Low reheating temperatures in monomial and binomial inflationary models

We investigate the allowed range of reheating temperature values in light of the Planck 2015 results and the recent joint analysis of Cosmic Microwave Background (CMB) data from the BICEP2/Keck Array and Planck experiments, using monomial and binomial inflationary potentials. While the well studied ϕ² inflationary potential is no longer favored by current CMB data, as well as ϕ^p with p>2, a ϕ¹ potential and canonical reheating (0w_re=) provide a good fit to the CMB measurements. In this last case, we find that the Planck 2015 68% confidence limit upper bound on the spectral index, n_s, implies an upper bound on the reheating temperature of T_re≲ 6× 10¹⁰ GeV, and excludes instantaneous reheating. The low reheating temperatures allowed by this model open the possibility that dark matter could be produced during the reheating period instead of when the Universe is radiation dominated, which could lead to very different predictions for the relic density and momentum distribution of WIMPs, sterile neutrinos, and axions. We also study binomial inflationary potentials and show the effects of a small departure from a ϕ¹ potential. We find that as a subdominant ϕ² term in the potential increases, first instantaneous reheating becomes allowed, and then the lowest possible reheating temperature of T_re=4 MeV is excluded by the Planck 2015 68% confidence limit.