Author ORCID Identifier

https://orcid.org/0000-0002-9103-4657

Date of Award

Spring 4-20-2023

Document Type

Thesis (Ph.D.)

Department or Program

Physics and Astronomy

First Advisor

Robert Caldwell

Second Advisor

Marcelo Gleiser

Third Advisor

Devin Walker

Abstract

The Lambda-cold dark matter (LCDM) model has become the standard model of cosmology because of its ability to reproduce a vast array of cosmological observations, from the earliest moments of our Universe, to the current period of accelerated expansion, which it does with great accuracy. However, the success of this model only distracts from its inherent flaws and ambiguities. LCDM is purely phenomenological, providing no physical explanation for the nature of dark matter, responsible for the formation and evolution of large-scale structure, and giving an inconclusive explanation for dark energy, which drives the current period of accelerated expansion.

Furthermore, cracks in its observational grounding have begun to form. When LCDM is used to interpret recent high precision measurements, tensions appear between individual experiments: the inferred values of the current cosmic expansion rate H0 and the amplitude of cosmic density fluctuations S8 based on early universe measurements of the cosmic microwave background (CMB), are in disagreement with the values measured in the local universe, probed by supernovae, weak lensing, and galaxy surveys. These tensions, if not caused by unaccounted for systematics, suggest that the model we use to interpret early universe data may be incomplete.

This thesis collects works investigating alternative cosmologies and new analysis techniques which aim to explain these tensions and ambiguities in LCDM, and provide new probes of beyond the standard model physics. I present four main projects here: I developed an assisted quintessence model of early dark energy (EDE), linking the early and late epochs of cosmic acceleration, which provides a solution to the H0 tension and coincidence problem of dark energy; I investigated the role that microphysics plays on the EDE solution to the Hubble tension, and found that EDE with an anisotropic shear can solve both the Hubble and S8 tensions simultaneously; I developed a method of using line-intensity mapping to constrain beyond the standard model physics, which I used to forecast constraints on non-CDM models and non-Gaussianity; and I derived the cosmological perturbations and initial conditions for a dark energy model built from a three classical U(1) gauge fields coupled to a scalar field, which I will use to investigate the compatibility of a such a scenario with cosmological observations.

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