Open EFTs, IR Effects and Late-Time Resummations: Systematic Corrections
  in Stochastic Inflation
  Though simple inflationary models describe the CMB well, their corrections
are often plagued by infrared effects that obstruct a reliable calculation of
late-time behaviour. We adapt to cosmology tools designed to address similar
issues in other physical systems with the goal of making reliable late-time
inflationary predictions. The main such tool is Open EFTs which reduce in the
inflationary case to Stochastic Inflation plus calculable corrections. We apply
this to a simple inflationary model that is complicated enough to have
dangerous IR behaviour yet simple enough to allow the inference of late-time
behaviour. We find corrections to standard Stochastic Inflationary predictions
for the noise and drift, and we find these corrections ensure the IR finiteness
of both these quantities. The late-time probability distribution, ${\cal
P}(\phi)$, for super-Hubble field fluctuations are obtained as functions of the
noise and drift and so these too are IR finite. We compare our results to other
methods (such as large-$N$ models) and find they agree when these models are
reliable. In all cases we can explore in detail we find IR secular effects
describe the slow accumulation of small perturbations to give a big effect: a
significant distortion of the late-time probability distribution for the field.
But the energy density associated with this is only of order $H^4$ at late
times and so does {\em not} generate a dramatic gravitational back-reaction.
