Ph.D HBNI 2023

Acoustic peaks in the Cosmic Microwave Background (CMB) temperature spectrum as observed by the Planck satellite appear to be smoother than our expectation from the standard model lensing effect. This anomalous effect can be also mimicked by a spatially closed Universe with a very low value of Hubble constant that consequently aggravates the already existing discordance between cosmological observations. This thesis identifies a novel solution that removes these anomalies along with improved fit to Planck 2018 data compared to the standard model and at the same time alleviates existing tensions between different datasets in the estimation of cosmological parameters such as H 0 and S 8 . <br><br> We reconstruct a signature from the early Universe, a particular form of oscillation in the primordial spectrum of quantum fluctuations with a characteristic frequency, that solves all these anomalies. Interestingly, we find this form of the primordial spectrum resolves or substantially subsides, various tensions in the standard model of cosmology in fitting different observations, namely Planck CMB, clustering and weak lensing shear measurements from several large scale structure surveys, local measurements of Hubble constant, and recently estimated age of the Universe from globular clusters. We support our findings phenomenologically by proposing an analytical form of the primordial spectrum with similar features and demonstrate that it agrees remarkably well with various combinations of cosmological observations. <br><br> We support further our findings theoretically by identifying examples of single field inflationary trajectories beyond the slow-roll regime. A damped oscillation in the first Hubble flow function - or equivalently a feature in the potential - and the corresponding localized oscillations in the primordial power spectrum partially mimic the improvement in the fit of Planck data due to A lens or Ω K . Compared to the baseline model, this model can lead simultaneously to larger value of H 0 and a smaller value of S 8 , a trend which can be enhanced when the most recent SH0ES measurement for H 0 is combined with Planck and BK18 data. Large scale structure data and more precise CMB polarization measurements will further provide critical tests of this intermediate fast roll phase.