State of the Universe

Seminar series of the Cosmology and Astroparticle Physics group at DTP, TIFR

Previous Seminars


December 2021

Dec 24, 2021
02:30 pm (IST)

Title: 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.

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Dec 21, 2021
02:30 pm (IST)

Title: Light Dark Universe: Prospects for light particle searches

Tanmay Poddar (PRL, Ahmedabad)

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.

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Dec 14, 2021
02:30 pm (IST)

Title: Lepton Flavored Dark Matter: Two Scenarios

Shiuli Chatterjee (IISc, Bengaluru)

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.

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Dec 10, 2021
02:30 pm (IST)

Title: Global 21-cm signal and its implications

Atrideb Chatterjee (NCRA, Pune)

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.

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Dec 07, 2021
02:30 pm (IST)

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

Tomáš Šoltinský (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.

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Dec 03, 2021
02:30 pm (IST)

Title: On detecting CMB spectral distortions: Recombination to Reionization

Mayuri Rao (RRI, Bengaluru)

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.

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Dec 02, 2021
10:30 am (IST)

Title: 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.

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November 2021

Nov 26, 2021
02:30 pm (IST)

Title: 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.

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Nov 23, 2021
02:30 pm (IST)

Title: 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.

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Nov 12, 2021
03:30 pm (IST)

Title: Gas-rich dwarf galaxies as powerful dark matter detectors

Digvijay Wadekar (Institute of Advanced Study, 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.

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October 2021

Oct 29, 2021
02:30 pm (IST)

Title: 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.

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Oct 22, 2021
02:30 pm (IST)

Title: 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.

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Oct 08, 2021
02:30 pm (IST)

Title: The effect of inhomogeneous reionisation on the Lyman alpha forest

Margherita Molaro (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

Sep 24, 2021
02:30 pm (IST)

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

Shikhar Mittal (TIFR, Mumbai)

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.

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June 2021

June 25, 2021
02:30 pm (IST)

The Cosmological Principle and the missing rest frame of the Universe

Mohamed Rameez (TIFR, Mumbai)

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, Pune)

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.

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June 11, 2021
04:00 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.

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June 04, 2021
02:30 pm (IST)

Extracting the 21 cm EoR signal using MWA drift scan data

Akash Patwa (Raman Research Institute, Bangalore)

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.

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May 2021

May 28, 2021
11:30 am (IST)

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

Anish Ghoshal (The National Institute for Nuclear Physics, 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.

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May 21, 2021
05:00 pm (IST)

Cross-correlation studies with future CMB experiments

Anirban Roy (The 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.

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May 07, 2021
02:30 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.

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April 2021

April 30, 2021
02:30 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.

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April 23, 2021
02:30 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.

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April 09, 2021
02:30 pm (IST)

The radial acceleration relation in a LCDM universe

Aseem Paranjape (IUCAA, India)

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.

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March 2021

March 26, 2021
02:30 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.

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March 19, 2021
03:30 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.

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March 12, 2021
02:30 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.

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March 5, 2021
01:30 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.

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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.

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February 19, 2021
02:30 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.

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February 12, 2021
02:30 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.

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February 5, 2021
02:30 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.

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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.

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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)

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.

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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.

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January 08, 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.

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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.

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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.

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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.

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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.

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