RESEARCH Over the past two decades, the large-volume of high-precision astronomical data has transformed cosmology into a data-driven, precision science. The so-called ΛCDM concordance model, which describes the origin, composition and evolution of the Universe on large scales with a handful of parameters, has emerged as the “standard model” of cosmology and its parameters are constrained at the percent-level accuracy by the current cosmological data. However, key fundamental questions remain open. What is the origin of structure in the universe? What is the nature of the major components of the energy density of the universe, i.e. dark matter and dark energy? What are the properties of the cosmic neutrinos? The unprecedented volume and precision of the cosmological data in the coming years, in particular those probing the large-scale structure (LSS) of the universe, are expected to significantly improve our understanding of these key questions. Having a broad interest in theoretical cosmology and astrophysics, my research has been largely centred around the physics of the early universe and the origin of the cosmic structure. While my scientific career began investigating theoretical aspects of inflation (as well as the viability of some alternative models) and their imprints on the cosmic microwave background (CMB), in the last few years my research has been focused mostly on using the LSS as a tracer of the underlying dark matter distribution to learn about fundamental physics. I have also studied to a more limited extent other topics, such as the epoch of reionization and its impact on the cosmological inference from the CMB, higher-order statistics of the CMB temperature and polarization anisotropies, and abundance and clustering of primordial black holes. The LSS can be probed using several observables, which provide complementary views of the universe across cosmic times. While galaxy surveys directly map the LSS by measuring shape, spatial position and redshift of individual galaxies and quasars, intensity mapping surveys provide the dimensional map of the LSS by measuring the integrated emission from the intergalactic medium and the galaxies. On the other hand, weak gravitational lensing of the CMB maps the entire distribution of matter indirectly via its impact on the CMB photons. In the last few years, I have worked extensively on using the clustering statistics of galaxies to constrain early universe. In particular, I have studied various aspects of extracting cosmological information from measurements of higher-order clustering statistics of galaxies. More recently, I also became interest in studying the prospects of intensity mapping surveys to probe cosmology broadly and the early universe specifically. Looking ahead, I am also interested in studying the synergies between various probes of the LSS and how they can be used to enhance the information content of each individual probe. Here are more detailed description of some of my most recent works, that can be summarized in three categories: 1. Cosmology with Higher-order Clustering Statistics The volume and precision of data from upcoming galaxy surveys will finally make it possible to use the higher-order statistics to perform precision tests of cosmology. I have investigated some of the most crucial ingredients for accurate theoretical modelling of the galaxy bispectrum, and its covariance, critical to extracting the imprints of primordial non-Gaussianity and modification to gravity. This included quantifying the impact of the assumptions of galaxy bias, investigating the impact of general relativistic projection effects, assessing the performance of various theoretical models for the matter bispectrum in the context of modified gravity, and computing for the first time the super- sample covariance of the galaxy bispectrum. Additionally, recently I defined a new estimator (referred to as skew-weighted cross-spectra) for the three-point statistics in Fourier-space, which encode information of the bispectrum and is computationally more efficient that the full bispectrum. 2. Large-scale Structure as a Cosmological Collider Primordial non-Gaussianity provides an invaluable window into the physics of the very-early universe, at energy scales beyond the reach of any particle collider. In a series of papers I investigated the prospects of constraining the presence of new particles with nonzero spin during inflation via their imprints on clustering statistics of galaxies. I showed that combining the measurements of the power spectrum from two populations of galaxies in photometric surveys like LSST, can potentially constrain massive particles with spin equal to one, and the measurement of the galaxy bispectrum from spectroscopic surveys like EUCLID can potentially detect the presence of particles with higher spins and set constraints on their masses if they are massive. 3. Line Intensity Mapping as a Novel Probe of Cosmology Intensity mapping is an emerging technique in which measurements of spatial fluctuations in the intensity of molecular and atomic lines and the frequency of each line provide a 3-dimensional map of the structure of the universe, over the spatial scales and redshifts largely inaccessible to galaxy surveys. In two consecutive papers, I investigated for the first time the potential of intensity mapping with rotational lines of carbon monoxide (CO) and fine transition line of ionized carbon (CII), as a probe of primordial non-Gaussianity. I investigated how the combined observations of CO and CII, as well as their synergies with upcoming galaxy surveys improve the constraints from a single line. In an extensive analysis, I investigated the requirements for optimal survey design that is capable of constraining PNG at the level that allows distinguishing between various inflation models. |