Moduli evolution in heterotic scenarios

We discuss several aspects of the cosmological evolution of moduli fields i n heterotic string/M-theory scenarios. In particular we study the equations of motion of both the dilaton and overall modulus of these theories in the presence of an expanding Universe and under different assumptions. First w e analyse the impact of their couplings to matter fields, which turns out t o be negligible in the string and M-theory scenarios. Then we examine in de tail the possibility of scaling in M-theory, i.e., how the moduli would evo lve naturally to their minima instead of rolling past them in the presence of a dominating background. In this case we find interesting and positive r esults, and we compare them to the analogous situation in the heterotic str ing. (C) 2001 Elsevier Science B.V. All rights reserved.