The Early Universe: New Aspects of Cuscuton Theory, Modified Dispersion Relations and Pulsar Constraints on Vacuum Noise

Abstract

The modern cosmology started blossoming in the early 1900s, with the theoretical framework provided by Einstein's general relativity. Soon after that, with observational discoveries such as expansion of cosmos by Hubble or later the CMB (Cosmic Microwave Background), many hypotheses and postulations were made to reconcile the observational data and the theoretical frameworks, and to answer open questions in the field. LCDM model with inflation is a model which fits the observational data very well, and is one of the most widely accepted theories. However, this model still leaves us with many puzzles yet to be resolved. In this thesis, the theoretical frameworks and the mathematical tools to understand the modern cosmology will be introduced, along with a brief review of inflation. From there, we will explore three different approaches to address some of these unanswered questions. Furthermore, we will give an example of how observational data can constraint theories of early universe.