Largest temperature of the radiation era and its cosmological implications- art. no. 023508

The thermal history of the universe before the epoch of nucleosynthesis is unknown. The maximum temperature in the radiation-dominated era, which we w ill 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 implication s for many topics in cosmology. We show that weakly interacting massive par ticles (WIMP's) may be produced even if the reheat temperature is much smal ler 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 th e WIMP differs drastically from familiar results. We reexamine predictions of the relic abundance and resulting model constraints of supersymmetric da rk matter, axions, massive neutrinos, and other dark matter candidates, nuc leosynthesis constraints on decaying particles, and leptogenesis by decay o f superheavy particles. We find that the allowed parameter space of supersy mmetric 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 in, only in the range 33 keV less than or similar tom(nu)less than or similar to6 (5) MeV, which is significantly smaller from the standard disallowed range 94 eV less than or equal tom(nu )less than or similar to2 GeV (this implies that massive neutrinos may stil l 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) G eV from the usually cited limit of about 10(12) GeV; and that efficient out -of-equilibrium grand unified theory GUT baryogenesis and/or leptogenesis c an take place even if the reheat temperature is much smaller than the mass of the decaying superheavy particle.