AMPLIFICATION OF GRAVITATIONAL-WAVES IN RADIATION-DOMINATED UNIVERSES- RELIC GRAVITONS IN MODELS WITH MATTER CREATION

The formation of a background of primordial gravitational waves is inv estigated in Friedmann-Robertson-Walker (FRW) universes endowed with a diabatic matter creation. The condition for conformal invariance of th e gravitational wave equation on this scenario is established. The rel ation between conformal non-invariance and the process of superadiabat ic amplification of waves is clarified. It is shown that, in such mode ls, the amplification process may occur even during a radiation-domina ted phase. Generalizing previous work, the spectrum of the relic gravi tons associated with transitions between arbitrary cosmic eras is obta ined. The analysis is not restricted to the low-frequency band. It is shown that, for a certain range of the parameters involved, the sudden transition approximation between eras can still give accurate results at frequencies higher than it is usually assumed. A simple expression for the spectrum in this range is obtained. Constraints on the matter creation rate are derived from the possible influence of the relic gr avitons on the nucleosynthesis of He-4. [S0556-2821(98)01604-X].