Non-equilibrium evolution of quantum fields during inflation and late
  accelerating expansion
  To understand mechanisms leading to inflation and late acceleration of the
Universe it is important to see how one or a set of quantum fields may evolve
such that the classical energy-momentum tensor behave similar to a cosmological
constant. In this work we consider a toy model including 3 scalar fields with
very different masses to study the formation of a light axion-like condensate,
presumed to be responsible for inflation and/or late accelerating expansion of
the Universe. Despite its simplicity, this model reflects hierarchy of masses
and couplings of the Standard Model and its candidate extensions. The
investigation is performed in the framework of non-equilibrium quantum field
theory in a consistently evolved FLRW geometry. We discuss in details how the
initial conditions for such a model must be defined in a fully quantum setup
and show that in a multi-component model interactions reduce the number of
independent initial degrees of freedom. Numerical simulation of this model
shows that it can be fully consistent with present cosmological observations.
For the chosen range of parameters we find that quantum interactions rather
than effective potential of a condensate is the dominant contributor in the
energy density of the Universe and triggers both inflation and late
accelerating expansion. Nonetheless, despite its small contribution in the
energy density, the light scalar field - in both condensate and quasi free
particle forms - has a crucial role in controlling the trend of heavier fields.
Furthermore, up to precision of our simulations we do not find any IR
singularity during inflation. These findings highlight uncertainties in
attempts to extract information about physics of the early Universe by naively
comparing predictions of local effective classical models with cosmological
observations, neglecting inherently non-local nature of quantum processes.
