Unknown period between inflation and Big Bang nucleosynthesis

Characterize the thermal and dynamical history of the universe between the end of cosmic inflation and the onset of Big Bang nucleosynthesis, including the mechanisms and rates by which the inflaton’s energy is transferred to Standard Model particles and the timeline for reaching thermal equilibrium.

Background

Modern cosmology asserts that inflation leaves the universe cold and non-thermal, while Big Bang nucleosynthesis requires a hot, radiation-dominated plasma. The text emphasizes a gap in understanding the intervening epoch when the first particles are produced, motivating the need to elucidate reheating and preheating dynamics.

The chapter outlines reheating’s two stages and notes potential observational relics (e.g., gravitational waves), but stresses that the nonperturbative, nonlinear nature of the process demands detailed characterization to bridge inflationary initial conditions to BBN-era thermodynamics.