Updated dark energy view of inflation
Résumé
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 with . 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 the scalar spectral index 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 equation of state parameter of the inflaton from . To do so, we consider a class of single-field slow-roll inflationary models dubbed , where the hierarchy of Hubble slow-roll parameters is truncated at the th order. The models are tested through a Markov chain Monte Carlo 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 . is excluded observationally since it would predict that , in contrast with the recent Planck constraints, while we find that would prefer , but is disfavored by the addition of the BICEP2/Keck data. The overall best description for the data is provided by , which yields a 68% upper bound of . 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 . This provides a cautionary tale for drawing conclusions about the nature of dark energy on the basis of the nonobservation of a deviation from .
Mots clés
98.80.-k
98.80.Es
95.36.+x
expansion: acceleration
inflation: slow-roll approximation
inflation: model
Monte Carlo: Markov chain
power spectrum: scalar
field theory: scalar
satellite: Planck
dark energy
equation of state
inflaton
cosmological constant
constituent
hierarchy
history
BICEP
cosmological model
Hubble constant
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