A cosmological mechanism for stabilizing moduli

In this Letter, we show how the generic coupling of moduli to the kinetic e nergy of ordinary matter fields results in a cosmological mechanism that in fluences the evolution and stability of moduli. As an example, we reconside r the problem of stabilizing the dilaton in a non-perturbative potential in duced by gaugino condensates. A well-known difficulty is that the potential is so steep that the dilaton field tends to overrun the correct minimum an d to evolve to an observationally unacceptable vacuum. We show that the dil aton coupling to the kinetic or thermal energy of matter fields produces a natural mechanism for gently relaxing the dilaton field into the correct mi nimum of the potential without fine-tuning of initial conditions. The same mechanism is potentially relevant for stabilizing other moduli fields. (C) 2000 Published by Elsevier Science B.V. All rights reserved.