Physics of Universe and Galaxies

Research

Understanding Galaxy formation and the co-evolution of black-holes and galaxies in the background of Large Scale Structure (LSS) is one of the key goals of contemporary cosmology and a major thrust area for all LSS experiments. My research focuses on using modern extragalactic surveys to study the joint formation of galaxies and super-massive black holes, together with probing the fundamental ingredients of cosmology: dark matter and dark energy. In particular, I focus on establishing the physics that links luminous galaxies and Active Galactic Nuclei (AGN) to the underlying cosmic web of dark matter. Using my extensive expertise in working with galaxy surveys and data on the Cosmic Microwave Background (CMB), I investigate where in the cosmic web the active black holes are found, the environmental effects of the local dark-matter distribution on star formation, and how this may influence the cosmological parameters derived from such galaxy samples. I use these precise connections between luminous matter and underlying dark matter to probe the evolution of dark energy, together with relativistic effects (e.g gravitational redshift), primordial non-Gaussianity, hierarchy and masses of neutrinos along with constraining the role of black holes in structure formation. More specifically, I will address three main problems which will be key to fundamental physics and cosmology in the coming decades:

Cosmological statistics: How can we model, with sub-percent accuracy, the impact of complex astrophysics and observational inhomogeneities, while using galaxies/AGN to trace the clustering properties of dark matter? see A
Galaxy Physics: What are the interlinked processes that terminate star formation across cosmic time, and how do these affect our ability to study fundamental physics from galaxy surveys? see A, B
AGN Physics: What are the key interactions of black holes with their host galaxy and dark matter halo, and how do these affect the activity of the central AGN and its spectral energy distribution? Can these introduce signatures similar to primordial non-Gaussianity? see A, B

I will study these questions using new large galaxy redshift surveys, employing my expertise in statistical analyses and constructing realistic mock datasets that include the relevant theoretical processes. This will allow me to address following two outstanding challenges in the extragalactic cosmology:

69% of the Universe consist of dark energy and 26% is dark matter, both of which cannot be explained with standard model physics but are the main drivers of the formation and evolution of the Universe.
The physics of galaxies and Active Galactic Nuclei (AGN) formation is not very well understood.

In summary, I will use the constraining power of large galaxy surveys to understand galaxy formation processes using novel statistical frameworks. I will then use this deeper understanding of galaxy formation processes to create mock observations of large scale surveys. This will be an essential part of the cosmological pipeline to calibrate theoretical template and estimate astrophysical systematics in cosmological inferences.

In the following tabs you can find more details of my research activites with relevant references

Relativistic effect in Large Scale Structure

We studied several relativistic effects like gravitational redshift, special relativistic beaming, transverse Doppler redshift, light cone effect, luminosity distance perturbation and other second order effects, which can affect large scale structure as observed by galaxies, using simple phenomenology modelling with tools such as N-body simulation, general relativistic perturbation theory and halo models. We have studied relative amplitude of these effects, detected a signal in SDSS DR12 data and compared linear vs nonlinear predictions. You can find simple descriptions of these things in here.

This project resulted in following papers:

Combining Weak Gravitational Lensing and RSD with LSS (EG)

I have been involved in the study of a unique combination of gravitational lensing with large scale structure and redshift space distortions (RSD) to measure a quantity called EG. This has some unique features. For example, it is independent of galaxy bias and amplitude of matter fluctuation. This also has a very simple theoretical prediction and it is easy to extend this prediction for a wide range of modified gravity theories. This provides a unique test of General Relativity at Large Scale. I have published several papers with my collaborators related to theoretical prediction and measurement of EG. Following is the list of papers I have on the above topic.

The first constraints on many of the Hordenski's model parameters with RSD

We looked at an extension to LCDM by including dark energy and modified gravity. We focused on Hordensiki theories for modified gravity with linear perturbation regime and used the RSD data from 6 Large Scale Structure surveys to provide some of the first constraints on a few model classes. We also improved latest constraints at that time on many of the model parameters. We also clarified some of the inconsistencies while constraining such models in the literature and pointed out how such data can also provide an indirect constraint on redshift of reionization. I have also contributed to a similar analysis focusing on different aspects of such an extension. The following are papers coming out of these contributions:

Redshift Space Distortion in Large Scale Structure

I have led and contributed to several of the analyses on Large Scale Structure focusing on various aspects of modelling galaxy velocity fields. These studies also contributed to the SDSS-III final cosmological results and folded into SDSS-III latest precision cosmology constraints. You can find further description about this effect and how this helps us constrain cosmology over here. The papers from these works are as follows:

The current best constraint on neutrino mass comes from cosmological data sets. I have been pondering about some aspects of neutrino mass's effect on large scale structure. Some animations and descriptions are available here. https://sites.google.com/site/salamcosmology/research/neutrino-mass