Non-parametric Reconstruction of Some Cosmological Parameters

Mukherjee, Purba (2022) Non-parametric Reconstruction of Some Cosmological Parameters. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Purba Mukherjee (14IP011)) 14IP011.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

The present thesis is devoted to the non-parametric reconstruction of some cosmological parameters using diverse observational data sets adopting the Gaussian Process regression. The Universe is assumed to be spatially homogeneous and isotropic, thus described by the FLRW metric. An assessment of the cosmic distance-duality relation, reconstruction of the kinematical quantities and exploring the possibility of a non-gravitational interaction in the cosmic dark sector, between dark energy and dark matter, have been carried out. In case of chapters 2, 3 and 4 a spatially flat Universe has been assumed at the outset. We have relaxed this flatness assumption in chapter 5 and investigated the possible effect of a non-zero spatial curvature from the Planck 2018 survey. The first chapter provides a brief introduction to cosmology. In the second chapter, a reconstruction of the cosmic distance duality relation (CDDR) has been discussed. As cosmography is strongly dependent on the validity of CDDR, the reliability of CDDR is evaluated with the increasing quality and quantity of present observational data. Indication towards a non-violation of CDDR in the late-time Universe is obtained. In the third chapter, a non-parametric reconstruction of the cosmic deceleration and jerk parameters have been discussed. Reconstruction for the effective equation of state parameter has also been carried out for the modelindependent datasets. A fitting function for the reconstructed j as a polynomial in z, in the low redshift range 0 < z < 1, has been obtained. The fourth chapter is devoted to exploring the possibility of a nongravitational interaction between dark matter and the dark energy sector. A non-parametric reconstruction of interaction function Q in the cosmic dark sector has been attempted. Three cases for the dark energy EoS have been considered. These are the decaying vacuum energy L with w = -1, the wCDM model and the Chevallier-Polarski-Linder (CPL) parametrization of dark energy. An analytic expression for the reconstructed Q as a polynomial in z has been provided. The evolution of the dark matter and dark energy density parameters Wm and WD have also been checked in the presence of this interaction term. The thermodynamics considerations are also studied in the presence of this interacting scenario. In the fifth chapter, attempts have been made to revisit a non-parametric reconstruction of the cosmic deceleration parameter q using various combinations of recently updated background datasets, following an improved analysis. The growth rate measurements from the redshift-space distortions have been utilized to investigate the effect of matter perturbations. The reconstructed quantities mimic the LCDM behaviour in the very low redshift range. Results indicate that the LCDM model is well consistent and included at the 2s level in the domain of all the reconstructions. Finally, chapter six contains the concluding remarks and relevant discussion regarding the overall work presented in the thesis.

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