PhD Dissertation- New theoretical ideas in cosmology driven by high precision observations.

Abstract: The standard ΛCDM cosmological was successful in accounting for the predictions of light elements in the Universe and anisotropies in the Cosmic Microwave Background (CMB).  However, the recent high precision data has indicated the presence of fundamental problems in the ΛCDM such as missing large angle correlation in CMB and the early appearance of supermassive black holes and galaxies. In this thesis, using the Hubble diagram of HII galaxies, angular diameter distance inferred from strong gravitational lenses, H(z) inferred from cosmic chronometer and angular diameter distance inferred from quasar cores, we show that Rh =ct an alternative FLRW cosmology with zero active mass condition is favored better over ΛCDM. In addition, using the linear perturbation theory based on interacting dark matter - dark energy model we show that the predicted halo mass function in Rh =ct universe is consistent with the observations throughout redshifts 10 > z > 4. Whereas the standard ΛCDM cosmological model is inconsistent with observations, in particular it under predicts  the number of halos by more than 4 orders of magnitude. Additionally, the structure formation in ΛCDM is strongly dependent on the inflationary mechanism to account for the generation of scale-invariant primordial fluctuations. The inflationary mechanism drives all the modes to exist and re-enter the horizon. In particular, the small-scale modes enter the horizon during radiation dominated epoch whereas large-scale modes enter the horizon during matter dominated epoch, thus providing a mechanism for different growth rates at different epochs. Whereas the Rh =ct universe doesn’t have such a complicated mechanism as the model does not have a horizon problem and does not incorporate inflation into its expansion history, so modes never exit the horizon. Moreover, growth of structures in Rh =ct doesn’t require different handling of small- and large-scale modes. The Rh =ct model with its simple structure formation mechanism fits the observations quite well, whereas ΛCDM with its complicated mechanism under predicts the number of halos by at least four orders of magnitude. All the tests thus far completed have shown support favoring Rh =ct better than over ΛCDM indicating Rh=ct universe is far more viable than ΛCDM cosmological model