A Cautionary Tale: Dark Energy in Single-Field, Slow-Roll Inflationary
  Models
  The current epoch of accelerated cosmic expansion is postulated to be driven
by dark energy, which in the standard model takes the form of a cosmological
constant with equation of state parameter $w=-1$. We propose an innovative
perspective over the nature of dark energy by drawing a parallel with
inflation, which we assume to be driven by a single scalar field, the inflaton.
The inflaton was not a cosmological constant, as indicated by the fact that
inflation ended and by the Planck satellite's constraint of $n_s\neq 1$ at
$8\sigma$ confidence. Therefore, it is interesting to verify whether its
equation of state parameter was measurably different from $-1$. We analyze this
question for a class of single-field slow-roll inflationary models, where the
hierarchy of Hubble slow-roll parameters is truncated at different orders.
Based on the latest Planck and BICEP2/Keck data, we obtain a $68\%$ upper bound
of $1+w < 0.0014$ for the three-parameter model, which gives the best
description to the data. This provides a cautionary tale for drawing
conclusions about the nature of today's dark energy based upon the
non-detection of a deviation from $w=-1$ with current and upcoming cosmological
surveys.
