Axions with Variable Masses

Abstract

Axions with variable masses, in the context of a scalar--tensor gravity theory, give a large entropy production during the matter era. The subsequent axion dilution is proportional to their present energy density. Depending on the parameters ($β_I,β_V$) of the model, this dilution relaxes or even eludes the cosmological bound on the axion mass, therefore opening the so--called ``axion window".