An updated dark energy view of inflation

Abstract

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.