Late-time quantum backreaction from inflationary fluctuations of a nonminimally coupled massless scalar

We consider the late-time one-loop quantum backreaction from inflationary fluctuations of a non-minimally coupled, massless scalar field. The scalar is assumed to be a spectator field in an inflationary model with a constant principal slow-roll. parameter. We regulate the infrared by matching onto a pre-inflationary radiation era. We find a large late-time backreaction when the nonminimal coupling xi is negative (in which case the scalar exhibits a negative mass term during inflation). The one-loop quantum backreaction becomes significant today for moderately small nonminimal couplings, xi similar to -1/20, and it changes sign (from negative to positive) at a recent epoch when inflation lasts not much longer than what is minimally required, N greater than or similar to 66. Since currently we do not have a way of treating the classical fluid and the quantum backreaction in a self-consistent manner, we cannot say decidedly whether the backreaction from inflationary quantum fluctuations of a nonminimally coupled scalar can mimic dark energy.