Light scalars on cosmological backgrounds
  We study the behaviour of a light quartically self-interacting scalar field
$\phi$ on curved backgrounds that may be described with the cosmological
equation state parameter $w$. At leading order in the non-perturbative 2PI
expansion we find a general formula for the variance
$\langle\hat{\phi}^2\rangle$ and show for several previously unexplored cases,
including matter domination and kination, that the curvature of space can
induce a significant excitation of the field. We discuss how the generation of
a non-zero variance for $w\neq-1$ can be understood as a process of
self-regulation of the infrared divergences very similarly to what is known to
occur in de Sitter space. To conclude, the appearance of an effective mass due
to self-interaction is generic for a light scalar in curved space and can have
important implications for reheating, vacuum stability and dark matter
generation.
