Cosmology

The study of origin, composition, evolution and fate of the Universe

Sukhdeep Singh Gill

I am a research scholar at the Department of Physics and Centre for Theoretical Studies, Indian Institute of Technology Kharagpur, India. I work in theoretical and computational cosmology, a vast field that beautifully interweaves physics and mathematics.

Broadly, my research focuses on understanding the higher-order correlations in the large-scale structure of the Universe, through galaxy clustering and HI 21cm fluctuations. I am also interested in constraining cosmological and galaxy formation parameters, and in developing simulations for cosmological studies. Currently, I am working under the supervision of Prof. Somnath Bharadwaj, investigating the bispectrum (a three-point correlation statistic) of cosmological fields such as the galaxy and HI distributions. I am also a member of 21-cm cosmology (Cosmic Dawn, EoR and Post-EoR) SKA-India consortium.

"I think nature’s imagination is so much greater than man’s, she’s never going to let us relax." R.P. Feynman

Research at a glance:

Statistics of Large-Scale Structure (LSS): Power Spectrum and Bispectrum
Galaxy Clustering
21cm Cosmology and Radio Astronomy
Epoch of Reionization and Post-Reionization Studies
Dark Energy and the Accelerated Expansion of the Universe
Cosmological Parameter Estimation from Survey Data

Currently, I am pursuing my PhD under the supervision of Prof. Somnath Bharadwaj (website). My research investigates higher-order statistics of cosmological fields, with a particular focus on the bispectrum of galaxy and HI distributions.

While two-point statistics, like the power spectrum, have long been the cornerstone of the LSS studies, higher-order statistics offer a richer source of cosmological information. By moving beyond the power spectrum, we can unlock deeper insights into the Universe's structure and evolution, maximizing the information gain from galaxy surveys and 21-cm observations.

The bispectrum, as the next higher-order statistic, is sensitive to non-Gaussian features in the underlying distribution. It plays a crucial role in understanding late-time cosmic evolution — where gravitational instability induces non-linearity — and in probing the very early Universe through constraints on primordial non-Gaussianity, offering a window into inflationary physics.

In my PhD, we have developed fast and efficient bispectrum estimators [1,,2]. A novel feature of our estimator is its ability to compute all non-zero multipole moments of the anisotropic redshift-space bispectrum [2]. Our methods are directly applicable to upcoming and ongoing galaxy surveys such as DESI, Euclid, and LSST. We have studied the bispectrum monopole of SDSS galaxies [3] and plan to extend the study to higher-order bispectrum multipoles.

Moreover, we have pioneered the development of an efficient estimator for the 21-cm HI angular bispectrum [4] and multifrequency angular bispectrum (MABS), capable of working directly on the radio-interferometric data from instruments like uGMRT, MWA, and SKA. In another study [5], we forecast the expected 21-cm Epoch of Reionization bispectrum using simulated signals, demonstrating its potential as a powerful diagnostic of the beginning and end phases of reionization.

"Somewhere, something incredible is waiting to be known". Carl Sagan

Evolution of the Universe, beginning with the singularity at the Big Bang and continuing to the formation of galaxies observed today (image: eso).

"What we know is a drop, What we do not know is an ocean". Sir Isaac Newton

Cosmic web observed by a typical galaxy survey. Each point here represents a galaxy. The clustering pattern of galaxies encodes lots of physics !! (image: 2dfgrs)

Murchison Widefield Array (MWA) telescope. It observes sky in radio frequencies. Of particular interest is redshifted 21 cm (or 1420 MHz) signal, emitted by neutral hydrogen in its ground state. This signal carries the valuable information from dark ages (redshift=1100) to post-reionization (z=0), making it a key target for future cosmological studies.

Simulation of the Epoch of Reionization (EoR, redshift=13-7), when the high energy photons from first luminous objects - such as stars, galaxies and quasars - began to 're'-ionize the neutral hydrogen in the intergalactic medium (IGM). This epoch marks the last phase transition of the Universe.

Depiction of Redshift Space Distortion (RSD). RSD is an observational effect that causes the apparent distribution of matter to appear squashed or elongated along the line of sight. It introduces a preferred direction in an otherwise isotropic Universe. As a result, statistical measures such as the power spectrum and bispectrum become anisotropic along the line of sight.

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