A Cautionary Tale: Dark Energy in Single-Field, Slow-Roll Inflationary Models

TL;DR

This paper investigates whether the inflaton's equation of state during inflation could have been measurably different from -1, highlighting the challenges in using current data to distinguish dark energy models from a cosmological constant.

Contribution

It analyzes single-field slow-roll inflation models to constrain the inflaton's equation of state using latest cosmological data, emphasizing the difficulty in detecting deviations from -1.

Findings

Upper bound of 1+w < 0.0014 at 68% confidence

Current data cannot definitively distinguish dark energy from a cosmological constant

Inflationary models are consistent with w close to -1 within observational limits

Abstract

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.