  The thermal history of the universe before the epoch of nucleosynthesis is
unknown. The maximum temperature in the radiation-dominated era, which we will
refer to as the reheat temperature, may have been as low as 0.7 MeV. In this
paper we show that a low reheat temperature has important implications for many
topics in cosmology. We show that weakly interacting massive particles (WIMPs)
may be produced even if the reheat temperature is much smaller than the
freeze-out temperature of the WIMP, and that the dependence of the present
abundance on the mass and the annihilation cross section of the WIMP differs
drastically from familiar results. We revisit predictions of the relic
abundance and resulting model constraints of supersymmetric dark matter,
axions, massive neutrinos, and other dark matter candidates, nucleosynthesis
constraints on decaying particles, and leptogenesis by decay of superheavy
particles. We find that the allowed parameter space of supersymmetric models is
altered, removing the usual bounds on the mass spectrum; the cosmological bound
on massive neutrinos is drastically changed, ruling out Dirac (Majorana)
neutrino masses $m_\nu$ only in the range 33 keV $\simlt m_\nu\simlt$ 6 (5)
MeV, which is significantly smaller from the the standard disallowed range 94
eV $\simlt m_\nu\simlt$ 2 GeV (this implies that massive neutrinos may still
play the role of either warm or cold dark matter); the cosmological upper bound
on the Peccei-Quinn scale may be significantly increased to $ 10^{16}$GeV from
the usually cited limit of about $10^{12}$GeV; and that efficient
out-of-equilibrium GUT baryogenesis and/or leptogenesis can take place even if
the reheat temperature is much smaller than the mass of the decaying superheavy
particle.
