State of the Universe

Seminar series of the Cosmology and Astroparticle Physics group at TIFR

Previous Seminars

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 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 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 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 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 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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