Warming up cold inflation
  The axion is a well-motivated candidate for the inflaton, as the radiative
corrections that spoil many single-field models are avoided by virtue of its
shift symmetry. However, axions generically couple to gauge sectors. As the
axion slow-rolls during inflation, this coupling can cause the production of a
non-diluting thermal bath, a situation known as "warm inflation." This thermal
bath can dramatically alter inflationary dynamics and observable predictions.
In this paper, we demonstrate that a thermal bath can form for a wide variety
of initial conditions. Furthermore, we find that axion inflation becomes warm
over a large range of couplings, and explicitly map the parameter space for two
axion inflation potentials. We show that in large regions of parameter space,
axion inflation models once assumed to be safely "cold" are in fact warm, and
must be reevaluated in this context.
