Curing inflationary degeneracies using reheating predictions and relic gravitational waves

Abstract

It is well known that the inflationary scenario often displays different sets ofdegeneracies in its predictions for CMB observables. These degeneracies usually arise either because multiple inflationary modelspredict similar values for the scalar spectral index n S and the tensor-to-scalarratio r, or because within the same model, the values of {n S , r}are insensitive to some of the model parameters, making it difficult for CMB observations alone to constitute a unique probe of inflationary cosmology. We demonstrate that by taking into account constraints on the post-inflationary reheating parameters such as the duration of reheating N re, its temperature T re and especially its equationof state (EOS), w re, it is possible to break this degeneracy in certain classes of inflationary models where identical values of {n S , r} can correspond to different reheating w re. In particular, we show how reheating constraints can break inflationary degeneracies in the T-model and the E-model α-attractors. Non-canonical inflation is also studied. The relic gravitational wave (GW) spectrum provides us with another tool to break inflationary degeneracies.This is because the GW spectrumis sensitiveto the post-inflationary EOS of the universe. Indeed a stiff EOS during reheating (w re > 1/3) gives rise to a small scale blue tilt in the spectral indexn GW = d log ΩGW/d log k > 0, while a soft EOS (w re < 1/3) results in a red tilt.Relic GWs therefore provide us with valuable information about the post-inflationary epoch, and their spectrumcan be used to cure inflationary degeneracies in {n S , r}.