CHALLENGES AND CURRENT QUESTIONS IN COSMOLOGY

Abstract

The last several decades proved to be a golden age for cosmology. From the observational front, we have significant breakthroughs including the discovery of Cosmic Microwave Background (CMB), detection of late time cosmic acceleration, and the mapping of Large Scale Structure (LSS) formation. From the theoretical front, a robust Λ-dominated Cold Dark Matter (ΛCDM) model was developed to explain those phenomena with remarkable precision. However, there remain considerable challenges and questions to address within each front. On the one hand, there are persistent tensions in different observational datasets such as σ8 tension arising from the conflicting measurements of the amount of matter clustering in the universe. On the other hand, theoretical frameworks still grapple in explaining the true nature of the dark sectors of ΛCDM model: dark energy and dark matter. This thesis consists of three separate studies that aim to address these issues by investigating the shared mechanism of two cosmic accelerations, the dark matter properties, and the σ8 tension.

In the first study, we explore a quintessential inflation within an α-attractor formalism that connects inflation with dark energy. Cosmic acceleration in the early universe, known as inflation, is essential to explain the observed properties of CMB and predicts the existence of primordial gravitational waves. Cosmic acceleration in the late universe, driven by dark energy, is again essential for understanding the universe’s geometry and structure formation. We demonstrate a robust relation between the present value of dark energy equation of state w0 and the strength of the primordial gravitational waves r, valid for a broad range of initial conditions. With the chosen α-attractor potential, we identify the scalar field as the thawing field thus explaining the large time gap between the two cosmic accelerations and revealing a tight relation between the observables of dark energy and inflation. Thus, we established a medium where a likelihood of observing a dynamical dark energy increases at the expense of detecting primordial gravitational wave and vice versa.

The second study examines deviations of dark matter properties from the standard model. ΛCDM assumes the dark matter to cluster with the same gravitational strength as baryons and evolve with the pressureless equation of state. However, any deviations in both characteristics, if observed by future cosmic growth measurements, may shed light on the origin of the longstanding tensions. We take a model-independent approach by binning the deviations in redshift and computing the constraints by three different cosmic surveys, which combined, can cover z = [0, 4]. The analysis shows 3 − 14% and 3 − 23% level constraints on the clustering and equation of state deviations, respectively. In obtaining these results, we neglected the general sound speed along with the viscous sound speed and leave these effects for future work.

The third study investigates the implication of a modified gravity model on the structure growth, in particular, on σ8 tension. With this model, we derived the modified background quantities and perturbation equation. Using redshift space distortion data, we performed MCMC analysis, successfully demonstrating the decrease in the σ8 tension from 3σ, as observed between the Planck mission and galaxy redshift measurements, to 1σ. It must be stressed that any proposal to alleviate one tension should not aggravate the other. However, we left the model implications on the Hubble tension for future work.

This thesis explores theoretical and observational challenges by offering new insights into the cosmic acceleration, dark matter properties, and persistent σ8 tension. Together, we believe these studies may contribute a glimpse of comprehension to the mysteries of our universe and possibly point towards new research directions.