The following gives an overall view of my research directions. Each bold underlined item takes you to the details of my research activities and publications in that area. Here is a summary of my research directions and future goals.  

The last few decades have witnessed significant progress in the field of observational cosmology. The study of hydrogen -- the most abundant element in the universe -- at the earliest times, assures unprecedented vistas into cosmology and astrophysics. The epoch of Cosmic Dawn, when the first stars and galaxies were born -- often seen as the 'final frontier' of research in observational cosmology today -- is within easy reach of future ground- and space-based facilities. Intensity mapping (a technique in which large cosmological volumes can be mapped without the need to resolve individual objects) has emerged as a novel, complementary tool to extract exciting physics and astrophysics from cosmological observations. Multi-messenger studies enabled by gravitational waves promise fascinating insights into the nature of the very first galaxies and black holes. An accurate understanding of the astrophysics allows to exploit the tremendous potential of next generation surveys to constrain theories of gravity and fundamental physics on the grandest scales. We are thus on the threshold of the richest available cosmological dataset in the coming years, facilitating the most precise constraints on theories of Fundamental Physics.

My research uses a unique combination of analytical, statistical and simulation tools suited to extract the maximum information from the multi-messenger data from the Cosmic Dawn to the present day.

Lensing of the Cosmic Microwave Background : At the earliest epochs, radiation decoupled from the neutral baryonic matter in the first major phase transition of the observable universe known as the epoch of recombination, which occurred about 300,000 years after the Big Bang (redshifts about 1100). This primordial radiation is observable today as the cosmic microwave background (CMB). Analyzing the lensing of the CMB power spectra provides exciting insights into the intervening universe between us and the epoch of recombination.

The Epoch of Reionization : Most of the baryonic material in the universe was (and even now, is) in the diffuse component between galaxies, known as the intergalactic medium (IGM). The IGM was almost fully composed of neutral hydrogen gas at the end of the epoch of recombination, with small (about 10%) amounts of neutral helium and remained so (a period known as the dark ages of the universe) until the first stars and galaxies formed about a few hundred million years later. These luminous sources contributed ionizing photons to complete the second major phase transition in the observable universe known as cosmic reionization. Today, we see an almost completely ionized IGM. Investigating this transition hence offers valuable clues towards the physics of the baryonic matter in the universe and the first stars and galaxies.

After Reionization : At later epochs (about a few billion years after the Big Bang; redshifts 1.5 to 4), the 21-cm line emission from neutral hydrogen (HI) is expected to be a powerful probe of the gas content of the universe. Mapping the distribution of HI has been carried out using a number of approaches : galaxy surveys, 21-cm intensity mapping experiments without resolving the individual galaxies, and higher-redshift Damped Lyman Alpha (DLA) observations. These studies enable one to constrain the neutral gas evolution in the post-reionization universe. Hence, the study of this period forms an important probe of the later stages of evolution of the universe.

The period between redshifts 2-4 is also important from the point of view of stellar and galaxy evolution. It is during this time that star formation in the universe reached its peak. Several important tracers, like the carbon monoxide (CO) molecule and ionized carbon (CII) can be used to constrain the distribution of star formation. This is also especially important to constrain reionization, since the ALMA telescope has now provided us with individual galaxy detections in these tracers at very early times (redshifts 6-9), which provide essential synergies with next generation 21 cm experiments.

The cosmological constant problem: As an aside from doing more observationally-oriented cosmology, I also made an excursion into a fundamental physics problem during my research! The current universe (the last 6 billion years of cosmic time) is characterized by the domination of the cosmological constant, which leads to the late-time accelerating phase of the universe. The extremely small value of the cosmological constant is a major puzzle in current cosmology (known as the cosmological constant problem). Resolving the cosmological constant problem is one of the important challenges of present-day cosmology.