Interacting dark sectors and cosmological tensions
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Abstract
In addition to the particles of the Standard Model, the Cosmological Standard Model (ΛCDM) contains a significant amount of dark matter in the form of Cold Dark Matter (CDM) and dark energy in the form of Einstein’s cosmological constant (Λ). ΛCDM has been well-tested and successfully predicts the large-scale distribution of matter in our universe (known as the large-scale structure, LSS), as well as the power spectrum of fluctuations in the temperature of the Cosmic Microwave Background (CMB) radiation, left over light from the Big Bang. Recent data, in both the LSS and the CMB, have revealed some tensions with the ΛCDM model, motivating possible extensions of ΛCDM that can reconcile the tensions. A possibility is to consider dark radiation, in addition to the radiation (the photons and neutrinos) that is contained in ΛCDM. Dark radiation may alleviate the current tension in the value of Hubble's constant (H0). The H0 tension results from a discrepancy between direct measurements of the expansion rate of the universe (via astronomical objects), and the value that is obtained from CMB measurements in the context of ΛCDM. In this dissertation, I show that a low-mass dark fermion that is not populated in the early universe can come into thermal equilibrium with the neutrinos after Big Bang Nucleosynthesis (BBN) through a process of neutrino - dark fermion oscillation and scattering. If the dark fermion is also coupled to other dark particles then this entire dark sector can become thermally populated without upsetting the predictions from BBN. If this dark sector contains massive particles as well as radiation, the massive particles can generate a "step" in the energy density of radiation, at a redshift associated with the mass of the particle, and they can introduce interactions between the dark matter and dark radiation. In this dissertation, I construct explicit example models, named WZDR and WZDR+, which demonstrate these features. WZDR+ in particular is shown to significantly reduce the H0 tension as well as the discrepancy between direct measurements of the clustering of matter on 8 Megaparsec scales (S8) and the value predicted from ΛCDM when fit to the CMB.