LAUSR

In this paper we develop a formalism for studying the nonrelativistic limit of relativistic field theories in a systematic way. By introducing a simple, nonlocal field redefinition, we may transform the description of a given relativistic theory in terms of a real-valued, self-interacting scalar field into an equivalent theory, describing a complex scalar field. Our low-energy effective theory incorporates corrections from nonvanishing momentum as well as from the backreaction of fast-oscillating terms on the behavior of the dominant, slowly varying component of the field. Possible applications of our new approach include axion dark matter, though the methods developed here should be applicable to the low-energy limits of other field theories as well.