MODEL-INDEPENDENT PROPERTIES AND COSMOLOGICAL IMPLICATIONS OF THE DILATION AND MODULI SECTORS OF 4D STRINGS

We show that if there is a realistic 4D string, the dilaton and moduli supermultiplets will generically acquire a small Mass approximately O (m3/2), providing the only vacuum-independent evidence of low-energy p hysics in string theory beyond the supersymmetric standard model. The only assumptions behind this result are (i) softly broken supersymmetr y at low energies with zero cosmological constant, (ii) these particle s interact with gravitational strength and the scalar components have a flat potential in perturbation theory, which are well-known properti es of string theories. (iii) They acquire a VEV of the order of the Pl anck scale (as required for the correct value of the gauge coupling co nstants and the expected compactification scale) after supersymmetry g ets broken. We explore the cosmological implications of these particle s. Similar to the gravitino, the fermionic states may overclose the Un iverse if they are stable or destroy nucleosynthesis if they decay unl ess their masses belong to a certain range or inflation dilutes them. For the scalar states it is known that the problem cannot be entirely solved by inflation, since oscillations around the minimum of the pote ntial, rather than thermal production, are the main source for their e nergy and can lead to a huge entropy generation at late times. We disc uss some possible ways to alleviate this entropy problem, that favour low-temperature baryogenesis, and also comment on the possible role of these particles as dark matter candidates or as sources of the baryon asymmetry through their decay.