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Monday, 01 May 2023
Time Speaker Title Resources
09:30 to 10:30 -- Introduction
16:00 to 17:00 Joseph Silk (The Institut d'Astrophysique de Paris, France and Johns Hopkins University, USA) The Future of Cosmology

I will review the prospects for future progress in cosmology. I will give examples of futuristic lunar experiments. One seeks the dark ages signature via low-frequency radio astronomy on the far side of the Moon, This could provide a  robust probe of our cosmic origins via inflation. Such concepts could be implemented in the coming decades of lunar exploration.

Tuesday, 02 May 2023
Time Speaker Title Resources
16:00 to 17:00 Joseph Silk (The Institut d'Astrophysique de Paris, France and Johns Hopkins University, USA) From nuclear star clusters to intermediate-mass black holes

Nuclear star clusters can be the missing link in the formation of massive black holes.  Million solar mass black holes can form within 100 Myr in the heaviest and most compact nuclear star clusters.  There are potential gravitational-wave signatures of the massive black-hole formation process. 

Thursday, 04 May 2023
Time Speaker Title Resources
16:00 to 17:00 Joseph Silk and students What are the limits of our knowledge?

Interaction session between Prof. Joseph Silk and students

Friday, 05 May 2023
Time Speaker Title Resources
16:00 to 17:00 -- Student-Lecturer interaction session
Monday, 08 May 2023
Time Speaker Title Resources
09:30 to 10:00 -- P1: Measurement of Cluster halo density profile and estimation of the Splashback Radius
10:00 to 10:30 -- P2: Extracting cosmological information from the large-scale structure signal: Baryon Acoustic Oscillations and Redshift Space Distortions
10:30 to 11:00 -- P3: Reionization constraints using 21 cm observations
11:30 to 12:00 -- P4: Identification of substructure in large stellar surveys
12:00 to 12:30 -- P5: Reanalysis of Cosmic Shear Measurements from the Dark Energy Survey
12:30 to 13:00 -- P7: Measurement of the weak lensing signal of galaxy clusters and their halo density profile
Tuesday, 09 May 2023
Time Speaker Title Resources
09:30 to 10:00 Ravi Sheth (University of Pennsylvania, USA) Optimal Transport Reconstruction and Peaks Theory for Energy

I will review recent progress in using optimal transport methods to reconstruct the initial positions of biased tracers such as galaxies from their present day, redshift-space distorted positions.  This is particularly useful for estimating the cosmic distance scale.

10:00 to 10:30 Arka Banerjee (IISER Pune, India) Beyond-Gaussian statistics for cosmological clustering - k-Nearest Neighbor Distributions

Abstract: Statistical measurements of the clustering of galaxies is one of the main observables from cosmological surveys, containing information both about the initial conditions of the Universe - such as the physics of inflation - as well as about those components that drive the expansion of the Universe today - Dark Energy, the nature of Dark Matter, and massive neutrinos. While most cosmology analyses in the past have focused on two-point functions of the galaxy distribution to probe these questions, there has been growing recognition that there is significant information in higher order N-point functions of the highly nonlinear galaxy distributions. In this talk, I will introduce the formalism for a new measure of cosmological clustering - the k-Nearest Neighbor distributions. These distributions are formally sensitive to all N-point functions and computationally inexpensive to measure. I will discuss how these statistics can also be easily extended to describe cross-correlations of different cosmological datasets. Finally, I will discuss the potential improvements in constraints on various cosmological parameters when using these statistics over two-point functions and current efforts in measuring these on actual data.

10:30 to 11:00 Supranta Boruah (University of Arizona, USA) Bayesian Field-Level Inference with Weak Lensing Data

Traditional analysis of weak lensing data relies on 2-point statistics of galaxy shapes. However, for late-time density fields that are strongly non-Gaussian, such analyses are suboptimal. In this talk, I will discuss a field-level analysis pipeline for analyzing weak lensing datasets. Because it utilizes information from the entire field, such analysis is optimal at a given scale. I will talk about some recent progress and challenges in this effort. In particular, I will talk about our efforts to enable forward-modeled mass mapping on the curved sky — a step that is essential for analyzing current and future weak lensing data. I will further discuss the gain in cosmological information that is enabled by such a field-level analysis.

11:45 to 12:00 Biswajit Biswas (CNRS, France) Maximum-A-Posteriori Solution with Deep Generative Networks for Source Separation (MADNESS)

The upcoming Stage IV surveys will observe the Universe up to an unprecedented depth and area of coverage. In the Legacy Survey of Space and Time (LSST), the Vera Rubin Observatory will observe billions of galaxies over 10 years in 6 photometric bands. The increase in depth will allow us to study fainter objects but at the same time, it will result in an increased probability to observe the overlapping of physically separated objects. For example, around 63% of objects LSST objects are likely to be blended. The effect of blending will be one of the major systematics to face while constraining Cosmology with probes such as weak lensing, which is one of the main goals of LSST. To mitigate the impact of blending, it becomes essential to find a way to separate the overlapping sources at the pixel level. We propose an alternative to classical deblending approaches that exploits recent advances in the field of deep learning to make the algorithm fast and scalable on GPUs. In the context of the LSST Dark Energy Science Collaboration (LSST DESC), we have developed a Python package called MADNESS that obtains the maximum-a-posteriori solution using deep neural networks. The architecture consists of a combination of a variational autoencoder and a normalizing flow to impose data-driven prior in the latent space instead of symmetry constraints on reconstructions like traditional deblenders. In my talk, I will demonstrate the performance of MADNESS on simulated realistic galaxy fields and show the improvement in performance over the state-of-the-art deblender - scarlet - in terms of flux and morphology.

12:00 to 12:15 Geet Rajio Mankar (IISER Pune, India) Validating the Cosmological Principle

The Cosmological Principle is a key feature of the very successful Standard model of Cosmology. It stands on to major pillars, isotropy and homogeneity at large scales. These pillars have been questioned recently using quasars as tracers of the Large scale structure (LSS). To check the validity of the Cosmological Principle, we use the Million Quasars Catalogue 7.2 to probe the LSS for isotropy and homogeneity. To do this we use techniques based on Shannon Entropy. We use these techniques to measure the ( i ) homogeneity, and ( ii ) isotropy along radial and transverse directions. We discover that the difference between isotropic and homogeneous distribution and the LSS as traced by the catalogued quasars, using this method, is negligible (~ 0.001) beyond ~250Mpc/h. Thus, the Cosmological Principle remains validated at large scales.

12:15 to 12:30 Purba Mukherjee (ISI Kolkata, India) Data-driven reconstruction in cosmology

I shall focus on the reconstruction of some cosmological parameters using Gaussian processes regression on mainstream observational datasets. I will talk about the non-parametric reconstruction of the dark energy equation of state and cosmography. I will explore the possibility of a non-gravitational interaction in the cosmic dark section and the constancy of the speed of light. I will discuss the prospects of GP, and its application in constraining the Hubble constant, the matter density parameter, and spatial curvature in a model-independent way. Further, I will compare some of the GP results with those from a neural network reconstruction, focusing on the advantages of both methods and their respective limitations. In conclusion, I will highlight the application of these ML techniques as a forecasting tool in future surveys. Based on arXiv: 2202.07886, 2207.07857, 2209.01113, 2302.00867, 2301.12708

12:30 to 12:45 Amit Kumar (IUCAA, India) Environmental Effects in Galaxy Clusters

Subaru HSC weak lensing of SDSS redMaPPer cluster satellite galaxies: Empirical upper limit on orphan fractions:(Kumar et al,2022, MNRAS) Gravitational lensing can directly estimate the matter distribution around objects. We measure the weak lensing signal around SDSS redMaPPer cluster satellite galaxies, induced on the shapes of galaxies background to them. We choose satellites where their central galaxy is defined with a probability $P {\rm cen}>0.95$ in the redshift range, $0.1\leq z\leq 0.33$. For shape measurements, we use galaxies from the Subaru Hyper Suprime-Cam (HSC) survey. In order to understand the effect of the various environmental processes on matter distribution (mainly dark matter), we bin our satellite galaxies by their distance from the cluster center. Then we compare the matter distribution around satellites to a sample of galaxies that do not reside in clusters but have colors and magnitudes similar to the satellites. We see hints of a difference in the mass of the subhalo of the satellite compared to the halo masses of galaxies in our control sample, especially for the innermost cluster-centric radial bin $0.1< r < 0.3$ $[h^{-1}{\rm Mpc}]$. We use this observed mass difference to put a first ever direct upper limit on the prevalence of orphan galaxies that have lost most of their dark matter, primarily due to multiple pericentric passages. However, these upper limits could be relaxed if there is substantial contamination in the satellite galaxy sample.

12:45 to 13:00 Sindhu Satyavolu (TIFR, India) Need for Obscured Growth of Supermassive Black Holes in the First Billion Years

The proximity zones of quasars with redshifts z > 6 are not only unique probes of the growth of their central supermassive black holes but also the only parts of the Universe at these redshifts where the cosmological small-scale structure can be measured. Considering the five-fold increase in the number of known high-redshift quasars in the past five years, and upcoming surveys enabled by JWST, LSST, Euclid, and WFIRST, it is of interest to leverage this power of proximity zones for the physics of black holes and the intergalactic medium. Proximity zone measurements in the last few years have led to inferences of surprisingly short lifetimes for quasars for which the Mg II-based masses are greater than 10^9 solar mass, challenging black hole growth models. Nonetheless, the theoretical models used to infer the lifetimes from the measurements of the proximity zone sizes make several simplifying assumptions about the quasar and its environment. Using high-dynamic-range cosmological radiative transfer simulations, we study the robustness of such assumptions. In this talk, I will show that the patchiness of reionization can relieve the tension between the proximity zone sizes and black hole masses to some degree, but not fully. I will then demonstrate that what is needed to explain the small proximity zone sizes is quasar variability with short duty cycle and brief bright periods. This conclusion, when combined the measurements of black hole masses of these quasars, directly leads to a prediction of significant obscuration in high-redshift quasars. I will end my talk by arguing that this prediction can be directly tested using JWST.

14:45 to 15:15 Elisabeth Krause (University of Arizona, USA) Weak Lensing without Shape Noise

Weak gravitational lensing imprints a coherent distortion onto the observed shapes of distant galaxies. At the image level, this gravitational shear is degenerate with the intrinsic shape of galaxies, and the weak lensing signal-to-noise from an individual galaxy is of order 0.01. I will describe Kinematic Lensing, a new technique combining imaging and galaxy kinematics to measure weak lensing with signal-to-noise of order one per galaxy.

15:15 to 15:45 Pratika Dayal (University of Groningen, Netherlands) The Emergence of Galaxies in the Epoch of Reionization and their Large-Scale Effects: Advance and Implications

Galaxy formation in the first billion years mark a time of great upheaval in our cosmic history: the first sources of light in the Universe, these galaxies ended the 'cosmic dark ages' and produced the first photons that could break apart the hydrogen atoms suffusing all of space starting the process of cosmic reionization. At the forefront of astronomical research, the past few years have seen cutting-edge instruments such as JWST and ALMA provide tantalising glimpses of such galaxies chaotically assembling in an infant Universe. I will show how this data has provided an unprecedented opportunity to pin down the reionization state of the Universe, understand the physical properties of early galaxies, and study the key physics driving their formation and evolution. As we look forward towards the era of 21cm cosmology, I will highlight the crucial and urgent synergies required between 21cm facilities (such as the SKA) and galaxy experiments (JWST, E-ELT and Subaru to name a few) to understand the physics of the epoch of reionization that remains a crucial frontier in the field of astrophysics and physical cosmology. Time permitting, I will try to give a flavour of how the assembly of early galaxies, accessible with the forthcoming JWST, can provide a powerful testbed for Dark Matter models beyond "Cold Dark Matter".

15:45 to 16:15 Priyanka Singh (Yale University, USA) New challenges to our understanding of the circum-galactic medium

The Circum-Galactic Medium (CGM) is a massive gas reservoir containing the majority of the baryonic matter in galaxies. The CGM is arguably the next frontier of investigating gas physics in galactic halos as it bears crucial information about galactic evolution and cosmology. The CGM poses a potential solution to the galactic missing baryon problem. It also provides additional information on the astrophysical feedback processes such as supernovae and active galactic nuclei, pushing the gas out to large distances from the galactic center. However, the CGM is also the most poorly constrained component of a galaxy. A multi-wavelength analysis of the gas can help us better understand the distribution and dynamics of the CGM. The combination of ongoing and future sky surveys such as South Pole Telescope at millimeter, eROSITA in X-rays and Dark Energy Survey in optical will significantly improve the existing constraints on the CGM properties. Newly emerging tools such as kinetic Sunyaev-Zel'dovich effect and fast radio bursts will push our observational capabilities to lower mass galaxies where the impact of astrophysical feedback processes significantly alters the galactic eco-systems.

17:00 to 18:00 -- Topical Discussion
Wednesday, 10 May 2023
Time Speaker Title Resources
09:30 to 10:00 Joseph Mohr (LMU and Max Planck Institute for Extraterrestrial Physics, Germany) Galaxy Cluster Studies with the Largest Cosmological Surveys

Galaxy clusters are rare and massive collapsed halos that have been successfully used to study cosmology, baryonic processes and the nature of dark matter, among other things. Galaxy cluster catalogs can be constructed from large area, multi-wavelength surveys. The most promising approach is to employ an intracluster medium signature (X-ray emission or thermal Sunyaev-Zel’dovich effect- SZE) to produce a candidate list and to then use deep, multi-band optical survey data to measure redshifts, remove contaminants and constrain the cluster halo mass using weak gravitational lensing. Numerical simulations and mock observations are additional crucial ingredients. The required X-ray and SZE surveys have existed now for several years, but ongoing cluster cosmological studies are only now seeing the full benefits of catalog cleaning and halo mass constraints from deep, multi-band optical survey data acquired with, e.g., the Dark Energy Camera (DECam) or the Hyper-Suprime- Camera (HSC). Current results include the largest cluster samples in SZE and X-ray yet, the best ever constraints on mass—observable relations and the promise of resolving the current S8 tension between structure formation based and primary CMB probes. The upcoming Euclid and Rubin datasets will offer tremendous advantages over the existing DECam and HSC datasets, but only if the challenges of improved calibration of shear and photometric redshifts can be met. In addition, significant improvements in the understanding of baryonic processes and their impact on cluster halos will be needed to reach the full potential of these upcoming, largest cosmological surveys.

10:00 to 10:30 Hironao Miyatake (Nagoya University, Japan) HSC Year 3 Weak Lensing Cosmology Results

The accelerating expansion of the universe is one of the most mysterious phenomena. Cosmic acceleration implies the existence of dark energy or the breakdown of Einstein’s general relativity. Either way, revealing the source of cosmic acceleration can result in a paradigm shift in modern physics. Weak gravitational lensing is a subtle, coherent distortion of distant galaxy images due to gravitational potential, allowing the direct measurement of the spatial distribution of dark matter. Weak lensing is one of the most powerful cosmological probes because of its capability to measure the nature of cosmic acceleration through the evolution of the large-scale structure of the universe. Hyper Suprime-Cam (HSC), a newly developed prime focus camera at Subaru Telescope, started a wide, deep galaxy imaging survey in 2014, covering 1,100 sq. degrees of the sky down to the i-band limiting magnitude of 26. The wide field of view, light-gathering power, and superb image quality of HSC make it possible to measure weak lensing distortions with unprecedented precision. In this talk, I will present cosmology results from the Subaru Hyper Suprime-Cam Survey Year 3 data, mainly about cosmological constraints from cosmic shear and the combination of galaxy-galaxy lensing and clustering.

10:30 to 11:00 Hector Gil Marin (ICC University of Barcelona, Spain) A Tale of 2 (or more) h's

I plan to give a brief review on how the hubble constant is measured from different techniques, and which assumptions each of these methods implicitly assume. 
In particular I will focus on a new method which is able to determine h from galaxy clustering data without assuming any sound-horizon scale physics. Such novel determination of h is key to rule out or confirm early dark energy models as a potential solutions of the low-high h measurements from Planck and Cepheids, respectively.

11:45 to 12:00 Shikhar Mittal (TIFR, India) Lγα Radiative Transfer at Cosmic Dawn

The coupling of Lyman-alpha photons with cosmological hydrogen is the critical process that enables the 21-cm signal from Cosmic Dawn. Motivated by the claimed detection of this signal by EDGES and the upcoming attempt to repeat their measurement by REACH, we investigate this Lyman-alpha coupling using RAMSES and RASCAS. This allows us to improve upon previous work in the literature, which either assumed optically thin Lyman-alpha RT or implemented this only approximately by assuming that the cosmological gas distribution is cold and has uniform density. In my talk, I will describe the RAMSES and RASCAS set-up that we have developed for this problem. I will then present the resultant predictions of the Cosmic Dawn signal.

12:00 to 12:15 Akanksha Kapahtia (NCRA, India) Seminumerical simulations of the epoch of Helium reionization

We present an efficient semi-numerical approach for modeling the reionization of the second electron of helium (HeII) while taking into account the spatial patchiness and the resultant impact of photoheating on the IGM. First, we model quasars as sources of ionization consistent with observed luminosity function data. We consider quasar properties such as their Spectral Energy Distribution described by the index \alpha^SED, a maximum cut-off magnitude (M^max) and a quasar lifetime (t q) . The resultant ionizing photon field is included in the Semi numerical Code for ReionIzation with PhoTon Conservation (SCRIPT) , (originally developed to model hydrogen reionization) after modifying it for HeII and incorporating inhomogenous HeII recombinations . The corresponding thermal history of the IGM is modelled analytically via a sub-grid prescription and is parameterized by the reionization temperature for HeII -- (T^re). The above procedure yields the HeII ionization history and the evolution of the equation of state of the IGM . We show that our simulations are consistent with observations for a reasonable set of values of our free parameters.

12:15 to 12:30 Janakee Raste (NCRA, India) Studying the end of EoR with the 21-cm Bispectrum

Recent constraints from Lyman-α and CMB data suggest a significantly delayed reionization scenario in which IGM is ionized to 50% at redshift z ~ 7. In these models, reionization ends at z ~ 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 simulation of the IGM calibrated to the CMB and Lyman-α forest data. In this talk, I will discuss effects of these neutral hydrogen islands on the 21cm signal. 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 at 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 21cm bispectra from the EoR. I will further discuss the sensitivity of various interferometric instruments to make predictions about the observability of these statistical signals. I will argue that 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.

12:30 to 12:45 Khandakar Md Asif Elahi (IIT Kharagpur, India) Towards 21-cm Intensity Mapping with uGMRT III: Foreground Removal

We analyse a $100$ MHz bandwidth (centering $444\,\rm{MHz}$) high resolution $(24.4\,\rm{kHz})$ uGMRT data aiming neutral hydrogen (\ion{H}{i}) $21$-cm intensity mapping (IM) in the redshift range $z=1.88-2.61$. We use the Cross TGE which grids the two cross-polarizations RR and LL separately and cross-correlates them to get an unbiased estimate of the multi-frequency angular power spectrum (MAPS) $C {\ell}(\nu a,\nu b)$. The measured $C {\ell}(\nu a,\nu b)$ is found to be foreground dominated. We identify the foreground eigenvectors using a Kosambi-Karhunen–Lo\`eve transform (KKLT). The foreground contribution in the MAPS is subtracted from the measured $C {\ell}(\nu a,\nu b)$ to obtain the residual $[C {\ell}(\nu a,\nu b)] {\rm{res}}$, which is used to constrain the $21$-cm signal. We still find some amount of residual foregrounds (or systematics) in $[C {\ell}(\nu a,\nu b)] {\rm{res}}$, which is, however, free from any negative systematics. We place upper limits on $\Delta^2(k)$ the mean-squared brightness temperature and $[\Omega {\ion{H}{i}}b {\ion{H}{i}}]^2$ the neutral hydrogen abundance parameter in the $k$-range $0.1<k

12:45 to 13:00 Avinanda Chakraborty (Presidency University, India) Constraining Quasar Feedback Models with the Atacama Large Milimeter Array by Probing Sunyaev-Zel'dovich Signals

The thermal Sunyaev-Zeldovich (SZ) effect is the spectral distortion of the cosmic microwave background (CMB) radiation by energetic electrons. The SZ effect can be used as a direct potential probe of the energetic outflows from quasars that are responsible for heating the intergalactic medium. Here we use the GIZMO meshless finite mass hydrodynamics cosmological simulation (Dave et al. 2019) which includes dark matter and gas dynamics, radiative cooling, star formation, black hole growth, and different feedback prescriptions (SIMBA), to compute the SZ effect arising from quasar feedback for different feedback modes. From these theoretical simulations, we perform mock observations of the Atacama Large Millimeter Array (ALMA) to characterize the feasibility of direct detection of the quasar-SZ signal. In this work, for all the systems we get enhancement of the SZ signal when there is radiative feedback only, while the signal gets suppressed when the jet mode of feedback is introduced. From our mock ALMA maps, we note that SZ signals from systems with jet feedback go below the detection threshold of ALMA. Our work for the first time puts detection constraints on models of AGN feedback that are widely used in cosmological simulation.

14:45 to 15:15 Susmita Adhikari (IISER Pune, India) From the Edge of a Dark Matter Halo

Halos are the densest regions of dark matter in the universe, the detailed structure, evolution and distribution can shed light on some of the most fundamental questions in Cosmology. In this talk I will focus on how the observed distribution of matter and galaxies within halos on different scales ranging from the inner scale radius to the splashback radius in the transition region, can be used to understand halo and galaxy co-evolution, the nature of gravity and dark matter.

15:15 to 15:45 Uri Keshet (Ben Gurion University, Israel) Stacking galaxy clusters

Superimposing many galaxy-cluster images brings out subtle features that are difficult to detect otherwise, especially if the clusters are first normalized to their characteristic radii in order to utilize their approximate self-similarity. I will review such stacking analyses, focusing in particular on the peripheries of clusters. Here, stacking has facilitated the detection of virial shocks in gamma-rays, radio, and Sunyaev-Zel'dovich signals.

16:00 to 17:00 Nick Kaiser (ENS, France) The History of Gravitational Lensing in Cosmology

In this talk I review the development of gravitational lensing in cosmology - a subject which is entering a "golden age" with the advent of Euclid and LSST.  I start with Newton, who, it seems, understood light deflection but didn't get cosmology right, and follow the trail to Einstein's calculation of light bending by the sun and its generalisation to light propagation in a lumpy universe.  I describe early exciting, but puzzling, applications to quasar-galaxy associations and galaxy-galaxy lensing and how, in particular, Tony Tyson's group's measurement of distortion of the "cosmic wallpaper" by galaxy clusters triggered the development of modern weak lensing. Finally, I shall discuss the insight that Raychauhuri's approach to light propagation brings to this subject.

Thursday, 11 May 2023
Time Speaker Title Resources
09:30 to 10:00 Cullan Howlett (University of Queensland, Australia) Cosmology and cosmography with galaxy peculiar velocity surveys

Direct measurements of galaxy peculiar velocities, i.e., their motions with respect to us beyond that expected from just the expansion of the Universe, are undergoing a resurgence. Five years ago, individual surveys were limited to a few thousand galaxies and to within the z~0.05 Universe. With the newest state-of-the-art, the number of galaxies, and the cosmological volume they cover, will increase by more than an order of magnitude. In this talk I will present the motivation and mechanisms for carrying out these measurements, focusing on both how they can be combined with galaxy redshifts to provide the most precise tests of General Relativity on large scales, and how they can be used to create detailed cosmographic maps of hidden structures in the nearby Universe. As an example, I will discuss the construction and new results from our recently released Sloan Digital Sky Survey peculiar velocity catalogue. I will then finish the talk by highlighting the revolutionary results we expect from the next generation of peculiar velocity surveys being carried out with the Dark Energy Spectroscopic Instrument (DESI) and 4-Metre Multi-Object Spectroscopic Telescope (4MOST).

10:00 to 10:30 Nishikanta Khandai (NISER, India) The Distribution of Cold Gas in the Local Universe

We use a volume common to the SDSS and ALFALFA surveys to constrain the conditional neutral hydrogen (HI) mass and velocity-width functions, conditioned on optical color and/or magnitude. This allows us to further constrain the distribution function of the cosmic HI density in the color-magnitude plane of galaxies. We find that the distribution has a long tail towards faint blue galaxies and luminous red galaxies.  About 10% of the HI budget is associated with low surface brightness galaxies whereas 18% is locked up in the red population of galaxies. We use these abundances and recent stacking results from the ALFALFA survey to constrain the halo mass function of HI-selected galaxies. The transition halo mass scale in the halo mass - HI mass relation  is smaller by ~ 1.4 dex to that of the halo mass - stellar mass relation, suggesting that baryonic processes like heating and feedback in larger mass halos suppress HI gas on a shorter time scale compared to star formation.  We show that our observationally calibrated HI halo model reproduces the clustering of HI selected galaxies.

10:30 to 11:00 Punyakoti Veena (Technion, Israel) Spin alignments and large-scale structure reconstructions using neural nets

I will present my work in two parts. Firstly, I will explore the role of the cosmic web environment in establishing the rotation of haloes and galaxies using large cosmological simulations. The origin of rotation in celestial objects is still not fully understood, particularly in the context of galaxies and their dark matter haloes. I will show correlations between the spin-axis of haloes/galaxies with the orientation of the cosmic filaments they are growing in, and explore the spin transition from parallel to perpendicular as a function of halo or galaxy mass with respect to the spine of the host filament.

Secondly, I will present a neural network method for reconstructing the underlying 3D cosmological density and velocity fields from incomplete observed galaxy distributions, which can provide valuable information on cosmological parameters. I will show how neural network reconstructions are related to different conventional statistical estimators, and compare the performance of our reconstruction method with the traditional Wiener filter. I will highlight the advantages of the neural network approach, particularly in capturing non-linear features, and discuss the impact of neural networks on the future of the field with the huge expected inflow of data.

11:45 to 12:00 Harshda Saxena (IIT Bombay, India) Kinetic Field Theory: Effects of modified gravity theories with screening mechanisms on non-linear cosmic density fluctuations

In a mean-field approximation within Kinetic Field Theory, it is possible to derive an accurate analytic expression for the power spectrum of present-day, non-linear, cosmic density fluctuations. It depends on the gravity theory and the cosmological model via the expansion function of the background space-time, on the growth factor derived from it, and on the gravitational coupling strength, which may deviate from Newton's constant in a manner depending on time and scale. In earlier work, we introduced a functional Taylor expansion around general relativity and the cosmological standard model to derive the effects of a wide class of modified-gravity theories on the non-linear power spectrum, assuming that such effects need to be small given the general success of the standard model. Here, we extend this class towards theories with small-scale screening, modelling screening effects by a suitably flexible interpolating function. We compare the Taylor expansion with exact solutions and find good agreement where expected. We find typical relative enhancements of the non-linear power spectrum between a few and a few ten per cent in a broad range of wave numbers, in good qualitative agreement with results obtained from numerical simulations.

12:00 to 12:15 Sankarshana Srinivasan (University of Manchester, UK) Probing Modified Gravity using Data from Large Scale Structure Surveys

Model-independent constraints on modified gravity models hitherto exist mainly on linear scales. I present the first N-body simulations of modified gravity models based on a consistent parameterisation that is valid on all scales. I investigate the impact of a time-dependent modification of the gravitational force on the matter power spectrum and consequently on weak-lensing observables, with particular focus on the constraining power gained by including non-linear scales. I describe a fitting function validated against simulations for that can predict the non-linear matter power spectrum of the simulations for a wide range of parameters and discuss how this pipeline has been implemented for Euclid modified gravity forecasts. This paves the way for a full model-independent test of modified gravity using all of the data from such upcoming surveys.

12:15 to 12:30 Sukhdeep Singh (IIT Kharagpur, India) Effect of Peculiar Velocities on Cosmic Bispectrum: EoR Case Study

Bispectrum, the lowest order statistics sensitive to non-Gaussianity, of the cosmic field distribution in real and redshift space provides a great deal of cosmological information regarding the physics of the very early Universe, subsequent growth of structures, and constraining various cosmological parameters. It is a function of closed triangles formed by three k-modes. Measuring the bispectrum from the observed cosmic fields is important to understand the underlying field's features better. However, in observed field distribution, there is induced anisotropy along the line of sight direction due to their peculiar velocities. So, in redshift space, the bispectrum depends on the orientation of triangles in addition to their shape and size, in contrast to real space bispectrum, which depends only on the shape and size of the triangle only. We quantify the triangle's orientation with Euler angles and quantify the anisotropy of the bispectrum by decomposing it into spherical harmonics. We study the multipole moments of the redshift space bispectrum and develop an FFT-based fast estimator to calculate these moments from data from ongoing and upcoming galaxy surveys like LSST, DESI, EUCLID, etc. I will also show the results for the evolution of bispectrum monopole and quadrupole moments of simulated Epoch of Reionization signal from redshift 7 to 13.

12:30 to 12:45 Shouvik Roy Choudhury (IUCAA, India) Neutrino Self-Interactions. Hubble Tension, and Inflation

In the Lambda-CDM model, Planck data provides a value of the Hubble constant H0 equal to 67.5 km/s/Mpc, which is around 5-sigma away from the locally measured value from Type Ia Supernovae, H0 equal to 73 km/s/Mpc. This Hubble tension is currently the biggest discrepancy in cosmology. Here we looked at the cosmological model that incorporates massive neutrino non-standard interaction mediated via a heavy scalar, which had previously shown a lot of promise in solving the Hubble tension in the strong interaction regime where the coupling strength is approximately 10^9 times the weak interaction coupling. However, with the latest Planck 2018 data, we found that the Planck high-multipole CMB polarization data disfavours such strong interactions. We also test the viability of Natural Inflation and Coleman-Weinberg Inflation in the presence of massive neutrino non-standard interactions, against CMB, BAO, and LSS data, with a particular focus on Planck and BICEP/Keck data. These inflationary models are ruled out at more than 2-sigma in the standard Lambda-CDM model with current cosmological data. But interestingly, we find that predictions from these inflationary models are allowed within 2-sigma when we include massive neutrino non-standard interactions.

12:45 to 12:48 Yashi Tiwari (IISc, India) Understanding Large Scale CMB Anomalies Using Generalised Non-Minimal Derivative coupling during inflation

We study the observational implications of a class of inflationary models wherein the inflaton is coupled to the Einstein tensor through a generalised non-minimal derivative coupling (GNMDC). Such a coupling can be realized in the framework of Horndeski theories or generalised Galileon theories and leads to novel and distinguishable inflationary predictions. In particular, we explore whether such models can provide a possible explanation to the large-scale anomalies such as the power suppression and other localized features associated with the CMB temperature anisotropies at low multipoles or large angular scales. For a specific choice of the GNMDC coupling function, we find that these models can lead to suitable localized features in the power spectrum on large scales. We work in the regime of parameter space such that we avoid the gradient instability and the superluminal propagation of scalar perturbations. An interesting aspect of our analysis is that a class of inflationary models, such as the hilltop-quartic model, results in a better agreement with the Planck data in the presence of an additional GNMDC term. Further, we compare the GNMDC model with the data using CosmoMC and find that these models provide a considerable improvement over the best-fit reference ΛCDM model with a featureless, power law, primordial spectrum.

12:48 to 12:51 Rashmi Sagar (IIT Indore, India) A Deep Look into the Epoch of Reionisation: uGMRT Band-2 Deep Field Observations

This study utilizes the ELAIS N1 deep field with a 24-hour observation time and 120 MHz bandwidth to achieve a central background RMS noise of 233 μJy/beam and a resolution of approximately 11.5 arcseconds. The radio catalog contains 1232 sources with a flux density greater than 1.1 mJy, and their flux density accuracy and positional offset are cross-matched and compared to other radio catalogs to ensure precision. Additionally, the normalized source count derived from the radio catalog generated in this study agrees well with the source count in previous catalogs of various fields and the LOFAR catalog of the same field. Moreover, this study compares two techniques for estimating the statistical fluctuations in galactic diffuse synchrotron emission over a wide frequency range of 120-250 MHz using power spectrum estimators: Image-based and Visibility-based estimators. The Image-based estimator is found to be more effective for studying the diffuse emission in our galaxy at a lower frequency. This study provides valuable insights into the EoR/CD and demonstrates the improved proficiency of the uGMRT in band-2.

12:51 to 12:54 Ankit Kumar (IIA, India) Formation of bulgeless galaxies in Illustris TNG50 cosmological simulation

The cosmological simulations of structure formation can now resolve galaxies at a few tens of parsec scale with sophisticated gas physics and feedback processes, thanks to the development of computing resources. In this workshop, I will discuss the challenges of bulgeless galaxy formation in the cosmological context and present our recent results on the formation of bulgeless galaxies in the state-of-art cosmological simulation, Illustris TNG50. We selected all the Illustris TNG50 galaxies having a total mass greater than 10^9 M⊙ and developed an automated pipeline to calculate various physical parameters and perform the photometric decomposition of galaxies. We identified a catalogue of bulgeless galaxies based on the goodness of the fitting. Using different comparisons with observations, we confirm that the TNG galaxy formation model is capable of producing observed characteristics of bulgeless galaxies in the low redshift Universe.

14:45 to 15:15 Yen-ting Lin (ASIAA, Taiwan) Application of Constrained Simulations in Understanding Galaxy-Halo Connection : the case for Halo Assembly Bias"

The halo assembly bias, a phenomenon referring to dependencies of the large-scale bias of a dark matter halo other than its mass, is a fundamental property of the standard cosmological model. By utilizing a constrained simulation that faithfully reproduces the observed structures larger than 2 Mpc in the local universe, for a sample of 634 massive clusters at z<0.12, we found their counterpart halos in the simulation and used the mass growth history of the matched halos to estimate the formation time of the observed clusters. This allowed us to construct a pair of early- and late-forming clusters, with a similar mass as measured via weak gravitational lensing, and large-scale biases differing at 3 sigma level, suggestive of the signature of assembly bias, which is further corroborated by the properties of cluster galaxies.  Our study paves a way to further detect assembly bias based on cluster samples constructed purely on observed quantities.

15:15 to 15:45 Biswajit Pandey (VBU, India) Do galaxies know about their large-scale environment?

I will discuss about an information theoretic framework to investigate the influence of large-scale environment on galaxy properties. We apply this to the data from the SDSS and find a non-zero mutual information (MI) between morphology and environment that decreases with increasing length-scales but persists throughout the entire length-scales probed. It is important to assess the statistical significance of these MI if any. We randomize the morphological information of the SDSS galaxies without affecting their spatial distribution and compare the MI in the original and randomized data sets. We also divide the galaxy distribution into smaller sub-cubes and randomly shuffle them many times keeping the morphological information of galaxies intact. The MI in the original SDSS data and its shuffled realizations are compared for different shuffling lengths. We find that a small but statistically significant (at 99.9 per cent confidence level) MI between morphology and environment exists up to the entire length-scale probed. This non-zero MI may be a product of the correlations of environment with the galaxy mass. We test this possibility by conditioning the stellar mass and still find a non-zero conditional mutual information (CMI). We compare our results with three different semi-analytic models implemented on the Millennium simulation and discuss the implications.

15:45 to 16:15 Andrea Maccio (NYU Abu Dhabi) Do We Need to go Beyond Cold Dark Matter?

Our current understanding of  galaxy formation is based on the presence of an elusive matter component: the Cold Dark Matter (CDM). This simple  model has ben challenged many times in the past decade, mainly by galaxy observations on small scales: from the abundance of satellites, to the distribution of dark matter within galaxies, and more recently by the discovery of galaxies "without" dark matter.In my talk I will first revise all these claims with the help of cosmological numerical simulations of galaxy formation from the NIHAO project. I will then discuss whether there is indeed an observationally motivated need to abandon Cold Dark Matter and move beyond such a simple model. 

17:00 to 18:00 -- Topical Discussion
Friday, 12 May 2023
Time Speaker Title Resources
09:30 to 10:00 Gilbert Holder (UIUC, USA) Cosmology with CMB Beyond the Power Spectrum

CMB measurements have been of tremendous utility for learning about cosmology. With current and upcoming experiments this will continue, as polarization measurements will provide a large amount of new information about the primary CMB, including the search for gravitational waves from inflation. At the same time, this new generation is starting to measure higher-order effects in the CMB, with the largest such one being the gravitational lensing of the CMB by lower redshift large scale structure. Other effects include fluctuations in the optical depth to Thomson scattering (“patchy tau”) as well as secondary fluctuations coming from moving electrons (the Sunyaev-Zeldovich effects). I will discuss these effects, using the South Pole Telescope as an example for what is possible with current and upcoming measurements.

10:00 to 10:30 Shivam Pandey (NYU, USA) Understanding Matter Distribution with the Sunyaev Zel'dovich Effect

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 ~ 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.

10:30 to 11:00 Sergey Koposov (University of Edinburgh, UK) Streams as Probes of Dark Matter Distribution in the Milky Way

One of the features directly predicted by the hierarchical galaxy formation are the stellar streams -- linear structures  produced by the disruption of accreted dwarf galaxies and stars clusters. The streams are valuable tracers of the Galactic potential as they approximately trace orbits in the potential. In last few years, the number of known streams have drastically increased. Using sophisticated statistical techniques we have been able to map these stream trajectories, motions and stellar densities. These analyses reveal a complicated picture where streams shows signs of many perturbations ranging from perturbations by low mass dark matter halos to large perturbation from the Magellanic Clouds and help us understand the distribution of dark matter in the Milky Way.

11:45 to 12:00 Manush Manju (TIFR, India) On the reliability of cusp-vs-cores and halo parameters in dark matter halos
12:00 to 12:15 Premvijay Velmani (IUCAA, India) The quasi-adiabatic relaxation of haloes in the IllustrisTNG and EAGLE cosmological simulations

Formation of a galaxy and subsequent astrophysical processes are known to affect the dark matter content of its host halo. I will talk based on my recent publication. In that work, we study the response of the dark matter for haloes spanning over four orders of magnitude using hydrodynamical cosmological simulation suites IllustrisTNG and EAGLE. In particular, we characterize the change in spherically averaged distribution using quasi-adiabatic relaxation framework, producing simple fitting functions that accurately capture the response in dwarf scale to cluster scale haloes all the way from their virial boundary to inner region that is just one-hundredth of their virial radii. We show that commonly employed schemes, which consider the relative change in radius $r f/r i-1$ of a spherical dark matter shell to be a function of only the relative change in its total enclosed mass $M i/M f-1$, do not accurately describe the measured response of most haloes in IllustrisTNG and EAGLE. Instead, letting $r f/r i$ have an additional dependence on the halo-centric radius allows us to isolate the response of the dark matter halo that is relatively more universal across mass scales and also between IllustrisTNG and EAGLE. We also account for a previously unmodelled effect, likely driven by feedback-related outflows, in which shells having $r f/r i\simeq1$ (i.e., no relaxation) have $M i/M f$ significantly different from unity. Our results immediately apply to several semi-analytical tools for modelling galactic and large-scale structure (baryonification schemes, rotation curves, total mass profiles, etc.). We also study how this response depends on different halo and galaxy properties, finding that it is primarily related to the halo concentration and the current active star formation rate. We discuss how these results can be extended to build a deeper physical understanding of the connection between dark matter and baryons at small scales.

12:15 to 12:30 Anoma Ganguly (TIFR, India) EDGES of the Dark Forest: A new absorption window into the composite dark matter and large-scale structure
12:30 to 12:45 Aritra Kumar Gon (TIFR, India) Probing reionisation through secondary CMB anisotropies: E and B modes from polarised kinetic Sunyaev Zeldovich effect.

We estimate the secondary E and B mode polarisation of the cosmic microwave background (CMB) originating from the transverse peculiar velocity of free electrons during the reionisation era. Interestingly, apart from having a blackbody part, there is a y-type (SZ) distortion in the frequency spectrum. This makes it distinguishable from primordial polarisation as well as other secondary sources, such as gravitational lensing and paves a way to beat the cosmic variance. Furthermore, it is also differentiable from other y-type signals such as from the thermal SZ effect as it involves polarised radiation. We show that this signal is sensitive to the reionisation optical depth as well as duration. Future detection of this signal can serve as a new probe to constrain different cosmological parameters related to reionisation and structure formation.

12:45 to 13:00 Nabendu Kumar Khan (TIFR, India) Accuracy of the small-scale structure of the Lyman-alpha forest in cosmological hydrodynamical simulations

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.

14:45 to 15:15 Alexandre Refregier (ETH Zürich, Switzerland) Cosmological HI Intensity Mapping with HIRAX
15:15 to 15:45 Hamsa Padmanabhan (Université de Genève, Switzerland) Getting the most out of cosmological surveys: synergies in intensity mapping

Mapping the Universe in the redshifted 21 cm line of atomic hydrogen offers tantalizing prospects to understand the first stars and galaxies. Accessing 21 cm radiation out to the Cosmic Dawn — about 300 million years after the Big Bang — promises at least five orders of magnitude more information than presently available from galaxy surveys and the Cosmic Microwave Background (CMB).  However, probing atomic hydrogen in the earliest Universe is extremely challenging, since it needs to be extracted from under the much stronger foreground emission from our Galaxy (about 10000 times the signal) and instrumental systematics which dominate the sources of error. One promising way to mitigate these challenges is by using a different tracer of structure in the Universe, in synergy with the 21 cm observations.  In the next couple of years, exciting ground-based and space-based experiments operating in sub-millimetre wavelengths, like the Fred Young Submillimetre Telescope (FYST) and the balloon-borne Experiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM),  will map the evolution of ionized carbon and oxygen in the early Universe. I describe how the cross-correlation of 21 cm and sub-millimetre surveys promises significant advantages to the science goals of both experiments. It eliminates the foreground and systematic challenges in the 21 cm mapping as well as the interloper contaminations in the sub-millimetre regime. In so doing, it leads to a much larger significance of detection of the tiny signal from the first stars, giving us key insights into the Cosmic Dawn.

15:45 to 16:15 Suman Majumdar (IIT Indore, India) Cosmic Dawn through 21 cm Bispectrum

The radiations from the first luminous sources drive the fluctuations in the 21-cm signal originating from the neutral hydrogen in the intergalactic medium (IGM) at Cosmic Dawn (CD). The astrophysical processes which are dominant in the IGM during this period are Lyα coupling, X-ray heating and photoionization. These processes together make the CD 21-cm signal highly non-Gaussian. The impact of these processes on the 21-cm signal and its non-Gaussianity vary depending on the properties of these first sources of light. One of the observable statistic that can be estimated from the radio interferometric observations of the CD and can quantify this non-Gaussianity present in the signal is the 21-cm bispectrum. In this talk, I will discuss how our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum combinedly provide the best measure of the signal fluctuations and its non-Gaussianity compared to the power spectrum. I will also show that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant IGM processes at different cosmic times. In the final segment of my talk I will show how with the help of an Artificial Neural Network-based emulator of the 21-cm bispectrum and power spectrum, we have demonstrated that the bispectrum can provide tighter constraints on the CD parameters compared to the power spectrum.

17:00 to 18:00 -- Topical Discussion