Massive Neutrinos, Light Relics, and Ultra-Light Axions in the Halo Bias

Cosmological data provide a powerful tool in the search for physics beyond the Standard Model (SM). Light but massive relics (LiMRs) which decoupled from the SM while relativistic contribute to the radiation energy budget, and are commonly searched through variations in the effective number Neff of neutrino species. In addition to this effect, LiMRs with masses on the eV scale (meV-10 eV) be-come non-relativistic before today, and thus behave as matter instead of radiation. This leaves an imprint in the clustering of the large-scale structure of the universe, as light relics have important streaming motions, mirroring the case of massive neutrinos. Likewise, ultra-light axions with masses approximately greater than the Hubble scale transition equation of state before today, including during matter domination. Further, ultra-light axion fields possesses macroscopic de Broglie wave-lengths of astrophysical scales. These two properties of ultra-light axions have important consequences for structure formation. In this work, we model the effects of massive neutrinos, LiMRs, and ultra-light axions in the scale dependent halo bias. We forecast how well current and upcoming cosmological surveys can probe massive neutrinos and LiMRs - considering minimal extensions to the SM by both fermionic and bosonic relic degrees of freedom. By combining current and upcoming cosmic-microwave-background and large-scale-structure surveys, we forecast the significance at which each LiMR, with different masses and temperatures, can be detected. We also explore how the mass and relic abundance of a single, ultra-light axion species imprints on the scale dependent bias. Accurate models of the scale dependent bias, which this work seeks to advance, will be critical tools in efforts to use galaxy data for exploring dark sector physics.