Ultralight axions in cosmology

Abstract

Ultralight axions (ULAs) play a tremendously important role in understanding several aspects of cosmology. The presence of the ULA field is promising to resolve several tensions of the standard A-cold-dark-matter model. Firstly, ULA with mass m_a, ≈ 10^-22 eV, which is usually referred to as fuzzy dark matter (FDM), has long been treated as a highly promising alternative to traditional cold dark matter (CDM) regime. Within the theoretical perspective, the ultralight mass of the FDM field can naturally be generated in the context of string compactifications. Expectedly, there may exist more than one species of ultralight axions with masses spanning many orders of magnitudes as in the string axiverse scenario. Along this direction, we propose the multiple-field FDM model with a specific focus on the two-field case. While the Milky Way and the dwarf spheroidal galaxies clearly fit solitons from an axion with mass m_a, ≈ 10^-22 eV, the ultrafaint dwarf galaxies can only be fitted with solitons coming from a second axion with mass m_a ≈ 10^-20 eV. The presence of this second axion also helps to resolve some of the other difficulties encountered in the single field FDM. Secondly, ULA with mass m_a, ≈ 10^-29 eV behaves as either dark energy or dark matter throughout cosmic history. This axion, when coupled to the Higgs field in the so-called axi-Higgs model, can uplift the electron mass in the early universe and alter recombination. As a result, the Hubble tension between early-time and late-time measurements is alleviated. The axi-Higgs model may also explain the ⁷Li puzzle in big-bang nucleosynthesis, the clustering S₈ tension with weak-lensing data, and the observed isotropic cosmic birefringence. The main focus of my thesis is todemonstrate the aforementioned potential of ULAs in cosmology.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Tao LIU (Supervisor)