An updated dark energy view of inflation
  The present epoch of accelerated cosmic expansion is supposed to be driven by
an unknown constituent called dark energy, which in the standard model takes
the form of a cosmological constant, characterized by a constant equation of
state w=-1. An interesting perspective over the role and nature of dark energy
can be achieved by drawing a parallel with a previous epoch of accelerated
expansion, inflation, which we assume to be driven by a single scalar field,
the inflaton. Since the Planck satellite has constrained the value of $n_s$
away from 1, the inflaton cannot be identified with a pure cosmological
constant, as is also suggested by the fact that inflation ended. Thus, it is
interesting to verify whether a hypothetical observer would have been able to
measure the deviation of the w of the inflaton from -1. To do so, we consider a
class of single-field slow-roll inflationary models dubbed HSR{i}, where the
hierarchy of Hubble slow-roll parameters is truncated at the i-th order. The
models are tested through a MCMC analysis based on combinations of the latest
Planck and BICEP2/Keck data sets, and the resulting chains are converted into
sets of allowed evolution histories of w. HSR{1} is excluded observationally
since it would predict that $n_s=1$, in contrast with the recent Planck
constraints, while we find that HSR{2} would prefer w>-1, but is disfavoured by
the addition of the BICEP2/Keck data. The overall best description for the data
is provided by HSR{3}, which yields a 68% upper bound of 1+w<0.0014. Therefore,
if the current era of accelerated expansion happens to have the same equation
of state as inflation during the observable epoch, then current and upcoming
cosmological observations will not be able to detect that w$\neq$-1. This
provides a cautionary tale for drawing conclusions about the nature of dark
energy on the basis of the non-observation of a deviation from w=-1.
