The quantum effective action is computed at finite temperatures in a Robertson-Walker type of metric suggested in Kaluza-Klein theories. The most interesting result is that above a critical temperature the effective gravitational constant becomes negative if the extra dimension is large enough. This provides a mechanism for the bounce from a contracting phase and may explain how the extra space has decoupled from the general expansion. The bounce mechanism also leads to a picture of the recycling universe, in which entropy produced after many cycles of expansion and contraction may solve the horizon and the flatness problem of the standard cosmology.

• Received 30 January 1984

DOI:https://doi.org/10.1103/PhysRevD.30.344