Testing new physics with precision observations

Abstract

There are persisting tensions in the Hubble constant and in the σ8 parameter that have yet to be resolved. Different models have been used throughout the years in an attempt to lessen these tensions and gain more knowledge about the Universe. ΛCDM is the theoretical framework that describes the origin, structure and evolution of the Universe. However, there are several tensions between the predictions of standard cosmology and the observations of various cosmological probes, such as the Hubble constant (H0) and the σ8 parameter. By investigating a reparametrisation of the ΛCDM, more information can be obtained on these tensions and new physics can be uncovered. The main objective of this research is to examine the viability of wCDM as a solution to the observed tensions within the standard cosmological model. The Cosmic Microwave Background (CMB) radiation, a powerful probe of the early universe, plays a central role in this study by analysing CMB anisotropies and temperature fluctuations in conjunction with high-precision data from Planck data. This project also takes the opportunity to examine the new Planck likelihood (Planck 2020) with the previous one (Planck 2018) and examine the new data of Dark Energy Spectroscopic Instrument (DESI) with the previously released data set of Baryon Acoustic Oscillations (BAO). The late-time data of Pantheon + SH0ES (SN+SH0ES) and Cosmic Chronometers (CC) data will also be used to serve as a way to add tighter constraints and, therefore, more accurate values with less uncertainties. In this project, the wCDM models showed that the newer datasets of Planck and BAO contained the models better than the previously released datasets. However, the wCDM model still favoured Planck 2018 and the older BAO data. Also, the tested models all had large uncertainties and degeneracies when only early-time data was taken, but the addition of the late-time data generally decreased the degeneracies and uncertainties. These key findings are discussed in further detail in Chapter 4