Class Year

2017

Document Type

Thesis

Honors Designation

High Honors

Department

Physics and Astronomy

Primary Advisor

Cosmin Ilie

Abstract

The thermal and expansion history of the universe between inflation and big bang nucleosynthesis is largely unconstrained. In standard cosmological theories, highly relativistic standard model particles dominate the universe’s energy density during this epoch. We consider a non-standard cosmology consisting of an era dominated by an oscillating scalar field that behaves similarly to matter. The growth of density perturbations is strongly enhanced in an early matter dominated era (EMDE), leading to potentially profound consequences regarding the dynamics of the early universe and the scales of the first bound dark matter structures. We investigate the applications and validity of the perfect and imperfect fluid approximations and find that the suppressing effects of frictional damping during kinetic decoupling are largely erased by the EMDE. Contrary to the standard cosmological model, we conclude that in an EMDE, free-streaming is the dominant mechanism for suppression in the cold dark matter transfer function. Thus, the free-streaming length is the significant lower bound on the size of current dark matter microhalos provided that kinetic decoupling occurs significantly before reheating. We present a remarkable closed-form analytic fit for the free-streaming length that depends on the ratio between the reheat temperature (TRH) and the lepton temperature at decoupling in standard cosmologies (Tkds).