ALTERING THE PRE-RECOMBINATION EXPANSION HISTORY WITH HIDDEN SECTOR PHYSICS

Although the standard model of cosmology, ΛCDM, accurately predicts a wide range of cosmological observables, discrepancies between early- and late-universe probes of the present-day expansion rate (H0) and small-scale clustering of matter point to the possibility that the ΛCDM model is incomplete. The maximum distance that sound waves traveled in the early universe, known as the sound horizon, subtends a precisely measured angle on the surface of the cosmic microwave background (CMB) that is used to infer a value of H0. Altering the pre-recombination expansion can reduce the size of the sound horizon and raise the CMB-inferred value of H0, reconciling CMB observations with direct measurements of the expansion rate from local probes. This dissertation investigates the observational signatures of several extensions to ΛCDM that introduce additional energy components to the early universe via a hidden sector - a theoretical set of particles that interact only weakly with the Standard Model. We first consider a hidden-sector particle that injects radiation into the early universe through its decay. We determine that current cosmological observations significantly constrain nonrelativistic particles that decay after Big Bang nucleosynthesis: the density of the decaying particles cannot exceed 3% of the total energy density of the universe. Next, we investigate the observational signatures of an axion-like scalar field that temporarily dominates the universe’s energy density well before matter-radiation equality. Such a period of scalar domination can enhance the growth of structure on sub-galactic scales without significantly altering the CMB. It has been proposed that present-day dark energy is sourced by a scalar field and that the H0 tension can be solved by the presence of a subdominant scalar field that becomes dynamical near matter-radiation equality. The detection of these unique signatures on sub-galactic scales would provide additional evidence for a collection of cosmological scalar fields, which is predicted by string theory.