Regular bouncing cosmological solutions in effective actions in four dimensions - art. no. 043503

We study cosmological scenarios resulting from effective actions in four di mensions which are, under some assumptions, connected with multidimensional , supergravity and string theories. These effective actions are labeled by the parameters omega, the dilaton coupling constant, and n which establishe s the coupling between the dilaton and a scalar field originating from the gauge field existing in the original theories. There is a large class of bo uncing as well as Friedmann-like solutions. We investigate under which cond itions bouncing regular solutions can be obtained. In the case of the strin g effective action, regularity is obtained through the inclusion of contrib utions from the Ramond-Ramond sector of superstring.