Modeling the dark sector with multi-field ultra-light scalars

Evidence suggests the existence of a dark sector of dark matter and dark energy that together make up much of the energy density of the Universe. The standard assumptions of modeling dark energy with a cosmological constant Λ and dark matter with weakly interacting massive particles (WIMPs) work well in many circumstances, but these models have their own difficulties: there is yet no experimental evidence for the existence of WIMPs, and the usual cosmological model under these assumptions (ΛCDM) has some serious discrepancies with observation, such as the Hubble and σ8 tensions. In light of these issues, it is prudent to consider other possibilities. In our analyses, we model the dark sector as a collection of (coupled) ultra-light scalar fields, first considering the mechanism that generates the requisite behavior and then showing how applications of these models can help alleviate the aforementioned tensions. One of these models principally analyzes the dark energy sector with two fields under a specific potential: it mimics a cosmological constant remarkably well on the classical level. The second of these models considers multiple coupled axion fields. Axions act as dark energy early on in cosmic time before transitioning to behavior as dark matter: this model considers situations where some fields have already transitioned while others have not, accounting for the entire dark sector. We argue that the use of these axion fields can be used to simultaneously alleviate both tensions while remaining consistent with observational data.