Reheating through the Higgs amplified by spinodal instabilities and gravitational creation of gravitons

If the post-inflationary equation of state parameter is stiff, the Universe may be reheated by the decay products the Higgs condensation formed during inflation, which generically have too large quantum fluctuations. One way to avoid this problem is to introduce a sufficiently large non-minimal coupling between the Higgs field to the Ricci scalar. Then, the Higgs effective mass term becomes negative at the transition from inflationary phase to the post-inflationary stiff phase, and the Higgs field experiences spinodal instabilities. This instability determines the energy density of radiation providing the dominant source of reheating. On the other hand, gravitons are also created gravitationally during the same transition phase, and if the energy density of gravitons is large relative to the energy density of radiation produced by the decay of the Higgs, it significantly alters the big bang nucleosynthesis, structure formation and the anisotropy in the cosmic microwave background radiation. Hence, this scenario can be constrained or tested by observational data.