Neutron-Proton Freeze-Out Under a Reduced Expansion Rate: The Shift in Temperature and Neutron Fraction

Every light-element abundance descends from one event: the moment the weak reactions converting neutrons and protons into each other became too slow to keep pace with expansion. The shift is systematic and grows as the bounce approaches: a bounce temperature of 1.2 MeV moves freeze-out to 0.739 MeV, a change of -7.6 percent from the standard 0.80 MeV, and the neutron fraction falls from 0.199 to 0.1738; at 3 MeV the freeze-out temperature is 0.791 MeV. Because helium, deuterium and lithium all inherit this number, the freeze-out shift is the single channel through which most bounce effects on abundances flow, and tabulating it separately makes the later abundance calculations auditable. Computed from stated assumptions, not measured.