Abstract

If axions are particularly light, and compose a significant fraction of the dark matter then there are interesting and potentially observable effects on the acoustic peaks and gravitational lensing of the CMB power spectrum, and on galaxy formation. These effects can be used to search for axions in cosmological and astrophysical data. Current constraints from Planck limit axions over eight orders of magnitude in mass to compose less than a few percent of the total dark matter. Upcoming “CMB Stage IV” lensing data will probe ten orders of magnitude in mass and could detect a percent level presence of axion dark matter at high statistical significance . This represents sub-percent level tests of the single-component standard cold DM model. Constraints from high redshift structure formation with axion DM place the current best lower bound on DM particle mass, m>1e-22 eV, which may be relevant to certain issues with the standard cold DM model in dwarf galaxies. This lower bound could be improved by up to four orders of magnitude using futuristic 21cm surveys. Given time, I will discuss the ideas and challenges to detect axion DM in this mass window, the possibility of forming black holes from the collapse of “axion stars”, and the origins of light axions in models of the string landscape.