Late-time quantum backreaction of a very light nonminimally coupled
  scalar
  We investigate the backreaction of the quantum fluctuations of a very light
($m \!\lesssim\! H_{\text{today}}$) nonminimally coupled spectator scalar field
on the expansion dynamics of the Universe. The one-loop expectation value of
the energy momentum tensor of these fluctuations, as a measure of the
backreaction, is computed throughout the expansion history from the early
inflationary universe until the onset of recent acceleration today. We show
that, when the nonminimal coupling $\xi$ to Ricci curvature is negative ($\xi_c
\!=\! 1/6$ corresponding to conformal coupling), the quantum backreaction grows
exponentially during inflation, such that it can grow large enough rather
quickly (within a few hundred e-foldings) to survive until late time and
constitute a contribution of the cosmological constant type of the right
magnitude to appreciably alter the expansion dynamics. The unique feature of
this model is in that, under rather generic assumptions, inflation provides
natural explanation for the initial conditions needed to explain the late-time
accelerated expansion of the Universe, making it a particularly attractive
model of dark energy.
