TNT Colloquium: Effective field theory of axion dark matter [Duke]

Axions are a strongly motivated candidate for dark matter. In relativistic field theory, axions are described by a real scalar field, whose self-interaction potential is a periodic function of the scalar field. If the dark matter in the universe is made of axions, then they are highly nonrelativistic. A collection of nonrelativistic axions can condense into a gravitationally bound Bose-Einstein condensate called axion stars. The existence of axion stars can make the detection of axions more challenging, but it would open up new possibilities. In this talk, I will describe such low-energy axions more simply using a classical nonrelativistic effective field theory with a complex scalar field called the axion EFT. We will use axion EFT to describe axion stars and also the inelastic loss process that decreases the number of nonrelativistic axions in axion stars. I will conclude with a discussion of recent work on axion EFT and axion stars by other research groups and some of the theoretical issues involving bound systems of axion condensate.