Cosmological Tensions: The observable universe is well-described by the Standard Model of Cosmology, known as the Lambda-Cold Dark Matter (ΛCDM) model. This simple six-parameter framework is based on Einstein’s General Theory of Relativity. However, in the current era of precision cosmology, the ΛCDM model faces significant challenges from emerging tensions and anomalies. One of the most prominent issues is the Hubble tension—a discrepancy between the Hubble constant measurements derived from the cosmic microwave background (CMB) assuming the standard ΛCDM model and those obtained from local distance ladder techniques at lower redshifts. Another crucial challenge is the S8 tension, which highlights the inconsistency in the parameter S8 (quantifying matter clustering at late times) between CMB predictions under ΛCDM and weak lensing measurements. Beyond these tensions, several anomalies persist in the CMB data from Planck, including the low quadrupole, lensing anomaly, and the cold spot. Additionally, baryon acoustic oscillation (BAO) measurements from eBOSS around Redshift 2.3 also reveal discrepancies that remain unexplained by the standard model. Recent observations by the James Webb Space Telescope of massive galaxies at remarkably early epochs have further posed challenges to our understanding of structure formation within the ΛCDM framework. My research interest lies in understanding these tensions and anomalies and exploring modifications to the standard model that could potentially resolve them while remaining consistent with a broad range of observational data.
Dark Energy: The present universe is undergoing an accelerated expansion driven by a less-known component called dark energy, which exerts negative pressure, counteracting gravity, and accounts for about 70% of the total energy budget. In the standard cosmological model, dark energy is represented by the cosmological constant (Λ), with a constant energy density throughout cosmic evolution. However, Λ faces significant theoretical challenges, particularly the fine-tuning problem, where its extremely small value lacks explanation within the framework of quantum field theory. In addition to these theoretical issues, emerging cosmological tensions, such as the Hubble and S8 tensions, are increasingly challenging our understanding of dark energy. This has led to growing interest in dynamical dark energy (DDE) as a compelling alternative to Λ. DDE models, especially those exhibiting phantom behaviour—where the equation of state drops below -1 and the energy density increases with expansion—have gained significant attention for their potential to alleviate these tensions. Furthermore, ideas like the negative values of dark energy density at early times, clustering dark energy, interacting dark energy etc, also have intriguing, non-trivial effects on the dynamics. I am interested in exploring the nature of dark energy, particularly the implications of dynamical scenarios on both the background evolution of the universe and the growth of cosmic structures. Notably, the first data release from the Dark Energy Spectroscopic Instrument (DESI) has indicated a preference for the dynamical nature of dark energy, further challenging our current understanding and opening new avenues for exploration.
Modified Gravity: Einstein's General Theory of Relativity (GR) is a highly successful theory of gravity, formulated in 3+1 dimensions with the metric tensor as its fundamental variable, leading to second-order field equations. It forms the foundation of the Standard ΛCDM model and has been remarkably consistent with laboratory experiments and solar system tests. Yet, despite its successes, our grasp of gravity on cosmological scales and in the extreme environments around compact objects remains incomplete. Further, GR's explanation of cosmological phenomena, such as the accelerated expansion of the universe, relies on the cosmological constant (Λ), which faces significant theoretical challenges. Additionally, the emergence of cosmological tensions complicates our understanding of dark energy (Λ) within GR, prompting interest in alternative theories for a more comprehensive framework. One simple approach to extending GR involves adding a scalar field and relaxing Lovelock’s theorem. Horndeski gravity, a generalized scalar-tensor theory with second-order field equations, offers a versatile framework that includes subclasses like non-minimal coupling, galileons, and k-essence, capable of describing various cosmological epochs. These theories provide alternatives to GR, influencing cosmic structure growth and introducing testable deviations. My research focuses on exploring these extensions, using observational data to examine their implications for cosmological challenges, such as accelerated expansion, cosmological tensions, and the nature of gravity on large scales and in strong-field regimes.
Cosmological Data Analysis: Modern cosmology has entered the precision era, driven by significant advancements in observational techniques for both ground-based and space-based probes. These advancements have resulted in a vast repository of cosmological data, with even more expected from upcoming next-generation surveys. In this context, data analysis has become an indispensable part of cosmological research, enabling scientists to probe the universe with unprecedented precision and depth. Data analysis in cosmology spans a wide range of tasks, from extracting meaningful cosmological information from raw data to employing statistical techniques for parameter estimation and model comparison. During my PhD projects, I have worked on utilizing Bayesian techniques to extract information about the parameters for various cosmological models. This involves determining best-fit values, assessing uncertainties and parameter degeneracies, and comparing models using data from CMB, BAO, and Supernova observations. I am keen to further develop my expertise in these statistical tools, which are essential for cosmological modelling. Additionally, I am interested in exploring machine learning techniques to interpret critical cosmological information from data in a model-independent manner, along with other applications like cosmological parameter estimation.
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