Interpolating the stage of exponential expansion in the early universe: Possible alternative with no reheating

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Abstract
In the standard picture, the inflationary universe is in a supercooled state which ends with a short time, large scale reheating period, after which the universe goes into a radiation-dominated stage. An alternative is proposed here in which the radiation energy density smoothly decreases all during an inflationlike stage and with no discontinuity enters the subsequent radiation-dominated stage. The scale factor is calculated from standard Friedmann cosmology in the presence of both radiation and vacuum energy density. A large class of solutions confirm the above identified regime of nonreheating inflationlike behavior for observationally consistent expansion factors and not too large a drop in the radiation energy density. One dynamical realization of such inflation without reheating is from warm inflation-type scenarios. However the solutions found here are properties of the Einstein equations with generality beyond slow-roll inflation scenarios. The solutions also can be continuously interpolated from the nonreheating-type behavior to the standard supercooled limit of exponential expansion, thus giving all intermediate inflationlike behavior between these two extremes. The temperature of the universe and the expansion factor are calculated for various cases. Implications for baryongenesis are discussed. This nonreheating, inflationlike regime also appears to have some natural features for a universe that is between nearly flat and open.