Cold and Nonthermal Production After a Bounce That Reheats Gently

Not every bounce reheats the universe to extreme temperatures. If the post-bounce reheating temperature is low, thermal production of the heavy particles that baryogenesis usually relies on is shut off, and the asymmetry must be made cold or nonthermally. This paper computes the trade-off. Thermally, a reheating temperature T_reh permits heavy-neutrino production only up to masses near 3 T_reh, so T_reh = 1e9 GeV caps thermal leptogenesis at 3.0e+09 GeV, below its usual minimum scale. Nonthermally, decays of a heavy field of mass 1e13 GeV with branching ratio 0.1 produce eta = 3.74e-12 at T_reh = 1e9 GeV and 3.74e-09 at 1e12 GeV, for reference CP asymmetry 1e-6 and efficiency 0.1; matching the observed eta at the reference parameters requires T_reh near 1.63e+11 GeV. Low-temperature bounces are therefore not excluded from baryogenesis, but they must use nonthermal production and they pay for it in parameter tuning. Computed from stated assumptions, not measured.