3.2 Cosmic Inflation 42 Hot Big Bang. Reheating is typically complete when the Hubble parameter H has fallen to the same order as the decay rate of the φ field The temperature at the point where reheating is complete is known as the Reheat Temperature and is given by There also exists the possibility of having a period of Preheating . This is where most of the energy density of the inflaton decays immediately (explo- sively) into radiation, due to non-perturbative effects. However, preheating is typically incomplete. Therefore, the final stages of inflaton decay are per- turbative. If Γ$_{φ}$ ≪ $^{H}$∗ , then preheating products are irrelevant, as the energy density of the Universe becomes dominated by the oscillating inflaton again. However, if Γ$_{φ}$ ≲ $^{H}$∗ , then the Hot Big Bang will begin after preheating. H ∼ Γ$_{φ}$ (3.56) T$_{reh}$ ∼ √ M$_{P}$ Γ$_{φ}$ (3.57)