationally expensive, taking weeks to months for a single simulation. Such a huge computational cost has paved the way for the development of approximate models that are now widely used in data analysis studies. However, these models can compromise accuracy in favor of speed. Recently, data-driven surrogate models have been shown to be capable of generating waveforms that are almost indistinguishable from NR in a fraction of a second. In this talk, I will summarize state-of-the-art NR surrogate models and discuss the application of these models to high-accuracy inference. We find that for many GW events, NR surrogates provide both more accurate and better-constrained parameter estimates in many of the key parameters including sky localization, mass ratio, and spin quantities. Furthermore, using numerical relativity surrogate models, we are able to constrain the properties of the remnant black hole and kick velocity. This information will help understand the environment and evolutionary pathways of black holes.

July 21, 2023
11:00 am (IST)

Screened modified gravity around small, dense sources and tests with hydrogen and muonium

Benjamin Elder (Hawaii University)


Abstract: New theories in physics, such as ones explaining dark energy and dark matter, tend to introduce new scalar particles. Such particles generically couple to Standard Model fermions, and lead to numerous physical observables, most notably a new force law. Hence, these theories typically fall under the umbrella of modified gravity, and tests of gravity and new forces are capable of searching for them. A particularly interesting class of new theories exhibit screening, a dynamical mechanism by which the new force is suppressed around extended objects. I will describe how atomic spectroscopy provides a window into testing screened theories. The discussion will particularly focus on muonium, as it is a system composed only of fundamental particles, and is therefore immune to some of the effects of screening. I will also discuss my work on some of the theoretical puzzles that extremely compact sources like fundamental particles and atomic nuclei present in these models.


June 2023

June 23, 2023
11:30 am (IST)

Reassessing the Constraints from SH0ES Extragalactic Cepheid Amplitudes on Systematic Blending Bias

Doron Kushnir (Weizmann Institute)


Abstract: The SH0ES collaboration Hubble constant determination is \(\sim 5\sigma\) different from the Planck value, known as the Hubble tension. The accuracy of the Hubble constant measured with extragalactic Cepheids depends on robust stellar-crowding background estimation, where \(\gamma=0.24\pm 0.05\) mag is the required unaccounted systematic blending bias to resolve the Hubble tension. I will discuss a method to constrain \(\gamma\) by comparing the light curves amplitudes of extragalactic and MW Cepheids. A careful application of this method leads to an estimate of \(\gamma = 0.013\pm0.057 \)mag. Although the obtained \(\gamma\) is consistent with zero, folding (in quadratures) it is \(\approx 3.0\sigma\) away from aligning with Planck.

June 16, 2023
2:30 pm (IST)

Probing the Cosmic Dawn using the redshifted 21-cm bispectrum

Mohammad Kamran (Uppsala University)


Abstract: In the evolutionary history of our Universe, the Cosmic Dawn and Epoch of reionization (CD-EoR) is the period when the formation and evolution of the very first luminous sources took place. These first sources transitioned the state of the Universe from cold and neutral to a fully heated and ionized one, together with introducing a high level of non-Gaussianity in the heating and ionization field. The CD-EoR 21-cm radiation from the abundant neutral hydrogen atom (HI) in the inter-galactic medium (IGM) is the direct tracer of heating and ionization processes in the IGM during this era. It thus carries the intrinsic non-Gaussian information present in the field. The power spectrum, which is one of the widely used and conventional statistics as a probe of the CD-EoR 21-cm signal in the current observations of this signal, cannot capture this non-Gaussianity. The bispectrum, being a potential probe of this non-Gaussianity, provides an opening for a comprehensive and correct interpretation of this signal. We, for the first time, explore how the bispectrum, probing the non-Gaussianity, can provide better insights into IGM physics during CD-EoR. This will help us in interpreting the future observations of the CD-EoR 21-cm signal using the upcoming Square Kilometer Array (SKA) once the interferometric detection of this signal has been made possible.

June 9, 2023
2:30 pm (IST)

A 3D Numerical Study of Anisotropies in Supernova Remnants

Soham Mandal (Purdue University)


Abstract: Supernova remnants (SNRs) develop fluid instabilities as they expand against the circumstellar medium (CSM). These instabilities are expected to contribute to anisotropies often observed in SNRs. We study this phenomenon by developing a suite of 3D hydrodynamical models using a novel numerical scheme that will be explained briefly. Construction of angular power spectra for these models reveals that our SNR models exhibit a dominant angular mode, which happens to be a diagnostic of their ejecta density profile. This talk will present results that suggest a Kolmogorov-like turbulent cascade is absent in SNRs. Instead, the fluid instability may be governed by independent growth rates for every angular mode. The talk will then go on to show that perturbations (or clumps) in the density field (whether imposed on the supernova ejecta or the CSM) do not influence the anisotropy of the remnant significantly unless they have a very large amplitude and form large-scale coherent structures. On the other hand, small scale structures are found to be independent of clumps and are solely governed by the growth and saturation of Rayleigh-Taylor instability.

June 9, 2023
11:00 am (IST)

Probing Particle Acceleration Processes in Astrophysical and Space Plasma Environments.

Bhargav Vaidya (IIT Indore)


Abstract: Magnetic fields, supersonic shocks, and turbulence are prevalent in astrophysical and space plasma environments. These processes play a crucial role in governing acceleration of particles to extremely high energies typically observed from such environments. Multiple processes of particle energisation collaborate in these environments, whose scales can span megaparsecs. It is crucial that we understand the origin of these highly energized particles and how their emission signatures manifest in observation. A numerical approach is the most suitable strategy for solving such a nonlinear problem. The disparity between the microscopic and macroscopic scales of particle acceleration poses a formidable challenge for modeling distinct physical processes. In an effort to decipher the interplay between these particle acceleration processes, I will present our latest results in adopting hybrid Eulerian-Lagrangian framework using the PLUTO code. The primary focus of my talk would be on the inter-play of particle energisation by supersonic shocks and turbulence, with implications for non-thermal emission signatures from large-scale jets of Active Galactic Nuclei (AGN). In addition, I shall also present a complementary approach of using the MHD-PIC method to emphasize the role of magnetic reconnection in particle energisation and discuss its efficacy in multiple current sheet systems, which are commonly observed in solar coronal regions and near Earths magnetosphere.

June 6, 2023
2:30 pm (IST)

Probing Axionic Instabilities in the late Universe via CMB-B mode

Subhajit Ghosh (Notre Dame)


Abstract: We study the cosmological signatures of a completely secluded dark sector consisting of axion-like particles (ALPs) with anomalous coupling to a dark Abelian gauge boson. The lighter ALP starts rolling during matter domination and produces dark photons through tachyonic instabilities. The resulting exponential growth in dark photon quanta sources tensor and scalar perturbations which are uncorrelated with the inflationary initial perturbation. These perturbations generate temperature and polarization (E and B mode) anisotropies in the CMB. We constrain the parameter space of the ALP-dark photon system using the CMB measurement from Planck and B mode constraints from the BICEP-Keck array. For most of the viable parameter space, the B mode signal is well within the reach of future B mode experiments. Additionally, this scenario exhibits intrinsic CP violation and produces non-zero EB correlation in the CMB spectrum. We analyze the CP violating signature in light of the recent measurement of cosmic birefringence from Planck data which shows striking deviation from CP symmetry.

June 2, 2023
2:30 pm (IST)

A Spin on Wave Dark Matter

Mustafa Amin (Rice University)


Abstract: What can we learn about the intrinsic spin of ultralight dark matter field from astrophysical observations? I will argue that the imprint of spin, ie. whether it is a scalar, vector or a tensor field, can be seen via (i) the initial density power spectrum, (ii) interference patterns in the density field inside dark matter halos, and through, (iii) (polarized) solitons with macroscopic intrinsic spin. Based on features in the initial power spectrum, I will provide a bound on the dark matter mass \(> 10^{-18}\) eV for post-inflationary production. With increasing intrinsic spin, interference patterns in halos are reduced (and the inner shapes of halos modified)-which can be probed by lensing and dynamical heating of stars. After introducing polarized solitons, I will show that the time-scale of emergence of solitons (within halos) increases with increasing spin, and briefly discuss electromagnetic and gravitational wave signatures from such polarized solitons. Time permitting, I might mention connections to "spinor" BECs in the laboratory.


May 2023

May 16, 2023
2:30 pm (IST)

Mapping the baryonic Universe: a new window into the cosmos

Hamsa Padmanabhan (University of Geneva)


Abstract: The evolution of the baryonic (normal) matter in the Universe is an excellent probe of the formation of cosmic structures and the evolution of galaxies. Over the last decade, considerable effort has gone into investigating the nature of baryonic material, theoretically and observationally. The technique of intensity mapping (IM), which measures the integrated emission from sources over a broad range of frequencies, is a promising probe of cosmological baryons. A particular advantage of IM is that it provides a tomographic, or three-dimensional picture of the Universe, unlocking significantly more information than available from traditional galaxy surveys. Astrophysical uncertainties, however, constitute an important systematic in our attempts to constrain cosmology with IM. I describe an innovative approach which allows us to fully utilize our current knowledge of astrophysics in order to develop cosmological forecasts from IM. Extensions of this model pave the way towards a comprehensive understanding of molecular gas evolution, allowing us to interpret results from upcoming surveys. This opens up the exciting potential of constraining physics beyond the LCDM model from future IM observations.


April 2023

April 28, 2023
2:30 pm (IST)

The GMRT-CATz1 Survey: Atomic Hydrogen in Galaxies at \(z\sim1\)

Aditya Chowdhury (NCRA)


Abstract: For many decades, a critical lacuna in our understanding of galaxy evolution has been that we knew nothing about the neutral atomic hydrogen (HI) content of high-z galaxies. Over the last few years, we have used the upgraded GMRT to carry out the GMRT-CATz1 survey, a 510-hour survey of the HI 21 cm line in star-forming galaxies at \(z=\)0.74-1.45. The survey led to the first measurement and characterisation of the HI content in galaxies at \(z\sim\)1 and an explanation to the long-standing puzzle of why the star-formation activity of the Universe declined rapidly after its peak at \(z\sim\)1-3. In this talk, I will describe the above as well as present other key results from the GMRT-CATz1 survey, including the first measurements of HI scaling relations, the HI mass function and the gas accretion rate at these redshifts. Finally, I will briefly mention our many ongoing efforts to use the upgraded GMRT to study HI in high-z galaxies, including a project to push HI 21 cm emission studies to \(z\sim\)2-3. For many decades, a critical lacuna in our understanding of galaxy evolution has been that we knew nothing about the neutral atomic hydrogen (HI) content of high-z galaxies. Over the last few years, we have used the upgraded GMRT to carry out the GMRT-CATz1 survey, a 510-hour survey of the HI 21 cm line in star-forming galaxies at z=0.74-1.45. The survey led to the first measurement and characterisation of the HI content in galaxies at z~1 and an explanation to the long-standing puzzle of why the star-formation activity of the Universe declined rapidly after its peak at z~1-3. In this talk, I will describe the above as well as present other key results from the GMRT-CATz1 survey, including the first measurements of HI scaling relations, the HI mass function and the gas accretion rate at these redshifts. Finally, I will briefly mention our many ongoing efforts to use the upgraded GMRT to study HI in high-z galaxies, including a project to push HI 21 cm emission studies to z~2-3.

April 25, 2023
2:30 pm (IST)

New statistical and ML-based methods for weak lensing and galaxy clustering analysis for Stage-IV surveys

Supranta Sarma Boruah (University of Arizona)


Abstract: With the increased observational precision of Stage-IV dark energy surveys, we will face new statistical and data analysis challenges. I will talk about two such challenges for weak lensing and galaxy clustering analysis and our efforts to solve these problems. First, preparing for standard 2-point analysis requires running hundreds (maybe even thousands) of MCMC chains, which will form a substantial computational bottleneck for the Stage-IV surveys. I will present a new iterative emulator method using neural networks that leads to fast and efficient inference in high dimensional parameter spaces - thus solving a major computational problem for Stage-IV data analysis. I will also talk about some recent investigations into the impact of systematics on LSST 3x2 pt analysis. Next, 2-point analyses are necessarily suboptimal as the late time cosmic density is highly non-Gaussian. I will talk about our efforts to build a Bayesian map-level inference method to analyse weak lensing data. I will talk about the recent progress, challenges and promises of such field-level analysis which is the optimal way to extract information from a data set at a given scale.

April 21, 2023
2:30 pm (IST)

On the Dynamics of Dwarf Galaxies and Nuclear Objects in a Fuzzy Dark Matter Halo

Dhruba Dutta Chowdhury (Hebrew University)


Abstract: Fuzzy Dark Matter (FDM), consisting of ultralight bosons, is an intriguing alternative to Cold Dark Matter (CDM). Unlike in CDM, FDM halos consist of a central solitonic core, surrounded by an envelope of order unity density fluctuations. The envelope density fluctuations also interact with the soliton causing it to wobble and oscillate. Using novel, high-resolution numerical simulations of an FDM halo corresponding to a particular boson mass, I will demonstrate that the gravitational potential fluctuations associated with the soliton's wobble, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and dwarf galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction and an equilibrium is reached. Similarly, dwarf galaxies continue to increase their sizes and central velocity dispersions. In addition, their kinematic structures become strongly radially anisotropic, especially in the outskirts. Dynamical heating also causes initially ellipsoidal galaxies to become more spherical over time from the inside out and gives rise to distorted, non-concentric isodensity contours. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, morphologies, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) can potentially constrain the boson mass.

April 14, 2023
2:30 pm (IST)

The Distribution of Neutral Hydrogen in the Local Universe

Saili Dutta (NCRA)


Abstract: Hydrogen is the most abundant element in the Universe. It can be characterized as being in the ionized, neutral atomic (HI), or molecular (\(\mathrm{H}_2\)) states. Though hydrogen gas is almost completely ionized in the post-re-ionization era, there are still traces of HI locked up inside galaxies, providing fuel for future star formation of the galaxy. HI is therefore a useful probe to study the formation and distribution of large-scale structures in the Universe. I will discuss how different populations of galaxies contribute to the total HI budget, \(\Omega_{\mathrm{HI}}\), in the local Universe. I will present how these populations of galaxies make up the total HI mass function, the HI width function, and the HI velocity function. Also, I will describe a model, based on these results, to populate HI in dark matter halos motivated and calibrated from recent observations.


March 2023

March 31, 2023
2:30 pm (IST)

Blue Isocurvature Perturbations and EFTofLSS based Forecast

Sai Chaitanya Tadepalli ( University of Wisconsin-Madison, USA)


Abstract: In this talk, we will present blue tilted axionic isocurvature spectra and possible constraints/sensitivities from experiments. Firstly, we will discuss the strongly blue tilted (nI\(\sim\)4) isocurvature spectrum generated during inflation for the mass parameter region just when the quantum oscillator modes are starting to be underdamped. Interestingly, there exist parametric regions with a strong resonant spectral behavior that leads to rich isocurvature spectral shapes and large amplitude enhancements. These can be particularly interesting with observational consequences, especially in relation to PBH generation in certain models. Lastly, we will examine the current 2-sigma evidence from last Planck/BOSS results and forecast constraints/sensitivities for LSS experiments such as Euclid (upcoming) and MegaMapper (proposed) using EFTofLSS based perturbative technique for a better theoretical error control. In the process we will comment upon the consistent renormalization requirements for mixed adiabatic and blue isocurvature primordial initial conditions.

March 24, 2023
2:30 pm (IST)

Role of fly-by interactions in the kinematic and morphological evolution of galaxies

Ankit Kumar (IIA, Bangalore)


Abstract: Galaxies are dynamic objects and are usually found in groups and clusters where they interact with each other gravitationally. Broadly speaking, galaxies go through two types of interactions: (1) mergers and (2) fly-bys. Galaxy fly-bys are as frequent as mergers and play a vital role in the formation and evolution of galaxies as they involve the exchange of significant amounts of mass and energy. As a part of my PhD thesis project, I have studied the effect of fly-by interactions and dark matter distribution on the evolution of disk galaxies. In this presentation, I will discuss the importance of fly-by interactions in the evolution of the Milky Way mass host (primary) galaxy. I will mainly focus on the evolution of bulges, the formation of spiral patterns and the origin of vertical breathing motion as observed in the Milky Way. I will also emphasize the importance of computational facilities to understand the basic physics behind galaxy formation and evolution.

March 23, 2023
4:00 pm (IST)

Neutrino flavor mixing, outflow hydrodynamics, and \(\nu p\)-process nucleosynthesis in supernovae.

Amol Patwardhan (SLAC)


Abstract: Neutrinos in core-collapse supernovae are the main carriers of energy and lepton number, and therefore play an important role in the explosion mechanism as well as in the synthesis of nuclides in these environments. In the aftermath of a supernova explosion, neutrino-induced heating drives outflows of baryonic matter from the surface of the nascent neutron star. The physical characteristics of these outflows, such as expansion timescale, entropy, and electron fraction, can significantly impact the synthesis of proton-rich isotopes via the \(\nu p\)-process. This could be of much relevance to a long-standing problem in nuclear astrophysics, pertaining to the origin of certain proton-rich nuclides in nature: \(^{92,94}\)Mo and \(^{96,98}\)Ru. In particular, self-consistently modeled subsonic outflows from explosions of massive progenitors can be shown to furnish \(\nu p\)-process yields consistent with observed Mo and Ru abundances. These isotopic yields can also be directly influenced by neutrino flavor mixing in the vicinity of the neutron star. In this talk, we examine this interplay between matter outflows, neutrino mixing, and nucleosynthesis in core-collapse supernovae.

March 17, 2023
10:00 am (IST)

Inferring the impact of feedback on the matter distribution using the Sunyaev Zel'dovich effect

Shivam Pandey (U Penn)


Abstract: Feedback from active galactic nuclei and stellar processes changes the matter distribution on small scales, leading to significant systematic uncertainty in weak lensing constraints on cosmology. In this talk, I will describe how the observable properties of group-scale halos can be used to constrain the impact of feedback on the matter distribution using Cosmology and Astrophysics with Machine Learning Simulations (CAMELS). By extending the results of previous work to smaller halo masses and higher wave numbers, \(k\), we find that the baryon fraction in halos contains valuable information about the impact of feedback on the matter power spectrum. We will also explore how the thermal Sunyaev Zel'dovich (tSZ) signal from group-scale halos can provide similar information. Using recent Dark Energy Survey (DES) weak lensing and Atacama Cosmology Telescope (ACT) tSZ cross-correlation measurements and models trained on CAMELS, we obtain \(10\%\) constraints on the effects of feedback on the power spectrum at \(k\sim 5\,h/\)Mpc. I will also demonstrate that with future surveys, it will be possible to constrain baryonic effects on the power spectrum to O(\(<1\%\)) at \(k = 1\,h/\)Mpc and O(\(3\%\)) at \(k = 5\,h\)/Mpc using the methods introduced here.

March 10, 2023
2:30 pm (IST)

EDGES of the Dark Forest

Anoma Ganguly (TIFR)


Abstract: We propose a new method to hunt for dark matter using dark forest/absorption features across the full electromagnetic spectrum, especially in the bands where there is a desert i.e. regions where no strong lines from baryons are expected. Such unique signatures can arise for dark matter models with a composite nature and internal electromagnetic transitions. In the presence of a background source, such as a quasar, such interactions in the dark matter halos can produce a series of closely spaced absorption lines, which we call the dark forest. The dark forest feature is a sensitive probe of the dark matter self-interactions and the halo mass function, especially at the low mass end. Moreover, the absorption of CMB photons by dark matter gives rise to a global absorption signal in the CMB spectrum which can explain the anomalous absorption feature detected by the EDGES collaboration.

March 3, 2023
2:30 pm (IST)

Gravitational wave memory- triggered supernova neutrino detection

Mainak Mukhopadhyay (Penn. State University)


Abstract: When a burst of neutrinos from a core-collapse supernova (CCSN) passes by the Earth, it causes a permanent change in the local space-time metric, called the gravitational wave (GW) memory. Long considered unobservable, this effect will be detectable in the near future, at deci-Hertz GW interferometers. I will present a novel idea, where observations of the neutrino GW memory from CCSNe will enable time-triggered searches of supernova neutrinos at megaton (Mt) scale detectors. This combination of a deci-Hz GW detector and a Mt neutrino detector will allow the latter to surpass its current sensitivity limits to detect a nearly background-free sample of \(\sim\) 3-30 supernova neutrino events per Mt per decade of operation, from large distances (\( \sim\) 10-100 Mpc), which will open a new avenue to studying supernova neutrinos.


February 2023

February 10, 2023
2:30 pm (IST)

Dark Neutrons: Cosmology, Astrophysics & Laboratory

Nirmal Raj (TRIUMF National Lab)


Abstract: New states that mix with the neutron, such as dark baryons and mirror neutrons, have been proposed to address dark matter, baryogenesis, the long-standing neutron lifetime anomaly, and the recent (albeit later disfavoured) XENON1T excess. First I show that such states are extensively probed by cosmological epochs and astrophysical systems where protons, neutrons and electrons play a central role. In particular, wide-ranging constraints arise from (1) Big Bang nucleosynthesis, (2) cosmic microwave background spectra, (3) the stability of nuclides in low-metallicity stars, (4) a novel mechanism to heat neutron stars by tapping the energy stored in their Fermi seas. Then I show that even more sensitive to dark neutrons would be a new "neutrons-shining-through-a-wall" search at a deep-underground accelerator facility such as the imminent IsoDAR experiment, and a reinterpretation of neutron disappearance searches at ultra-cold neutron facilities.

February 10, 2023
9:00 am (IST)

Novel systematics for 21 cm LIM cosmology as a cosmological probe in the post-reionization era

Heyang Long (Ohio State University)


Abstract: 21 cm intensity mapping (IM) has the potential to be a strong and unique probe of cosmology from redshift of order unity to redshift potentially as high as 30. The upcoming dedicated 21 cm IM experiments such as PUMA and SKA aims to map the Universe up to \(z\sim\) 6 and higher. But for the post-reionization 21 cm observations, the signal is modulated and the inference for \(\Lambda\)CDM model will be shadowed by several effects, like the hydrodynamic and thermodynamic response of IGM to the passage of ionization fronts during the Epoch of Reionization, as well as the streaming velocities between dark matter and baryons at the time of recombination. In this talk, I will present the impact of inhomogeneous reionization on the post-reionization 21 cm power spectrum and the forecasted induced shifts of cosmological parameters at redshifts \(3.5 \leq z\leq 5.5 \) in SKA1-LOW and PUMA. Furthermore, I will also showcase the estimated shifts of Baryon Acoustic Oscillation (BAO) in PUMA due to streaming velocity effect at \(3.5 \leq z \leq 5.5\).

February 3, 2023
2:30 pm (IST)

Studying the end of Epoch of reionization with the 21-cm Signal

Janakee Raste (NCRA, Pune)


Abstract: Recent constraints from Lyman\(-\alpha\)and CMB data suggest a significantly delayed reionization scenario in which IGM is ionized to 50% at redshift \(z\sim\) 7. In these models, reionization ends at \(z\sim\) 5.3, with large "islands" of cold, neutral hydrogen persisting in the IGM well below \(z =\) 6. We have studied these models using state-of-the-art radiative transfer simulations of the IGM calibrated to the CMB and Lyman-_ forest data. In this talk, I will discuss effects of these neutral hydrogen islands on the 21cm signal. In contrast with previous models, we find that thanks to the late end of reionization, the 21cm power at \( z=5-6 \) predicted by our simulations is several orders of magnitude higher than that in conventional models considered in the literature for these redshifts. While the power spectrum is often the primary statistics to study the 21cm signal, these neutral islands are lowest density voids and are expected to be highly non-gaussian. With this motivation, I will also discuss the unique signature of these islands on the 21cm bispectrum signal. The delayed end of reionization moves the window of opportunity for the 21cm signal observation to higher frequencies, which will make the observational efforts easier due to easier thermal noise characteristics and synergies with abundant multi-wavelength observations.


January 2023

January 27, 2023
2:30 pm (IST)

Unmasking the Universe with neural nets

Punyakoti G. V. (Technion, Israel)


Abstract: I will present a neural network method for reconstructing the underlying 3D cosmological density and velocity fields from discrete and incomplete observed galaxy distributions. These reconstructions are a powerful probe of cosmological parameters. One of the main aims of my talk is to demystify neural networks by clarifying their relation to different conventional statistical estimators. After discussing this, I will explicitly compare the performance of our reconstruction method with the traditional Wiener filter and highlight the advantages of the neural network approach, particularly in capturing nonlinear features. I will conclude with a discussion of the impact of neural networks on the future of the field.

January 13, 2023
2:30 pm (IST)

Mass Distributions of PBHs in the light of 21cm brightness temperature

Upala Mukopadhyay (ISI, Kolkata)


Abstract: The evaporation of primordial black holes (PBHs) via Hawking radiation influences the evolution of Inter - Galactic Medium by heating up the latter which consequently affects the 21cm signal from neutral Hydrogen atoms. In this work, we have considered EDGES observational data for 21cm line from the era of cosmic dawn to constrain the mass and the abundance of PBHs. In this context, two different PBH mass distributions namely, power law and lognormal mass distributions are considered to estimate the effects of PBH evaporation on the 21cm brightness temperature \(T_{21}\). The impacts of dark matter-baryon interactions on \(T_{21}\) are also considered in this work along with the influences of PBH evaporation. Furthermore, an attempt has been made to formulate a distribution function for PBH masses. This has been addressed by associating with every calculated \(T_{21}\) value corresponding to a pair of values of PBH mass and initial PBH mass fraction, a probabil- ity weightage considering the range of \(T_{21}\) (at \(z\sim 17.2\)) given by EDGES experiment.

January 6, 2023
2:30 pm (IST)

Effect of helium on hydrogen reionization

Shikhar Asthana (University of Cambridge)


Abstract: In the early universe, the birth of the first sources produced ample amounts of ultraviolet radiation capable of converting the intergalactic medium from neutral to ionized, a phase known as the epoch of reionization. The timing, duration and cause of reionization is still not well understood. However, over the past decade research into this has exploded with the advent of moment based radiative transfer simulations. It has been found that a late and rapid reionization model leads to the best fit to the current observations. However, these results were derived without the presence of helium in the simulation models, and this is the problem I worked on at TIFR. In my talk, I will discuss the implementation of helium into ATON, a fast GPU based radiative transfer program, the cosmological simulations (160 cMPc/h) we ran, the effects of helium on the observables of the lyman-alpha forest data, and what the simulation suite can further be used for.

January 3, 2023
2:30 pm (IST)

Probing the Cosmic Reionization using an efficient SCRIPT

Barun Maity (NCRA)


Abstract: According to the standard model of cosmology, the universe was mostly ionized and hot at very early stages. Then it cooled down with time and became predominantly neutral around 380,000 years after birth. Reionization is the era when the universe is again ionized by the photons coming from the first luminous sources. This is still one of the least understood phases in the evolutionary history of the Universe and is also known as one of the final frontiers in modern cosmology.
The ionization and thermal state of the intergalactic medium (IGM) during the epoch of reionization has been of interest in recent times because of their close connection to the first stars. We try to constrain the thermal and ionization history of the universe using a semi-numerical photon conserving model SCRIPT and a variety of observables like UV luminosity function, low-density IGM temperatures, CMB scattering optical depth, etc. We study the consequences of physical effects like inhomogeneous recombination and radiative feedback on the reionization phenomena, which is necessary for accurate modelling. We find that the model parameters are reasonably well constrained which can give insights into the reionization timeline. The bounds will certainly be improved with more observational data coming in near future.
As we track the inhomogeneities in the medium, we can also compute the large-scale 21cm power spectra which quantifies the fluctuations in the neutral hydrogen field. We check the prospects of 21cm power spectra as a tracer of reionization. Our study involves creating a mock data set corresponding to the upcoming SKA-Low, followed by a Bayesian inference method to constrain the model parameters. In particular, we explore in detail whether the inferred parameters are unbiased with respect to the inputs used for the mock and if the inferences are insensitive to the resolution of the simulation. We find that the model is reasonably successful on both fronts.


December 2022

December 30, 2022
2:30 pm (IST)

ICM & gaseous halos: cooling clouds, heating and transport mechanisms

Prakriti Pal Choudhury (University of Cambridge)


Abstract: Energy transport across a wide range of dynamical scales in the intracluster medium (and generally in gaseous halos) is one of the most interesting topics in current research and future interest. Hot baryons, visible in the X-rays, need to be stably sustained against radiative cooling over a large inner fraction of the cluster virial radius. A historical motivation has been the lack of sufficient observed cold gas in the cluster cores that is expected in the absence of efficient heating. Quantitatively, there is enough energy from active galactic nuclei to solve the problem at the simplest level, but the complexity of how that energy flows around is not well understood. Multiple transport mechanisms are being actively discussed including long wavelength, nearly isotropic sound waves, anisotropic heat conduction only along local magnetic fields (depending on the local temperature gradient), generation and dissipation of volume-filling turbulence, etc. While sound waves and turbulence have been strong contenders, thermal conduction has been claimed to be further suppressed by gyro-scale whistlers that scatter thermal electrons efficiently in the weakly magnetised ICM. In the latter scenario, thermal instability domain may be enhanced leading to excess and/or smaller scale cold gas. This further implies that observations may need to account for excess cold/mixed phase gas. In my talk, I will discuss these topics of energy transport in the ICM and the consequences.

December 23, 2022
2:30 pm (IST)

When Matter Effects matter for identifying Sterile mass ordering

Dibya Sankar Chattopadhyay (TIFR)


Abstract: We analytically calculate the neutrino conversion probability in the presence of sterile neutrinos, with exact dependence on \(\Delta m^2_{41}\) and with matter effects explicitly included. The terms involving sterile mixing angles \(\theta_{24}\) and \(\theta_{34}\) are separated out perturbatively, with the effects of the latter only arising due to matter effects. Furthermore, the neutrino conversion probability, written as a summation of terms of the sin(x)/x form, allows a physical understanding of matter effects and their resonance-like behaviour.
We focus on the identification of sterile mass ordering (sign of \(\Delta m^2_{41}\) at a long baseline experiment like DUNE. The analytic expressions derived bring out the complex interplay between sterile and matter contributions. We numerically calculate the sensitivity of DUNE to sterile mass ordering over a large range of \(\Delta m^2_{41}\). The dependence of this sensitivity, on \(\Delta m^2_{41}\) values and all mass ordering combinations, can be explained using our analytic expressions.

December 13, 2022
2:30 pm (IST)

If dark matter is fuzzy, the first stars form in massive pancakes

Mihir Kulkarni (University of Toledo)


Abstract: Fuzzy dark matter (FDM) is a proposed modification for the standard cold dark matter (CDM) model motivated by small-scale discrepancies in low-mass galaxies. Composed of ultra-light (mass \(\sim10^{-22}\) eV) axions with kpc-scale de Broglie wavelengths, this is one of a class of candidates that predicts that the first collapsed objects form in relatively massive dark matter halos. This implies that the formation history of the first stars and galaxies would be very different, potentially placing strong constraints on such models. I will describe our numerical simulations of the formation of the first stars in an FDM cosmology, following the collapse in a representative volume all the way down to primordial protostar formation including a primordial non-equilibrium chemical network and cooling for the first time. We find two novel results: first, the large-scale collapse results in a very thin and flat gas "pancake"; second, despite the very different cosmology, this pancake fragments until it forms protostellar objects indistinguishable from those in CDM. Combined, these results indicate that the first generation of stars in this model are also likely to be massive and, because of the sheet morphology, do not self-regulate, resulting in a massive Pop III starburst. We estimate the total number of first stars forming in this extended structure to be \(10^4\) over 20 Myr using a simple model to account for the ionizing feedback from the stars, and should be observable with JWST. These predictions provide a potential smoking gun signature of FDM and similar dark matter candidates.

December 9, 2022
2:30 pm (IST)

Evidence for strong progenitor age bias in supernova cosmology

Young-Wook Lee (Yonsei University , Seoul)


Abstract: Supernova (SN) cosmology is based on the assumption that the width-luminosity relation (WLR) and the colour-luminosity relation (CLR) in the type Ia SN luminosity standardization would not show zero-point offsets with progenitor age. Unlike this expectation, recent age datings of stellar populations in host galaxies have shown significant correlations between progenitor age and Hubble residual (HR). Here we show that this correlation originates from a strong progenitor age dependence of the zero-points of the WLR and CLR, in the sense that SNe from younger progenitors are fainter each at given light-curve parameters x1 and c. This 4.6 sigma result is reminiscent of Baade's discovery of the zero-point variation of the Cepheid period-luminosity relation with population age, and, as such, causes a serious systematic bias with redshift in SN cosmology. Other host properties show substantially smaller and insignificant offsets in the WLR and CLR for the same dataset. We illustrate that the differences between the high-z and low-z SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift may well be an artifact of over-correction in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is little evidence left for an accelerating universe, posing a serious question to one of the cornerstones of the concordance model.


November 2022

November 25, 2022
2:30 pm (IST)

The theory and applications of interferometric constants to astronomical measurements

Nithyanandan Thyagarajan (CSIRO)


Abstract: In an interferometer composed of \(N>=3\) elements, there are certain quantities that are invariant under local corruption factors like the medium of propagation and the instrument itself. I will describe this concept from the context of radio interferometry, and provide a prescription for determining the complete and independent set of invariants using 3-element closed loops as fundamental units while unifying different descriptions for various classes of these invariants. I will also highlight some of their key astronomical applications, including the inference of morphological parameters of the supermassive black holes at the centres of M87 and the Milky Way using planet-scale Very Long Baseline Interferometry, and in observational cosmology using redshifted 21 cm line to detect the formation of large-scale structures in the high-redshift Universe.

November 18, 2022
11:00 am (IST)

Neutrino forces and where to find them.

Mitrajyoti Ghosh (Cornell University, USA)


Abstract: In the non-relativistic limit, scattering of two particles by boson exchange can be described using a static potential, i.e, that of a force between them. The exchange of two fermions can also lead to a force, as if the two fermions behave like an effective boson. These forces are called "quantum forces", and the range of these forces is inversely proportional to the mass of the fermions being exchanged. In particular, the exchange of neutrinos leads to a long-ranged force due to neutrinos being the lightest fermions in the Standard Model. In this talk, I will review the neutrino force and discuss possible probes of this force. I will talk about scenarios where the effects due to the neutrino force can be enhanced, thereby providing sensitivity to neutrino physics parameters that have so far eluded us.

November 4, 2022
2:30 pm (IST)

Truth and Beauty in Constructing the Standard Model of Cosmology

Suvodip Mukherjee (TIFR)


Abstract: The era of observational cosmology using multi-messenger observations is taking its pace. I will discuss how this avenue can explore new frontiers that can shape our understanding of the standard model of cosmology. I will show some latest findings using the current gravitational wave data and a roadmap for discovering new physics from the upcoming multi-band observatories in the coming years.


October 2022

October 28, 2022
2:30 pm (IST)

Quasar feedback and the origin of extended Ly\(\alpha\) glow in \(z > 6\) quasars

Tiago Costa (Max Planck Institute for Astrophysics)


Abstract: The detection of quasars out to \(z \approx 7.5\) shows that supermassive black holes with masses of \(\approx 10^9 M_{\odot}\) have already assembled by the time the Universe was only \(\approx\) 680 Myr old. These observations strenuously test theoretical models of galaxy evolution, which have to explain how such rapid black hole growth comes about. I will start by reviewing results from state-of-the-art cosmological simulations that show that black hole growth to \(\approx 10^9 M_{\odot}\) can be accommodated by galaxy evolution models. These black holes, however, must evolve inside rare, massive dark matter haloes tracing extreme overdensities. Zooming-in on the quasar host galaxies, I will argue that the required rapid black hole growth is expected to ignite powerful quasar feedback in the form of large-scale outflows. I will illustrate how these outflows, which are characterized by a multi-phase structure resembling that of the interstellar medium, affect quasar environments in a myriad of ways. I will then demonstrate how the same cosmological simulations that succeed in reproducing \(\approx 10^9 M_{\odot}\) black holes also account for recent observations of bright, extended Ly\(\alpha\) nebulae around \(z > 6\) quasars. I will make the case that the uncanny match between theoretical models and observations is only possible if quasar feedback already operates efficiently in \(z > 6\) quasars. I will conclude my talk by highlighting new theoretical insights into the nature of Ly\(\alpha\) nebulae at \(z = 6\), explaining their detailed observational properties, their dominant physical mechanism, and the potential to detect them at \(z = 7.5\).

October 18, 2022
2:30 pm (IST)

Testing the cosmological principle through topological properties of cosmic microwave background fluctuations

Pratyush Pranav (ENS Lyon)


Abstract: Cosmology is transitioning from a theoretical discipline towards one with increased focus on observations, resulting in a massive surge of data that demands increasingly more sophisticated methods to glean meaningful information. In a related development, geometry and topology have witnessed a tilt from purely theoretical fields towards strong focus on application. A foray into "big data" quickly brings to front two of the central statistical challenges of our times -- detection and classification of structure in extremely large, high-dimensional, data sets. Among the most intriguing new approaches to this challenge is "TDA," or "topological data analysis," the primary aim of which is providing topologically informative pre-analyses of data, which serve as input to more quantitative analyses at a later stage. Algebraic and computational topology at the level of homology and persistent homology are the foundational pillars of TDA. These developments on the topological side are recent, and add value to the already existing geometric tools and methodologies employed in investigating the cosmological fields.
In the first part of my talk, I will present a summary of the theoretical and computational aspects of geometry and topology from the view point of data analysis. Subsequently, I will present an analysis of the topological properties of the temperature and polarization maps of the cosmic microwave background (CMB) radiation obtained by the Planck satellite. The CMB radiation represents the earliest visible light in the Universe, and contains a treasure trove of information about the initial conditions of the Universe. The CMB also represents the largest canvas on which to test the fundamental tenets of the cosmological principle. Within this context, I will discuss some of the anomalies that the CMB temperature and polarization maps exhibit with respect to the simulations based on the standard cosmological model, which assumes the initial fluctuation field to be an instance of an isotropic and homogeneous Gaussian random field.

October 14, 2022
2:30 pm (IST)

Detecting the First Stars with REACH and Bayesian Statistics

Dominic Anstey (University of Cambridge)


Abstract: The development of the universe between the emission of the CMB at the epoch of recombination and the emergence of modern structure is currently quite poorly understood. One of the most promising mechanisms for probing this period is the absorption of radiation at 21cm by neutral hydrogen, as the magnitude of absorption is affected by the properties of the surrounding gas and radiation field. As a result, the development of these properties can be traced if this absorption signal can be measured.
In this talk, I will give an overview of the current state of 21-cm cosmology and introduce the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH). I will also discuss of the challenges involved in detecting the 21-cm signal, particularly the presence of bright radio foregrounds that can overwhelm the signal and systematic distortions arising from the antenna, and my work on overcoming these as part of REACH, using Bayesian statistical techniques.

October 7, 2022
2:30 pm (IST)

The need for obscured supermassive black hole growth to explain high redshift quasar proximity zones.

Sindhu Satyavolu (TIFR)


Abstract: The existence of supermassive black holes (\(M_{\mathrm{BH}} \gtrsim 10^9 M_{\odot}\)) at a time when the Universe was only about a Gyr old is an open problem in astrophysics and cosmology. The duration for which a supermassive black hole (SMBH) is accreting as a quasar is defined as the quasar lifetime. Proximity zones of quasars probe the highly ionized regions around quasars and have been used to infer their lifetimes. The recent measurements of small proximity zones in a fraction of \(z\sim\) 6 quasars seemed to imply very short lifetimes, making it further challenging to explain the black hole masses at this redshift. In this talk, I will discuss the robustness of some of the assumptions that are usually made to infer quasar lifetimes from proximity zone sizes. I will show that thanks to the short equilibration time of gas inside the proximity zones, small proximity zones can be readily explained by quasars that vary in brightness with a short duty cycle of \(f_\mathrm{duty}\sim 0.1\) and short bright periods of \(t_\mathrm{on}\sim 10^4\) yr, even for long lifetimes. I will then show that reconciling this with black hole mass estimates requires the black hole to continue to grow and accrete during its obscured (when it's not visible as a quasar) phase. Further, I will show that incomplete reionization can impede the growth of proximity zones and make them smaller up to 30%, and the quasar host halo mass only affects proximity zones weakly and indirectly.


September 2022

September 23, 2022
2:30 pm (IST)

Do cosmological observations allow a negative \(\Lambda\)?

Anjan Sen (JMI)


Abstract: In view of the recent measurement of \( H_{0} \) from HST and SH0ES team, we explore the possibility of existence of a negative cosmological constant in the Universe. In this regard, we consider quintessence fields on top of a negative cosmological constant and compare such construction with \(\Lambda\)CDM model using different combination of CMB, SnIa, BAO and \(H_{0}\) data. Various model comparison estimators show that quintessence models with a negative \(\Lambda\) is either preferred over \(\Lambda\)CDM or performs equally well as \(\Lambda\)CDM model. This suggests that the presence of a negative \(\Lambda\) (AdS ground state) in our Universe, which can naturally arise in string theory, is consistent with cosmological observations.

September 20, 2022
2:30 pm (IST)

The cosmic web environment of dark matter haloes: for an enhanced understanding of structure formation

Sujatha Ramakrishnan (IUCAA)


Abstract: Cosmological interpretation of data from galaxies and cluster surveys relies on our understanding of hierarchical structure formation in which galaxies are hosted by dark matter haloes which are biased tracers of the dark matter distribution. More often than not, this picture leaves out details of the dark matter haloes such as its assembly history and the structure of the cosmic web of its environment. For e.g, we lack a first principles understanding of the halo assembly bias, which describes the correlation between internal properties of dark matter haloes and the large-scale halo clustering (or halo bias) at fixed halo mass.
Understanding and calibrating these effects also continues to be of interest from the point of view of galaxy formation and evolution, it will also be a potential systematic in the inferences made by upcoming large-volume surveys.
Using a suite of cosmological simulations, we quantify the halo's cosmic web environment by the tidal anisotropy at 4 times halo radius. We then show that the halo assembly bias comprises two statistical links: one between the halo's internal property and its tidal anisotropy and the other between the tidal anisotropy and the large-scale bias. We then use the Separate Universe technique to calibrate for the dependence of linear and quadratic halo bias on the tidal anisotropy as well as several other halo properties. Our calibration of the quadratic assembly bias is the first demonstration of the dependence of non-linear bias on the local web environment. Finally, we also describe an algorithm which makes use of the information in the cosmic web environment to increase the accessible dynamic range of large-volume simulations by an order of magnitude in halo mass.

September 16, 2022
2:30 pm (IST)

Cosmological signatures of Hidden sector containing dark radiation

Pranjal Ralegankar (UIUC)


Abstract: This talk is divided in two parts. In the first part I will describe the ramifications to hidden sector model building if future CMB surveys find no evidence of dark radiation. In particular, using Millicharged particle model as an example, I will show how CMB measurements can provide leading constraints on BSM portal interactions and how these constraints are largely independent of detailed hidden sector model.

In the second part of the talk, I consider the alternative possibility where we might detect dark radiation in future surveys. Then to find if the dark radiation ever had considerable interaction with Standard Model particles, we need to look for signatures of isocurvature. I show how we can use the distribution of Helium and Deuterium to search for such an isocurvature.

September 9, 2022
2:30 pm (IST)

E and B modes of the CMB y-type spectral distortions: Polarised kSZ effect

Aritra Kumar Gon (TIFR)


Abstract: We analyse the secondary E and B mode polarisation of the CMB originating from the transverse peculiar velocity of free electrons, during the reionisation and post-reionisation eras. Interestingly, the spectrum of this polarised kSZ effect can be decomposed into a blackbody part and a y-type distortion. The y-distortion part is distinguishable from the primary E and B modes and also the lensing B modes. Furthermore, it is also differentiable from the other y-type signals, such as the thermal SZ effect, which are unpolarised. We show that this signal is sensitive to the reionisation history, in particular to how fast reionisation happens. The E and B modes of y-type distortion provide a way to beat the cosmic variance of primary CMB anisotropies and are an independent probe of the cosmological parameters.

September 6, 2022
2:30 pm (IST)

Weak lensing to Doppler boosting to cross-correlations: novel probes of the high-redshift Universe

Abhishek Maniyar (NYU)


Abstract: In recent years, cosmic microwave background (CMB) weak lensing, cosmic infrared background (CIB), and line intensity mapping (LIM) have emerged as a powerful tool to probe fundamental physics and galaxy evolution.
1. In the first part of the talk, I will present a brief introduction to the quadratic estimators for the weak lensing. Then I will show that a linear combination of lensing maps from the cosmic microwave background (CMB) and from line intensity maps (LIMs) allows to exactly null the low-redshift contribution to CMB lensing, and extract only the contribution from the Universe from/beyond reionization. This would provide a unique probe of the Dark Ages, complementary with 21 cm. I will quantify the interloper bias (which is a key hurdle to LIM techniques) to LIM lensing for the first time, and derive a "LIM-pair" estimator which nulls it exactly.
2. Then I will present a new observable: Doppler boosted CIB emission (DB-CIB). It is analogous to the kinematic Sunyaev-Zeldovich effect (kSZ) but has its origins in an emitted signal rather than a scattered signal like kSZ. This subtle difference allows us to use DB-CIB as a probe of the peculiar velocity field without the infamous 'optical depth degeneracy' problem. I will show some results for prospects of observing the DB-CIB and its potential applications.
3. Finally, I will briefly present an idea I am working on which utilises the kSZ effect to extract 'only' the patchy component of the reionisation i.e. the high redshift contribution using the so called projected-field estimators.


July 2022

July 29, 2022
2:30 pm (IST)

The Hubble Constant with supernovae: Strong lensing and the distance ladder

Suhail Dhawan (University of Cambridge)


Abstract: The Hubble tension is arguably the largest open question in modern cosmology. It could be a sign of new cosmological physics or unknown sources of systematics. To definitively answer this question, we need percent level measurements from independent cosmological probes. I also will present our recent work on calibrating the absolute luminosity of Type Ia supernovae from the wide-field Zwicky Transient Facility (ZTF). This distance ladder is uniform in that both the calibrator and Hubble flow SNe Ia are observed with a single, untargeted survey, which sidesteps the two largest systematics in the local distance ladder, i.e. photometric cross-calibration and selection biases depending on host environment. I'll preview the upcoming work on building this distance ladder with modern space telescopes in the JWST era. I will also talk about strongly lensed supernovae as an exciting route to precision measurements of H0. I will review the results from our recent work estimating time-delays, extinction and lensing magnification for the first, resolved strongly lensed Type Ia supernova, iPTF16geu and the ongoing search with ZTF. Finally, I'll present some ongoing work on using hierarchical Bayesian models to predict SN distances for percent precision in future H0 measurements.

July 19, 2022
2:30 pm (IST)

Spectral distortion as a probe for late/early time physics

Sandeep Acharya (University of Manchester)


Abstract: I will talk about Cosmic Microwave Background (CMB) and Cosmic Infrared Background (CIB) distortions and what we expect to learn about pre- and the post-recombination universe by studying them. I will talk about tSZ effect from CMB and CIB, kSZ from quasars during reionization. I will also talk about photon injection from dark matter decay and their signature from 21 cm signal and possible consequences for experiments like EDGES and ARCADE.


June 2022

June 17, 2022
4:00 pm (IST)

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

Kailash Sahu (Space Telescope Science Institute)


Abstract: We recently reported the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (T~270 days), high-magnification microlensing event MOA-11-191/OGLE-11-461, in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Combining the photometric, astrometric, and spectroscopic measurements, we obtain a lens mass of 7.1 +/- 1.3 solar mass and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a "natal kick" from its supernova explosion. Our mass measurement is the first ever for an isolated stellar-mass BH using any technique.

June 10, 2022
4:30 pm (IST)

Connecting the Extremes: A Story of Supermassive Black Holes and Ultralight Dark Matter

Peter Denton (Brookhaven National Laboratory)


Abstract: Dark matter (DM) is known to interact gravitationally. If it is ultralight then there will be unique macroscopic effects that may be probable in the same environments that we see the evidence for DM. I will discuss how ultralight DM can be fermionic, evading the Tremaine-Gunn bound, and the new relevant constraints including those from supermassive black holes. Finally, I will present a specific model that addresses some interesting hints/anomalies in terms of early supermassive black hole formation, ultrafaint dwarf galaxies, and possible gravitational wave signatures.

June 3, 2022
2:30 pm (IST)

Background of radio photons from primordial black holes

Shikhar Mittal (TIFR)


Abstract: We compute the isotropic radiation background due to the radio emission from gas accretion onto supermassive primordial black holes (PBHs). We find that these PBHs can readily produce radio background explaining not only the magnitude but also the power-law index of the excess radio background observed by the ARCADE2 and LWA1 experiments at frequencies less than similar to 1 GHz. To estimate the radio luminosity of a single accreting black hole we use the empirical relation known as the fundamental plane of black hole activity which connects the luminosity in the radio band to the X-ray band and the mass of black hole. The synchrotron mechanism of relativistic jets and the first-order Fermi acceleration mechanism in shocks support the power-law index observed by ARCADE2/LWA1 experiments.


May 2022

May 27, 2022
2:30 pm (IST)

Harnessing the Unresolved Lenses: Detecting Strong Lenses and Measuring Time-Delays from Unresolved Light Curves

Satadru Bag (Korea Astronomy and Space Science Institute)


Abstract: Strong gravitational lensed systems with variable sources, like supernovae (SNe), quasars (QSO), can be the next frontier in cosmic probes. One can obtain crucial constraints on cosmological parameters, such as the value of Hubble constant, evolution of dark energy etc, independent of other probes by measuring the time delays between the images. Lensed SNe and QSOs have their own advantages, e.g. lensed SNe are extremely rare as compared to lensed QSOs but the former have much better understood light curves with the time scale of a few months only. The ongoing and the upcoming time-domain surveys like ZTF, Rubin, Roman will observe a lot of lensed systems with both kinds of sources. However, many will have the images spatially unresolved, with the observed lightcurve a superposition of time-delayed image fluxes. We investigate whether the unresolved sources can be recognized as lensed given only the lightcurve information and whether time delays can be extracted robustly.

In this talk, I will discuss a few such interesting techniques that can identify the unresolved lensed systems of both source kinds (SN and QSO). Most importantly, these techniques are very much generic and, hence, do not assume any particular property of the sub-classes of the sources, such as the type of SNe, the flux variability of QSOs etc. These techniques can be very useful in detecting the lensed systems in wide-field surveys and in measuring the time delays simultaneously to improve our understanding of the cosmos.

May 20, 2022
2:30 pm (IST)

Propagation of decaying neutrinos in matter: an analytic treatment

Dibya Sankar Chattopadhyay (TIFR)


Abstract: We present compact analytic expressions for neutrino propagation probabilities in matter, with effects from invisible decay of neutrinos included. These will be directly relevant for long-baseline, reactor and atmospheric neutrino experiments.

The inclusion of decay leads to effective Hamiltonian being non-Hermitian, with the Hermitian component representing oscillation, and the anti-Hermitian component corresponding to invisible decay. Due to a possible mismatch between the effective mass and the decay eigenstates of neutrinos, these two components need not commute. In fact, in presence of matter, these two components will invariably become non-commuting. We overcome this by employing the techniques of inverse Baker-Campbell-Hausdorff (BCH) expansion, and the Cayley-Hamilton theorem applied in the 3-flavor framework: We also derive the probabilities in the One Mass Scale Dominance (OMSD) approximation.

The analytic results obtained provide physical understanding into possible effects of neutrino deay as it propagates through Earth's matter.We show that the non-intuitive feature of decay increasing the value of \(P_{\mu\mu}\) at its first and second oscillation dip maybe explained by our analytic expression.

May 6, 2022
2:30 pm (IST)

Joint Modelling of Astrophysics Systematics for Cosmology with LSST

Nikolina Šarčević (Newcastle University)


Abstract: LSST will provide an unprecedented wealth of astronomical data, with which we will be able to tightly constrain the values of the parameters of our cosmological model, notably those which describe the poorly understood dark energy component. As weak lensing and galaxy clustering measurements provide a way to infer key cosmological quantities such as the dark matter distribution, the evolution of cosmic structure, and the expansion history of the Universe, detailed and rigorous analysis is necessary in order to glean as much information as possible from LSST measurements of these effects. This project will develop a consistent and reliable joint modelling framework for the intrinsic alignment of galaxies, galaxy bias and photometric redshift uncertainties: three key systematic effects impacting weak lensing and galaxy clustering. The outcome of this investigation will directly enable rigorous weak lensing and galaxy clustering constraints on cosmological parameters with LSST.


April 2022

April 29, 2022
2:30 pm (IST)

Accessing the Cosmological Collider

Thomas Flöss (University of Groningen)


Abstract: Over the past decades, cosmological inflation has become a well-established paradigm for describing the early universe. Besides explaining the homogeneity and isotropy of the observed universe, it provides a simple mechanism for generating the initial conditions of the large scale structure of the universe. I will discuss how inflation can be regarded as a cosmological particle collider and how 21-cm observations from the Dark Ages could allow us to study its phenomenology, highlighting the various hurdles along the way.

April 22, 2022
2:30 pm (IST)

Forward Modeling Approaches to Cosmological Inference - reconstruction of cosmological fields with differentiable simulations

Chirag Modi (University of California, Berkeley)


Abstract: In this talk, I will present the forward modeling approaches to cosmological inference for the next generation of surveys. In the first part of the talk I will motivate these approaches with the focus on a specific method- reconstruction of the cosmological fields. I will present two examples where this enhances the information that can be extracted from the large scale structure surveys, one for galaxy clustering surveys and the other for 21-cm intensity mapping. In the second part of the talk, I will focus on the challenges for forward modeling approaches and the necessity of differentiable simulations to overcome them. I will present FlowPM- a differentiable particle mesh code in TensorFlow and demonstrate how such simulations can be combined with machine learning tools to speed up reconstruction of cosmological fields by upto an order of magnitude.

April 8, 2022
4:00 pm (IST)

Solving Reionization with Resolved Lyman-alpha

Rohan Naidu (Harvard University)


Abstract: Reionization was the last great phase transition of the Universe, when the first galaxies announced their presence by transforming the vast reservoirs of cold, neutral gas across the cosmos into a hot, ionized state. The most pressing questions around reionization hinge on the unknown ionizing photon escape fractions (LyC fesc) of star-forming galaxies -- i.e., the fraction of photons that evade absorption by gas and dust in galaxies so they may reionize the intergalactic medium (IGM). Direct measurements of LyC fesc are impossible at high redshift (z>3-4) due to the opacity of the intervening IGM. In this talk I will present recent breakthroughs on the LyC fesc problem. I will describe how high-resolution (R>4000) Lyman-alpha (LyA) line profiles may be used to infer LyC fesc. Based on these findings I will present a LyA-linked framework to understand the cosmic ionizing background from z~2-8. I will end by previewing two Cycle 1 JWST programs I am leading -- these programs perform deep spectroscopy of ionized bubbles towards the beginning and the end of reionization.

April 1, 2022
4:30 pm (IST)

Studying stars at the reionization epoch with gravitational lensing

Jose M Diego (Instituto de Física de Cantabria)


Abstract: The recent discovery of strongly lensed stars such as Icarus, Warhol, Godzilla, and more recently Earendel has pushed the limits of the maximum distance at which stars can be studied individually. Currently Earendel holds the record for the most distant star at an estimated redshift of 6.2 and promises a wealth of even more distant stars to come with the newly launched JWST telescope. Studying individual stars at redshifts larger than six is no longer science fiction but belongs to the realm of what is possible to achieve with current technology. In this talk I will review the recent discoveries of strongly lensed stars, and the unique opportunities offered by this new type of observations to study not only the formation and evolution of the first stars, but also to constrain models of dark matter.


March 2022

March 11, 2022
2:30 pm (IST)

Constraining Reionization with the HERA 21cm power spectrum observations, and with Fast Radio Bursts

Stefan Heimersheim (University of Cambridge)


Abstract: In this talk I want to highlight two interesting probes of cosmic reionization — the recent 21cm power spectrum limits from the HERA telescope, and future high-redshift Fast Radio Burst observations.

21cm cosmology provides a unique window into the early universe, allowing us to observe neutral Hydrogen gas via its 21cm spin-flip transition, in principle allowing us to probe redshifts z=5 to 100. Using the recent HERA observations which provide the strongest limits on the 21cm power spectrum at z=8, we constrain models of reionization as well as properties of the IGM in the early universe.

Fast Radio Bursts are recently discovered, very bright, radio transient signals observed from outside our galaxy. These bursts are dispersed due to the free electrons present in the IGM, and thus we can use them to probe the free electron content of the universe. Future observations (e.g. SKA) are expected to measure thousands of FRBs from high redshifts, and, thus, allow us to directly probe the reionization history of the IGM. We show how this, still very uncertain, history can be probed model-independently, without assuming a particular shape.

March 4, 2022
2:30 pm (IST)

Searching for dark energy off the beaten track

Sunny Vagnozzi (University of Cambridge)


Abstract: Most of the efforts in searching for dark energy (DE) have focused on its gravitational signatures, and in particular on constraining its equation of state. However, there is a lot to be learned about DE by getting off the beaten track. I will first focus on non-gravitational interactions of DE with visible matter, leading to the possibility of “direct detection of dark energy” (analogous to direct detection of dark matter): I will argue that such interactions can and potentially may already have been detected in experiments such as XENON1T, while discussing some of their complementary cosmological and astrophysical signatures. I will then discuss early- and late-time consistency tests of LCDM, and how these may shed light on (early and late) DE, particularly in relation to the Hubble tension. I will present two such tests based on the early ISW effect and the ages of the oldest astrophysical objects in the Universe. If time allows, I will turn to more general ultralight particles (which may have to do with either dark matter or DE) and discuss new ways of probing these, using black hole shadows and planetary objects such as asteroids.


February 2022

February 25, 2022
2:30 pm (IST)

Lyman-alpha data as a probe of dark matter interactions

Deanna Hooper (University of Helsinki)


Abstract: Despite its remarkable success, the standard LCDM paradigm has been challenged lately by significant tensions between different datasets. This has reinvigorated interest in beyond-LCDM models, such as dark matter interacting either with standard model particles or with an additional dark sector. These interactions result in a suppression of the matter power spectrum on small scales, making them an ideal target to be constrained with Lyman-alpha data. In this talk I will discuss a method to use Lyman-alpha data that does not need the usual computationally-expensive hydrodynamical simulations. I will present recent bounds for dark radiation interactions, and show that there are reasonable hints for non-zero interactions between dark matter and neutrinos.

February 18, 2022
2:30 pm (IST)

The small-scale structure of galaxy cluster cores

Massimo Meneghetti (INAF-OAS Bologna)


Abstract: In the standard cosmological model, the matter content of the Universe is dominated by cold dark matter (CDM), collisionless particles that interact with ordinary matter (baryons) only through gravity. Gravitationally bound dark-matter halos form hierarchically, with the most massive systems forming through mergers of smaller ones. As structure assembles in this fashion, large dark-matter halos contain smaller-scale substructure in the form of embedded subhalos. I will show that observations of gravitational lensing can be used to map the inner mass distribution of cosmic structures such as galaxy clusters to test these predictions of the CDM paradigm.

Interestingly, the reconstructed granularity of cluster cores implies an excess of galaxy-galaxy strong lensing (GGSL) probability compared to expectations in the ΛCDM cosmological model (Meneghetti et al. 2020). The theoretical estimates on the GGSL probability are based on the analysis of hydrodynamical simulations, while the observational measurements are derived from parametric strong lensing reconstructions combining inputs from HST imaging and MUSE/VLT spectroscopy.

The reported mismatch may indicate an unidentified problem with either prevailing simulation methods or standard cosmology.

In the attempt to understand this issue, we analyse cluster-size halos simulated with different mass and force resolutions and implementing several AGN energy feedback schemes. We show that the feedback model have a significant impact on the properties of subhalos and on their ability to produce GGSL effects. However, none of the hydrodynamical simulations studied so far are in agreement with observations. They persistently have difficulty reproducing the stellar mass and the internal structure of cluster galaxies simultaneously.

February 16, 2022
2:00 pm (IST)

Challenges and Opportunities at non-linear scales of Large Scale Structure

Shadab Alam (University of Edinburgh)


Abstract: The initial metric perturbations in the universe were small O(10^-5) allowing linearised equations to give a very good description of the evolution of the Universe. The standard model predicts the evolution of the clustering statistics (e.g. P(k)) of the matter distribution. Under the simplified assumption, all the complex physics of galaxy formation is encompassed in a few parameters called the galaxy biases. We know that these assumptions fail as we approach scales equivalent to the size of a galaxy. While such solutions were appropriate for the past experiments, (e.g: SDSS), they fall short for the measurement precision of ongoing experiments (e.g: DESI, EUCLID, LSST) which are better by more than an order of magnitude. Therefore, one of the challenges is to understand the limits of the current models to a precision better than ongoing experiments to avoid biases in interpreting the underlying physics of the universe. This is challenging because of two reasons: first, the growth of density perturbations and its coupling to the velocity field becomes highly non-linear, and second, high precision true predictions require understanding the physics of galaxy formation. At the same time, there is a golden opportunity to improve upon the models by identifying the dominant failure modes. I will describe our efforts to validate and improve the theoretical models especially focusing on the redshift space two-point clustering statistics and its implications on our ability to derive physics from ongoing experiments.

February 11, 2022
2:30 pm (IST)

Large scale radio cosmology with 21cm intensity mapping

Steven Cunnington (University of Edinburgh)


Abstract: Neutral hydrogen (HI) has been shown to be a tracer of the underlying matter field in our Universe. Thus by probing the large scale clustering structure of HI, we can indirectly probe cosmic structure and from this, infer constraints on cosmological parameters. 21cm intensity mapping with radio telescopes is an efficient method for this purpose, although the success of this novel technique relies on overcoming a number of challenges which I will discuss. The ultra-large scales of our Universe host potential signatures for new physics, but conventional galaxy surveys struggle to probe these scales with statistical significance. 21cm intensity mapping should be better suited to this task since it can rapidly survey large cosmic volumes. However, to access the largest scales with the much anticipated Square Kilometre Array (SKA), the single-dish intensity mapping method will be relied on. I will therefore also discuss the work being undertaken with MeerKAT (an SKA pathfinder telescope) to successfully demonstrate single-dish intensity mapping, which represents an important milestone for using the SKA as a large-scale structure probe.


January 2022

January 28, 2022
2:30 pm (IST)

Constraints on primordial magnetic fields in the light of ARCADE-2 and EDGES observations

Pravin Kumar Natwariya (PRL)


Abstract: The 21-cm signal, due to the hyperfine transition in the neutral hydrogen atom, appears to be a treasure trove to study physics in the cosmic dawn era. The presence of an exotic physical process can inject energy into the intergalactic medium (IGM) and heat up the gas. Subsequently, it can modify the absorption amplitude in the global 21-cm signal during cosmic dawn. The absorption feature in the 21-cm signal can provide a robust bound on such a process of energy injection. Recently, the EDGES observation reported such an absorption signal in the redshift range 15-20. It is nearly two times larger than theoretical predictions based on the LCDM model. In order to explain the observation, one may require to increase the background radiation temperature above CMB or decrease the gas temperature. ARCADE-2 observation detected extra-galactic excess radio radiation in the frequency range of 3-90 GHz. The primordial magnetic fields (PMFs) can inject energy into IGM by ambipolar diffusion and turbulent decay. In the presence of decaying PMFs, the 21-cm signal can modify due to the heating of the gas. We will discuss the constraints on PMFs in the light of the EDGES low-band and ARCADE-2 observations.

January 21, 2022
2:30 pm (IST)

On the detection of a cosmic dawn signal in the radio background

Saurabh Singh (RRI)


Abstract: EDGES collaboration reported detection of an anomalous profile, which was interpreted to be 21-cm signature arising from cosmic dawn. Several non-standard theoretical models were proposed to explain its origins, including dark matter-baryon interactions and excess radio background. We deployed SARAS 3 radiometer on lakes in Southern India to examine this claim. In this talk, I will discuss the unique design and observing environment of SARAS 3. I will present the statistical methods used to test for presence of the reported signal. I will conclude with our inference about the non-detection of the reported profile in SARAS 3 data and discuss its implications.

January 20, 2022
2:30 pm (IST)

Improving galaxy cluster mass estimation with machine learning

Digvijay Wadekar (IAS, Princeton)


Abstract: Accurately estimating masses of galaxy clusters is needed to extract cosmological information from them. It is therefore crucial to find combinations of observable properties of clusters which have a low-scatter relationship with their masses. Machine learning (ML) tools provide a quick and efficient way of looking for low-scatter relations in abstract high-dimensional parameter spaces. I will present a new and a more accurate method for estimating cluster masses which combines observables from CMB and X-ray surveys. More generally, I will show how ML tools can be useful for estimating distances and masses of astrophysical objects, making mock catalogs, and extracting cosmological information from non-linear scales.

January 14, 2022
2:30 pm (IST)

Interpreting the high-redshift 21-cm signal observations

Sambit Giri (Zurich)


Abstract: About 100 million years after the birth of the Universe, the first generation of luminous sources formed. The photons from these sources heated and ionised the gas in the intergalactic medium (IGM). This period is known as the Epoch of Reionization (EoR). Exploring this era will help us understand the “seeds” of the modern-day Universe. The 21-cm signal, produced by the spin-flip transitions in neutral hydrogen, is a unique tracer of the IGM during the EoR. This signal can be found at the radio frequency band. Current radio telescopes have put interesting upper limits on the fluctuations in this signal. In this talk, I will discuss how we can derive physical constraints from the 21-cm signal observations and show what we have learnt from the current observations. In the coming times, the Square Kilometre Array (SKA) telescope will come online, which will be able to produce images of the distribution of the 21-cm signal during reionization. I will also present a forecast study using mock observations of SKA.


December 2021

December 24, 2021
2:30 pm (IST)

Perturbations In Some Dark Energy Models

Srijita Sinha (IISER Kolkata)


Abstract: Dark energy is the candidate that can produce effective negative pressure and make the galaxies and galaxy clusters move away from each other in an accelerated way. The structures of the Universe have evolved from some initial primordial fluctuations and depend on the background dynamics of different components of the Universe like dark matter, dark energy and others. The motivation is to investigate if some of the dark energy models can give rise to a suitable environment for the formation of the structures in the Universe.

December 21, 2021
2:30 pm (IST)

Light Dark Universe: Prospects for light particle searches

Tanmay Poddar (PRL)


Abstract: Strong constraints from direct detection experiments of WIMP dark matter and the small structure problems of the universe force the physicists to think of alternative candidates for dark matter particles. In this talk I will discuss such candidates like axions, light gauge bosons, sterile neutrinos, and gravitons and the prospects of searching for those particles.

December 14, 2021
2:30 pm (IST)

Lepton Flavored Dark Matter: Two Scenarios

Shiuli Chatterjee (IISc)


Abstract: I will discuss two cases of lepton flavored dark matter and entailing phenomenology. In the first part of the talk, I will discuss the connection between the stability of and the symmetries possessed by a lepton flavored dark matter (LFDM), systematically showing that many representations of LFDM are stable under the minimal flavor violation (MFV) hypothesis as long as there are no lepton number violating interactions. I will then discuss the production of the dark matter (DM) showing that an LFDM in the MFV framework naturally accommodates a freeze-in production, and finish with a discussion on its detection at present and future direct detection experiments. In the second part of the talk, I will discuss the robustness of neutron stars as probes of particle DM. Focusing on the case of lepton flavored DM, I will discuss the capture of such DM by muons leading to kinetic heating in old neutron stars. The temperatures of such old neutron stars can be probed at near-future telescopes like the James Webb Space Telescope (JWST). I will discuss our results showing that the capture rates and subsequently the temperatures of the neutron stars are crucially dependent on the DM properties as opposed to the astrophysical properties of the neutron stars, like equation-of-state, the velocity of the neutron star, DM halo distribution, etc.

December 10, 2021
2:30 pm (IST)

Global 21-cm signal and its implications

Atrideb Chatterjee (NCRA)


Abstract: The claimed discovery of the sky-averaged global 21-cm signal by the EDGES group at redshift \(z \simeq 12-22\) has opened up a new window to investigate the Physics of this epoch. The redshift and strength of this signal gives us an ideal opportunity to obtain constraints on the allowed Warm Dark Matter particle mass at redshifts currently inaccessible by any other means. We find that WDM models with \(m_X ≤ 3\) keV can be ruled out since the global signal obtained using them are unable to match either the redshift range or the amplitude of the EDGES signal. Another application of this discovery is to study “First stars” (Pop III) formed at this high redshift. Though this metal-free PopIII stars have long been postulated to explain the metallicity gap between Big Bang Nucleosynthesis and the metal-rich PopII stars, they lack any observational support. With the assumption that this signal is driven by a combination of PopII and PopIII stars we provide for the first time a data constrained estimate of the SFRD for PopIII stars at redshifts \(z \simeq 12-22\). Finally, for the first time, we manage to include this global 21-cm observation along with the CMB data (from Planck) and Reionization related observations to jointly constrain the cosmological and astrophysical parameters. To this aim, we develop a Markov Chain Monte Carlo (MCMC)-based parameter estimation package, CosmoReionMC, and find that inclusion of more data indeed enables us putting tighter constraints on some of the cosmological parameters compared to the previous constraints obtained from Planck Observations.

December 7, 2021
2:30 pm (IST)

The detectability of strong 21-cm forest absorbers in late reionisation models

Tomáš Šoltinský (University of Nottingham)


Abstract: A late end to reionisation at redshift z=5.3 is consistent with the observed spatial variations in the Lyα forest transmission and the deficit of Lyα emitting galaxies around extended absorption troughs at z=5.5. In this picture, large islands of neutral hydrogen should persist in the diffuse intergalactic medium (IGM) until z=6. In this talk I will present state-of-the-art models of the forest of 21-cm absorption lines that will arise from this neutral hydrogen, obtained using high resolution cosmological hydrodynamical simulations coupled with radiative transfer. I include the effects of redshift space distortions and hydrodynamical response to the reionisation on the simulated 21-cm forest spectra. These effects have typically been neglected in the existing 21-cm absorption models. I will show that strong 21-cm lines (>1% absorption) should persist in the spectra of high-redshift radio sources until z=6, for the case of only modest IGM pre-heating with gas kinetic temperatures of T~100K. Forthcoming observations of 21-cm forest absorbers at z=6-9 with SKA1-low or possibly LOFAR should therefore provide an informative lower limit on the still largely unconstrained soft X-ray background at high redshift.

December 3, 2021
2:30 pm (IST)

On detecting CMB spectral distortions: Recombination to Reionization

Mayuri Rao (RRI)


Abstract: The ΛCDM model of cosmology predicts inevitable, weak distortions in the spectrum of the Cosmic Microwave Background (CMB) from that of a blackbody. However, no such deviations have been measured to date. I present work on simulating realistic foregrounds which present astrophysical challenges to detect the faint global redshifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization. After discussing a suggested method for foreground separation, I will present ongoing efforts at the Raman Research Institute to detect this faint cosmological signal from space. I will discuss PRATUSH -- the proposed lunar orbiter experiment, which will make scientific measurements in the lunar farside using a custom-designed radiometer. I will also discuss a feasibility study for a ground-based detection of extremely weak, ripple-like additive features in the CMB spectrum created by photons emitted during cosmological recombination (900 < z < 7000). Detection and measurement of these CMB spectral distortions will enable a better understanding of the thermal and ionization history of the Universe and help us probe redshifts that have never been directly observed thus far.

December 2, 2021
10:30 am (IST)

Weak lensing: globally optimal estimator and a new probe of the high-redshift Universe

Abhishek Maniyar (CCPP, NYU)


Abstract: In recent years, weak lensing of the cosmic microwave background (CMB) has emerged as a powerful tool to probe fundamental physics. The prime target of CMB lensing surveys is the lensing potential, which is reconstructed from observed CMB temperature T and polarization E and B fields. In this talk, I will show that the classic Hu-Okamoto (HO02) estimator used for the lensing potential reconstruction is not the absolute optimal lensing estimator that can be constructed out of quadratic combinations of T, E and B fields. Instead, I will derive the global-minimum-variance (GMV) lensing quadratic estimator and show explicitly that the HO02 estimator is suboptimal to the GMV estimator.

Rapidly expanding field of the line intensity mapping (LIM) promises to revolutionise our understanding of the galaxy formation and evolution. Although primarily a tool for galaxy astrophysics, LIM technique can be used as a cosmological probe and I will point out one such application in rest of the talk. I will show that a linear combination of lensing maps from the cosmic microwave background (CMB) and from line intensity maps (LIMs) allows to exactly null the low-redshift contribution to CMB lensing, and extract only the contribution from the Universe from/beyond reionization. This would provide a unique probe of the Dark Ages, complementary with 21 cm. I will quantify the interloper bias (which is a key hurdle to LIM techniques) to LIM lensing for the first time, and derive a "LIM-pair" estimator which nulls it exactly.


November 2021

November 26, 2021
2:30 pm (IST)

Exclusion limit on Dark Matter-Neutrino Scattering Cross-section

Divya Sachdeva (IISER Pune)


Abstract: In this talk, I will discuss the strong new limits on the Dark Matter (DM)-Neutrino scattering crosssection for DM of mass less than 10 MeVs obtained using data from the Super-Kamiokande (SK) experiment. The key ingredient leading to this result is the observation that the diffused supernovae neutrinos flux and the cosmic electron flux are comparable in the energy range 1 - 50 MeV providing a significant boost to the cold DM particles and hence lead to an observable amount of signal in the SK energy range. Though we used SK as well as XENON1T data to derive bounds on DM-electron and DM-neutrino scattering cross-section, we found that the SK experiment provides the strongest constraint on DM-neutrino interaction.

November 23, 2021
2:30 pm (IST)

A strong broadband 21 cm cosmological signal from dark matter spin-flip interactions

Priyanka Sarmah (IIT Bombay)


Abstract: We explore a novel mechanism, where dark matter spin-flip interactions with electrons through a light axial-vector mediator could directly induce a 21 cm absorption signal which is characteristically different from the expected absorption features in the standard cosmology and in models with excess gas cooling, which have been broached to explain the recently observed anomalous signal in the EDGES experiment. We find generically that our model predicts a strong, broadband absorption signal extending from frequencies as low as 1.4~MHz (z~1000), from early in the cosmic dark ages where no conventional signal is expected, all the way up to higher frequencies where star formation and X-ray heating effects are expected to terminate the absorption signal. We find a rich set of spectral features that could be probed in current and future experiments looking for the global 21 cm signal. Large swathes of our model parameter space of interest are safe from existing particle physics constraints, however future searches for short range spin-dependent forces between electrons on the millimeter to nanometer scale have the potential to discover the light mediator responsible for our predicted signal.

November 12, 2021
3:30 pm (IST)

Gas-rich dwarf galaxies as powerful dark matter detectors

Digvijay Wadekar (IAS, Princeton)


Abstract: Gas-rich dwarf galaxies located outside the virial radius of their host are relatively pristine systems and have ultra-low gas cooling rates. This makes them very sensitive to heat injection by non-standard dark matter (DM). I will show that by requiring the gas heating rate by DM to not exceed the cooling rate of gas in the Leo T dwarf galaxy, one gets strong constraints on popular DM models like millicharged DM, axion like particles (ALPs) and primordial black holes (PBHs). For dark photon DM and for DM decay models, Leo T gives stronger constraints than all the previous literature. I will therefore show that observations of gas-rich dwarfs from current and upcoming optical and 21cm surveys opens a new way of probing DM. Towards the end, I will change gears and talk about application of machine learning techniques to cosmology.


October 2021

October 29, 2021
2:30 pm (IST)

Solar reflection of light dark matter

Timon Emken (Stockholm University)


Abstract: Direct detection experiments are searching for rare interactions between dark matter (DM) particles from the galactic halo and ordinary matter on Earth. If the mass of these dark particles is too low, their kinetic energy does not suffice to trigger terrestrial detectors leaving them incapable to observe DM. Processes that boost DM particles can therefore extend the observational reach of direct DM searches to lower masses. I will discuss the mechanism and phenomenology of solar reflection, where DM particles get accelerated via collisions with solar electrons or nuclei. Compared to standard halo DM, solar reflection not only allows to probe lighter masses, a solar reflection signal would also feature a novel modulation signature.

October 22, 2021
2:30 pm (IST)

Opportunities in detecting neutral current interactions of neutrinos

Bhavesh Chauhan (TIFR)


Abstract: In this talk, I will discuss avenues for detecting neutral current (NC) interactions of sub-GeV neutrinos. First, I will discuss our recent proposal for a deuterated liquid scintillator detector and its potential for detecting neutrinos from a galactic supernova. I will emphasise on the quenched proton spectrum from NC interaction and the secondary interactions of recoil neutrons. Moving on, I will present our forecast for NC mediated neutrino-proton elastic scattering at JUNO. I will discuss the signal spectrum, backgrounds, and our preliminary threshold analysis. I hope to demonstrate that, with nothing but Standard Model physics, there is exciting data waiting to be discovered at neutrino detectors.

October 8, 2021
2:30 pm (IST)

The effect of inhomogeneous reionisation on the Lyman alpha forest

Margherita Molaro (University of Nottingham)


Abstract: Lyman-alpha absorption features in the spectra of bright, high-redshift quasars contain a wealth of information on the evolution of the intergalactic medium (IGM) during and soon after the final stages of reionisation. By comparing observations with simulations of these absorption features in different reionisation scenarios we are therefore able to obtain information on how this process unfolded.

One of the most significant challenges on the simulation side of this effort remains the implementation of photo-ionisation and -heating of the IGM by UV photons. Homogeneous UV background approximations are widely used to keep computational costs down, however it is believed they might miss crucial fluctuations in the ionisation and temperature fields, and therefore artificially alter the simulated Lyman alpha features. It is therefore important to establish whether constraints obtained from such models might be affected by biases.

In this talk, I will review a recent work directly comparing a homogeneous UVB approximation with an hybrid-RT prescription in the Sherwood-Relics suite of IGM simulations. This study has indeed confirmed that the different treatment of the UV radiation field leads to significant differences in the 1D Lyman alpha power spectrum, at both small and large scales. More importantly, it has highlighted that inhomogeneous reionisation effects should be accounted for in analyses of forthcoming high precision measurements of high redshift quasars if systematic biases are to be avoided. Conveniently, the same study also allowed us to develop a correction term to account for these effects that can be easily applied to existing simulations relying on homogeneous UV background approximations.


September 2021

September 24, 2021
2:30 pm (IST)

Constraining the abundance of primordial black holes using EDGES 21-cm signal

Shikhar Mittal (TIFR)


Abstract: Using the global 21-cm signal measurement by the EDGES collaboration, we derive constraints on the fraction of the dark matter that is in the form of primordial black holes (PBHs) with masses in the range \(10^{15}-10^{17}\) g. Improving upon previous analyses, we consider the effect of the X-ray heating of the intergalactic medium on these constraints, and also use the full shape of the 21-cm absorption feature in our inference. In order to account for the anomalously deep absorption amplitude, we also consider an excess radio background motivated by LWA1 and ARCADE2 observations. Because the heating rate induced by PBH evaporation evolves slowly, the data favour a scenario in which PBH-induced heating is accompanied by X-ray heating. Also, for the same reason, using the full measurement across the EDGES observation band yields much stronger constraints on PBHs than just the redshift of absorption. We find that 21-cm observations exclude \(f_{\mathrm{PBH}}\gtrsim 10^{-9.7}\) at 95% CL for \(M_{{\rm PBH}} = 10^{15}\) g. This limit weakens approximately as \(M_{\mathrm{PBH}}^4\) towards higher masses, thus providing the strongest constraints on ultralight evaporating PBHs as dark matter over the entire mass range \(10^{15}-10^{17}\) g.


June 2021

June 25, 2021
2:30 pm (IST)

The Cosmological Principle and the missing rest frame of the Universe

Mohamed Rameez (TIFR)


Abstract: The largest anisotropy of the Cosmic Microwave Background is the dipole, believed to originate from our relative motion at \(\sim 369~{\rm km}~ {\rm s}^{-1}\) with respect to the ‘rest frame of the Universe’. This should also cause a dipolar modulation in the number counts of distant sources. We test this with various all-sky catalogues: NVSS and SUMSS radio galaxies, WISE galaxies and CatWISE quasars, consistently finding a significantly larger dipole than expected. Thus we reject the exclusively kinematic interpretation of the CMB dipole with statistical significances as high as 4.9 sigma. This and other observations imply a bulk flow of matter in the local Universe, extending out to scales much larger than is typical in \(\Lambda\)CDM and showing no convergence to the CMB rest frame. A predicted consequence of such a bulk flow is a scale-dependent dipolar modulation in the deceleration parameter measured from within the flow. We look for this in the SDSS-II/SNLS-III Joint lightcurve analysis compilation of SN Ia data and find it at ~3.9 sigma statistical significance, while the evidence for any isotropic simultaneously drops to <1.4 sigma. These observations suggest that dark energy is an artefact of our idealized cosmological model and the Hubble ‘constant’ cannot be measured to a precision less than ~10%.

June 18, 2021
11:30 am (IST)

Quest for primordial B-modes in CMB: Foreground issues

Debabrata Adak (IUCAA)


Abstract: Detection of the primordial B-mode polarization in CMB is considered as one of the ultimate challenges of cosmology in the coming decades. B-modes contain an unequivocal signal of the gravitational waves generated during the epoch of inflation. There are several upcoming facilities that will be searching for B-modes from the ground, e.g., CMB-S4, Simons Observatory etc., and from space, e.g., LiteBIRD, PICO, CMB-Bharat etc. with unprecedented sensitivity. However, to claim the detection, the major challenge we have to face is removing the polarized dust and synchrotron emission from our Galaxy that obscure B-modes by order of magnitudes. Therefore, the foreground issue is a major focus of research for the quest for B-modes, which I will demonstrate over the course of this talk. I will discuss the modelling and estimating spectral properties of CMB foregrounds. I will also focus on developing a component separation algorithm to subtract foregrounds from CMB and the impact of the residual leakage of foregrounds on the detection of B-modes for future high sensitive instruments.

June 11, 2021
4:00 pm (IST)

LCDM extensions and consequences on cosmological observables and parameters constraints from current and upcoming surveys

Ziad Sakr (IRAP, Toulouse)


Abstract: The standard cosmological LCDM model success in accommodating for most of nowadays observations, still leaves some room for further extensions, albeit with small deviations, that are awaiting upcoming surveys, to be ruled out or to be further constrained. Moreover, two main growing tensions, the measurements of the matter fluctuation parameter known as sigma 8 and that of the expansion Hubble parameter, showing discrepancies among values constrained using local versus deeper probes, are hinting for the need for exploring alternatives to LCDM model. I will review some of the phenomenologically most common proposed extensions, per se or as solutions to the aforementioned tensions, such as dark energy different from the cosmological constant, a growth index from modified gravity or massive neutrinos, and express observables within these frameworks to test their ability to fix these discrepancies with current data or forecast on constraints from future new generation surveys. I will then reiterate the previous exercise with other models, such as modifying some of the dark matter properties or supposing a dynamical gravitational constant, an additional curvature-like component, or other modified gravity models such as f(R). I will finish with a more radical modification to LCDM with models that relax the Copernican principle and with it the homogeneity and isotropy hypothesis we usually assume when performing cosmological calculations and show constraints on related null tests from current and future datasets.

June 4, 2021
2:30 pm (IST)

Extracting the 21 cm EoR signal using MWA drift scan data

Akash Patwa (RRI)


Abstract: The detection of the redshifted hyperfine line of neutral hydrogen (H I) is the most promising probe of the epoch of reionization (EoR). We report an analysis of 55 h of Murchison Widefield Array (MWA) phase II drift scan EoR data. The data correspond to a central frequency \( \nu_0 = 154.24~{\rm MHz} \) (\(z \simeq 8.2\) for the redshifted H I hyperfine line) and bandwidth \( B = 10.24~{\rm MHz}\)⁠. As one expects greater system stability in a drift scan, we test the system stability by comparing the extracted power spectra from data with noise simulations and show that the power spectra for the cleanest data behave as thermal noise. We compute the H I power spectrum as a function of time in one and two dimensions. The best upper limit on the 1D power spectrum is \(\Delta^2(k)≃(1000~{\rm mK})^2 \) at \( k \simeq 0.2~{\rm h}~{\rm Mpc}^{−1}\) and at \(k \simeq 1~{\rm h}~{\rm Mpc}^{−1}\). The cleanest modes, which might be the most suited for obtaining the optimal signal to noise, correspond to \(k \gtrsim 1~{\rm h}~{\rm Mpc}^{−1}\). We also study the time-dependence of the foreground-dominated modes in a drift scan and compare it with the expected behavior.


May 2021

May 28, 2021
11:30 am (IST)

Constraining Sommerfeld-Enhancement via Inflationary Perturbations & CMB: Dark Matter & Neutrino Interactions

Anish Ghoshal (INFN, Rome)


Abstract: In ΛCDM cosmology, Dark Matter (DM) and neutrinos are assumed to be non-interacting. However, it is possible to have scenarios, where DM-neutrino interaction may be present, leading to scattering of DM with neutrinos and annihilation of DM into neutrinos. We investigate the viability of such scenarios in the light of cosmological data by making use of the PLANCK-2018 and BOSS-BAO 2014 dataset and constrain these processes in the light of the same. We also discuss a particle DM model where DM-neutrino interaction is present and map the constraints to the parameter space of the model.

May 21, 2021
5:00 pm (IST)

Cross-correlation studies with future CMB experiments

Anirban Roy (Cornell University, USA)


Abstract: The observation of the Cosmic Microwave Background (CMB) is a powerful probe to unravel many mysteries of the late-time Universe. I will review present constraints on the parameters related to the reionization process and galaxy clusters. In the near future, we will have access to low noise and high-sensitive CMB datasets and hence, the question arises of what new information we can get about the detailed physics of the reionization process and galaxy clusters. We propose a new reionization probe that uses CMB observations: the cross-correlation between fluctuations in the CMB optical depth which probes the integrated electron density, and the Compton y-map which probes the integrated electron pressure. I will discuss how this cross-correlation technique is used to put the first constraints on the temperature and size of the ionized bubbles from the Planck data. On the other hand, I will explain how different cross-correlation studies between CMB and galaxy surveys, can be used to constraint the density and pressure profile of galaxy clusters. I will conclude by discussing the prospects of these studies by upcoming CMB experiments, like Simons Observatory, CMB-S4, and CCAT-prime.

May 7, 2021
2:30 pm (IST)

Hints of new physics in dark matter and neutrino sector and cosmological Hubble anomaly

Subinoy Das (IIA, Bangalore)


Abstract: The 5 sigma mismatch between CMB and local distance ladder measurements of Hubble parameter is one of the greatest challenge to our well known Lambda CDM model of cosmology. I will discuss that we definitely need beyond standard model particle physics to appear before CMB epoch if the anomaly persists. I will also discuss how secret dark matter decay or secret neutrino interaction can play a crucial role around CMB epoch to alleviate this recent problem in cosmology.


April 2021

April 30, 2021
2:30 pm (IST)

An EAGLE view of the missing baryons

Toni Tuominen (Tartu Obersatory)


Abstract: A significant fraction of the predicted baryons remains undetected in the local Universe. To aid in the search of the missing baryons, we turned to the state-of-the-art hydrodynamical EAGLE simulation. I will present the spatial and thermal distributions of the simulated baryons within the filaments of the Cosmic Web. In particular, I will characterise the properties of the hot phase (log T(K) > 5.5) of the Warm-Hot Intergalactic Medium (WHIM). As observations of the diffuse intergalactic medium at these temperatures are extremely challenging, the hot WHIM remains largely undetected and is a candidate to solve the cosmological missing baryons problem. Indeed, the EAGLE simulation predicts a large fraction of baryons to be in this phase: while the filaments occupy only ≈ 5% of the full simulation volume, the diffuse hot intergalactic medium in filaments amounts to approximately 23% − 25% of the total baryon budget. I will demonstrate how we located the simulated hot WHIM and examined its radial temperature and density profiles by using the Bisous and MMF/NEXUS+ filament finders, and show how the galaxy luminosity density can aid in selecting the most promising filaments for future observations.

April 23, 2021
2:30 pm (IST)

Extracting the cosmological 21cm signal Using Artificial Neural Networks

Madhurima Choudhury (IIT Indore)


Abstract: One of the key science goals of most of the ongoing or upcoming low-frequency radio experiments is the detection of the redshifted H1 21cm signal to probe the cosmic Dark Ages, Cosmic Dawn, and Epoch of Reionization (EoR). This signal can be detected by averaging over the entire sky, using a single radio telescope, in the form of a Global signal as a function of only redshifted HI 21cm frequencies or as a power spectrum using interferometers. One of the major challenges faced while detecting this signal is the dominating, bright foreground. The success of such detection lies in the accuracy of the foreground removal. The presence of instrumental gain fluctuations, chromatic primary beam, radio frequency interference (RFI) and the Earth's ionosphere corrupts any observation of radio signals from the Earth. We propose to extract the faint signal from the cocktail of influences using artificial neural networks (ANNs). In this talk, I will present an ANN based framework to extract the 21cm signal parameters from mock observations of the 21-cm Global signal and power spectrum experiments.

April 9, 2021
2:30 pm (IST)

The radial acceleration relation in a LCDM universe

Aseem Paranjape (IUCAA)


Abstract: The `radial acceleration relation’ (RAR) between the total and baryonic centripetal acceleration profiles of galaxies has recently emerged as an intriguing new potential test of gravity at galactic scales. I will discuss the RAR in the framework of the Lambda-cold dark matter (LCDM) framework, presenting new analytical insights into the emergence of this relation from an interplay between baryonic feedback processes and the distribution of CDM in dark halos. I will show that, at high baryonic acceleration (i.e., in the baryon-dominated, inner halo region), the median RAR of central galaxies in LCDM is very sensitive to the backreaction of baryons on the dark matter profile of the host halo. On the other hand, the median at very low accelerations (halo outskirts) is determined by the abundance and distribution of diffuse gas affected by feedback processes. At all accelerations, the scatter around the median RAR is determined almost entirely by variations in host halo mass and concentration. These results follow from analytical arguments augmented by a realistic mock catalog of galactic rotation curves in a cosmological volume, and lead to a number of testable predictions of the LCDM paradigm at galactic scales.


March 2021

March 26, 2021
2:30 pm (IST)

A strong broadband 21 cm cosmological signal from dark matter spin-flip interactions

Vikram Rentala (IIT Bombay)


Abstract: In the standard cosmology, it is believed that there are two weak and distinct band-limited absorption features, near 20 MHz (z ~ 70) and 90 MHz (z ~ 15) in the global cosmological 21 cm signal which are signatures of collisional gas dynamics in the cosmic dark ages and Lyman-alpha photons from the first stars at cosmic dawn, respectively. A similar prediction of two distinct band-limited, but stronger, absorption features is expected in models with excess gas cooling, which have been invoked to explain the anomalous EDGES signal. In this work, we explore a novel mechanism, where dark matter spin-flip interactions with electrons through a light axial-vector mediator could directly induce a 21 cm signal which is characteristically different from either of these. We find generically, that our model predicts a strong, broadband absorption signal extending from frequencies as low as 1.4 MHz (z ~ 1000), from early in the cosmic dark ages where no conventional signal is expected, all the way up to 90 MHz, depending upon the epoch of star formation and X-ray heating. We will discuss a rich set of spectral features that could be probed in current and future experiments looking for the global 21 cm signal as well as some complementary laboratory tests of short range spin-dependent forces between electrons.

March 19, 2021
3:30 pm (IST)

The Thermal and Ionization state of the Universe in past 12.8 billion years

Prakash Gaikwad (University of Cambridge)


Abstract: The cosmic evolution of large-scale structures along with the astrophysical processes of galaxy formation and evolution leads to the (re)ionization of the low-density intergalactic medium (IGM). The changes in the thermal and ionization state of the IGM can be used to measure the reionization history. While much of the current and future work is focused on understanding and detecting the HI reionization (z>6), the epoch of HeII reionization (4>z>2) is observationally more accessible with existing telescopes. However, the main challenges lie in modeling the reionization and understanding the observational systematics.

In this talk, the speaker will discuss the modeling of reionization in the state-of-the-art cosmological hydrodynamical ( gadget-3) and radiative transfer (aton) simulations. The speaker will show their recent measurements of ionization and thermal parameters from the UV (HST-COS), optical (Keck, ESO) and infrared (MIKE) observations of Ly-alpha forest at 6>z>0. The implication of these measurements to reionization models, Galaxies/QSO properties and feedback processes will be discussed in the second part. Finally, the speaker will discuss the prospects of the upcoming space-based facility JWST, ground-based radio facility SKA and next-generation 30/40 m class optical telescopes TMT/E-ELT in measuring the reionization history and detecting the faint sources of reionization at z>6.

March 12, 2021
2:30 pm (IST)

The Purely Astrometric Quasar Survey: A Color-blind approach to quasar identification

Jens-Kristian Krogager (Institut d'Astrophysique de Paris)


Abstract: Quasars are widely used in astrophysics to probe various environments, ranging from large-scale studies of the intergalactic medium through Lyman-alpha absorption and clustering to galactic-scale studies of interstellar and circumgalactic media. Moreover, the details of quasar evolution and how this is reflected in the various observational phenomena are still poorly understood. One main limitation to quasar studies in general is the pre-selection for spectroscopic observations. The largest spectroscopic quasar samples to date are selected based on optical colors with inhomogeneous inclusion of multi-wavelength data to increase purity. Such identifications lead to potential biases in the derived quasar demographic and can have severe consequences for foreground absorption studies. Various other methods for selecting quasars based on different parts of the quasar energy distribution have been employed (e.g., radio, X-ray or IR), yet current samples are small. In this talk, I will highlight our efforts to obtain the first color-independent quasar sample by selecting candidates for observation purely based on astrometry from Gaia. This method allows us to select a sample of bright quasars with no assumption on the shape of the SED. The talk will focus on the implications for studies of foreground absorption systems as well as the quasar demographic in general.

March 5, 2021
1:30 pm (IST)

The Impact of Primordial Magnetic Fields on the First Galaxies, Reionization, and the Global 21cm Signal

Harley Katz (University of Oxford)


Abstract: Magnetic fields are important in nearly every astrophysical environment yet they are only just starting to be included in large scale cosmological simulations of galaxy formation. Magnetic fields are particularly interesting in the context of the epoch of reionization because decaying magnetic turbulence and ambipolar diffusion can both heat and ionise the gas, magnetic fields can be generated during reionization at the edges of ionization fronts, primordial magnetic fields can induce secondary perturbations on the matter power spectrum and enhance the number of dwarf galaxies, and magnetic fields can alter the structure of the ISM. I will present the first results from SPHINX-MHD, a suite of cosmological magneto-radiation hydrodynamics simulations designed to study the impact of primordial magnetic fields on the first galaxies and reionization and discuss how reionization observables can be used to constrain the properties of early magnetic fields.


February 2021

February 26, 2021
10:30 am (IST)

Probing Cosmic Reionization with Quasar Proximity Zones

Huanqing Chen (University of Chicago)


Abstract: Probing the epoch of reionization, which happened in the first billion years of the Universe, is an exciting research frontier of modern astrophysics and cosmology. Observing reionization is challenging, due to the saturation of Lya absorption of distant quasars and the scarceness of observable galaxies. However, the quasar proximity zones are ideal windows to study reionization: the enhanced radiation field reduces the saturation in Lya absorption, allowing us to delve into the details of IGM properties. Moreover, quasars are thought to reside in most massive halos, around which more galaxies than average form. At the same time, the radiation feedback from the quasar mimics the radiation feedback of cosmic reionization. Therefore, the study of quasar fields helps us to understand the feedback of cosmic reionization on galaxies. In this talk, I will show two highlights of my research: how to recover the density field in quasar proximity zones and how the radiation feedback from a quasar impacts galaxy formation. I will conclude my talk with a discussion of new exciting science we can do using quasar proximity zones. In the near future, data from JWST and thirty-meter class telescopes will revolutionize our understanding of the epoch of reionization and the formation of the first quasars/galaxies.

February 19, 2021
2:30 pm (IST)

Low Mass Black Holes from Dark Core Collapse

Anupam Ray (TIFR)


Abstract: Unusual masses of the black holes being discovered by gravitational wave experiments pose fundamental questions about the origin of these black holes. More interestingly, black holes with masses smaller than the Chandrasekhar limit (~ 1.4 M_{\odot}) are essentially impossible to produce through any standard stellar evolution. Black holes of primordial origin, with fine-tuned parameters and with no well-established formation mechanisms, are the most discussed explanation of these objects. The notable alternative proposals: implosion of a compact object due to a tiny black hole transit or cumulative accumulation of self-interacting, asymmetric fermionic dark matter, are either ineffective or appeal to fairly baroque dark matter models. In this talk, I will discuss a simple production channel of sub-Chandrasekhar mass black holes. Non-annihilating dark matter with non-zero interaction strength with stellar nuclei, a vanilla dark matter model, can naturally explain these low mass black holes. I will point out several avenues to test the origin of these low mass black holes, concentrating on the redshift dependence of the binary merger rate. I will show how redshift dependence of merger rate can be used by the future GW experiments to determine the transmuted origin of low mass black holes.

February 12, 2021
2:30 pm (IST)

Implications of the z ~ 5 Lyman-alpha forest for the 21-cm power spectrum from EoR

Janakee Raste (TIFR)


Abstract: Most ongoing experiments targeting to detect the neutral hydrogen (HI) 21-cm signal from Epoch of Reionization (EoR) aims to look for the signal at redshifts well above 6. This strategy is motivated by the traditional assumption that reionization ends at z ~ 6. However, recent observations of Lyman-alpha absorption troughs in spectra of high redshift QSOs suggest large spatial fluctuations of HI gas within the intergalactic medium (IGM) at redshifts z = 5–6. These observations, combined with the Cosmic Microwave Background (CMB) Thomson scattering optical depth observed by Planck Collaboration prefer a significantly delayed reionization scenario in which the reionization is 50% complete at redshifts as low as z ~ 7. In these models, reionization ends at z ~ 5, with large 100-Mpc "islands" of cold, neutral hydrogen persisting in the IGM well below z = 6. We study the effect of these neutral hydrogen islands on the 21-cm power spectrum by analyzing outputs of a state-of-the-art radiative transfer simulation of the IGM calibrated to the CMB and Lyman-alpha forest data. We calculate the power spectra of the 21-cm signal from these simulations and compare them with a more traditional reionization model in which reionization is completed by z = 6.7. Contrary to previous models, we find that thanks to the late end of reionization the 21-cm power continues at be high at z = 5-6 and this signal should be detectable by upcoming radio interferometric projects (HERA and SKA1-LOW) for reasonable integration times, assuming optimistic foreground models. We argue that the redshift range z = 5-6 is very attractive for 21-cm experiments due to easier thermal noise characteristics and synergies with abundant multi-wavelength observations.

February 5, 2021
2:30 pm (IST)

The Cosmic Ballet: spinning in the web

Punyakoti G. V. (TIFR)


Abstract: All the celestial objects, from small asteroids and planets to large galaxies and even the long filaments of the cosmic web are rotating. According to the classical tidal torque theory, in the early Universe, as matter began to clump together, the resulting anisotropic distribution of matter torqued up proto-galaxies. Simultaneously, the matter overdensities collapsed to form the large-scale filaments, clusters, walls and voids of the cosmic web that we see today. As a result, we expect a correlation between galaxy spin and the cosmic web. During the talk, I will discuss the role of the cosmic web environment in establishing the rotation of haloes and galaxies using large cosmological simulations and using NEXUS and Bisous formalisms to detect the cosmic web. I will present the correlations between the spin-axis of haloes/galaxies with the orientation of the cosmic web environment that they are growing in. I will also discuss in detail the spin transition from parallel to perpendicular as a function of the halo or galaxy mass with respect to the spine of the host filament and show how these trends evolve with cosmic time and filament properties.


January 2021

January 29, 2021
11:30 am (IST)

Self-interacting neutrinos in the light of cosmological and IceCube observations

Priyank Parashari (PRL)


Abstract: Over the past few decades, the huge influx of data from cosmological and particle physics observations have enabled us to understand and test the viability of the theories. These observations have also yielded some discrepancies which hint towards the new physics. One such discrepancy is the mismatch between the values of the Hubble constant (H0) obtained from the direct local measurements and from the Planck CMB observation within ΛCDM cosmology. Self-interaction between active neutrinos had been proposed as a solution to the H0 tension. Similar self-interaction can also explain the observed dips in the flux of the neutrinos in IceCube detectors. In this talk, I will explain the H0 tension and observed dips in IceCube as a signature of flavour specific self-interaction between active neutrinos. Self-interaction between sterile neutrinos had also been proposed to make light sterile neutrinos viable with cosmological observations. In this talk, I will also discuss the effect of self interacting light sterile neutrinos on the validity of the inflation models.

January 22, 2021
2:30 pm (IST)

Constraining the states of the intergalactic medium during the Epoch of Reionization using 21-cm observations

Raghunath Ghara (Technion, Israel)


Abstract: The redshifted 21-cm signal from neutral hydrogen in the intergalactic medium (IGM) is the most promising probe of the Epoch of Reionization. It has the ability to reveal many of the unknown facts about this epoch such as properties of the early sources of radiation, thermal and ionization states of the IGM. Radio telescopes such as LOFAR, MWA are providing stronger upper limits on the 21-cm power spectrum. I will be talking about how these measurements are used to constraint the states of the IGM during this epoch.

January 15, 2021
11:30 am (IST)

Higher-order clustering statistics in the Intergalactic Medium using Lyman-α forest

Soumak Maitra (IUCAA)


Abstract: The Intergalactic Medium (IGM) is a reservoir of the majority of baryonic content of the Universe and is intimately connected with the evolution of cosmic structures and various astrophysical processes. The matter distribution in the IGM manifests itself observationally in the form of HI Lyman-α forest absorption in the spectra of distant QSOs. Clustering studies of Lyman-α forest has been essential in understanding the matter distribution as well as thermal and ionization state of the IGM. Considerable work has been done involving two-point clustering statistics (two-point correlation or power spectrum) of Lyman-α forest, but higher-order clustering statistics remain largely unexplored. While higher-order clustering statistics have been explored in the case of galaxy clustering, Lyman-α forest will be able to characterize the underlying matter distribution at small scales and high redshift. The speaker will discuss higher-order clustering statistics, specifically three-point correlation statistics of Lyman-α forest, and its dependence on various astrophysical parameters. The speaker will also talk about the recent detection of non-gaussianity in low-redshift (z < 0.48) Lyman-α forest.

January 8, 2021
11:30 am (IST)

Generalized unitarity limits on the mass of thermal dark matter in (non-) standard cosmologies.

Disha Bhatia (IACS)


Abstract: We use the S-matrix unitarity to put constraints on the upper mass of a thermal dark matter whose annihilations proceed via generic k to 2 processes. These annihilations may be either within the standard model sector or dark sector. The bounds are first evaluated assuming radiation dominated universe during the freeze-out. We then extend the analysis to freeze-out during matter-dominated universe before the BBN era. Due to entropy dilution, the upper bound on the mass relaxes. We also find that reheating temperatures higher that O(200 GeV) for k=4, O(1 TeV) for k=3 and O(50 TeV) for k=2 are strongly disfavoured by the combined requirements of unitarity and dark matter relic abundance.


December 2020

December 18, 2020
11:30 am (IST)

Primordial Black Holes from a tiny bump/dip in the Inflaton potential

Swagat Mishra (IUCAA)


Abstract: Primordial Black Holes, formed in the early universe during the hot Big Bang phase, have important implications for a number of cosmic conundra. This talk is about the formation of primordial black holes. Scalar perturbations during inflation can be substantially amplified by tiny features in the inflaton potential. A bump-like feature behaves like a local speed-breaker and lowers the speed of the scalar field, thereby locally enhancing the scalar power spectrum. A bump-like feature emerges naturally if the base inflaton potential contains a positive local correction term, which leads to a large peak in the curvature power spectrum and to an enhanced probability of black hole formation. Remarkably this does not significantly affect the scalar spectral tilt on CMB scales. Consequently such models can produce higher mass primordial black holes (M_PBH ≥ 1 M_sun) that are important for LIGO observations. This is in contrast to models with `near inflection-point potentials' in which generating higher mass black holes severely affects the CMB observables. Interestingly, the results remain valid if the bump is replaced by a dip implying a negative local correction to the inflaton potential. With a generic choice of plateau base potential, the amplification of primordial scalar power spectrum can be large enough leading to a significant contribution of primordial black holes (PBHs) to the dark matter density at the present epoch.

December 11, 2020
11:30 am (IST)

Uncovering the nature of Fast Radio Bursts through multi-wavelength studies

Shriharsh Tendulkar (TIFR)


Abstract: Fast Radio Bursts (FRBs) are millisecond-timescale radio transients originating from cosmological distances (~Gpc) that have been discovered a little more than a decade ago. At these distances, they have to be a trillion times more luminous than the brightest radio pulses observed from Galactic pulsars. The engine and emission mechanism that can produce such luminosities is still unknown despite ~80 different theories. Over the past few years, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB backend has detected hundreds of FRBs including a dozen repeating FRBs and a few of the nearest FRB sources. The repeating nature of these FRBs, allows for precise localization with radio interferometers and a detailed study of their environment and nature with multi-wavelength observations. I will introduce the broad questions about the nature of FRBs and their promise as tools for cosmology. I will focus on discussing recent multi-wavelength results on two of the best studied FRBs — FRB 121102 and FRB 180916 — from CHIME/FRB as well as a larger collaboration which shed new light on the nature of these sources.


November 2020

November 27, 2020
11:30 am (IST)

Exploring KSZ velocity reconstruction with N-body simulations and the halo model

Utkarsh Giri (Perimeter Institute)


Abstract: KSZ velocity reconstruction is a recently proposed method for mapping the largest-scale modes of the universe, by applying a quadratic estimator v̂ to the small-scale CMB and a galaxy catalog. We implement kSZ velocity reconstruction in an N-body simulation pipeline and explore its properties. We find that the reconstruction noise can be larger than the analytic prediction which is usually assumed. We revisit the analytic prediction and find additional noise terms that explain the discrepancy. The new terms are obtained from a six-point halo model calculation and are analogous to the N(1) and N(3/2) biases in CMB lensing. We implement an MCMC pipeline which estimates fNL from N-body kSZ simulations and show that it recovers unbiased estimates of fNL, with statistical errors consistent with a Fisher matrix forecast. Overall, these results confirm that kSZ velocity reconstruction will be a powerful probe of cosmology in the near future, but new terms should be included in the noise power spectrum.

November 13, 2020
10:30 am (IST)

Peculiar velocities in the local Universe and its role in observational cosmology

Supranta Sarma Boruah (The University of Arizona)


Abstract: The study of peculiar velocity in the low redshift Universe plays an important role in observational cosmology in two main ways: i) It introduces correlated errors in the measurement of the redshifts, potentially making up a non-negligible fraction of the systematic error budget in the measurement of the expansion history, ii) Peculiar velocities are the only probe of growth of structure in the very low-redshift Universe. In this talk, I will present some recent results on both of these aspects. First, I will present some results from our investigation into the role of peculiar velocity corrections in the measurement of H0. I will show that assumptions on the line-of-sight peculiar velocity can lead to significant differences in the measured value of H0. Then, I will present results of constraints on cosmological parameters from velocity-velocity comparison. The constraints on fσ8 from peculiar velocities are competitive and complementary to probes of structure growth at high redshifts.