Theoretical Constraints on Phantom Dark Energy and Non-singular Bouncing Cosmology

Abstract

In general relativity, energy conditions can restrict the allowed geometries. One of the most prominent conditions is the classical null energy condition (NEC). However, quantum field theories can violate this condition which prompts the need for quantum null energy conditions. The smeared null energy condition (SNEC) is a proposed semilocal conjecture, that modifies NEC by averaging `energy density' along a null geodesic, with the averaging weight being a real, positive smearing function. The SNEC allows for `the null energy density' to be negative over short smearing scales. This can have insightful implications for phantom dark energy models and bouncing cosmologies by introducing bounds on dark energy equation-of-state parameters and an inequality between the duration of the bouncing phase and the growth rate of the Hubble parameter.

In the research presented in this thesis, we studied the application of the SNEC on flat FLRW cosmology with pressureless matter and dark energy as sources and showed for which regions of parameter space the SNEC is compatible with an evolving phantom dark energy. What makes it interesting is that it provides us with a theoretical framework to see where it is in agreement with the recent Dark Energy Spectroscopic Instrument (DESI) observation findings, which suggest the violation of the classical NEC. Furthermore, we applied the SNEC to very early universe nonsingular bouncing cosmology and specifically found a bound where these models in general and more specifically the cuscuton model agree with the SNEC.