NUMERICAL EXPERIMENTS ON STRING COSMOLOGY

We investigate some classical aspects of fundamental strings via numer ical experiments. In particular, we study the thermodynamics of a stri ng network within a toroidal universe, as a function of string energy density and space dimension. We find that when the energy density of t he system is low, the dominant part of the string is in the form of cl osed loops of the shortest allowed size, which correspond to the momen tum string modes. At a certain critical energy density corresponding t o the Hagedorn temperature, the system undergoes a phase transition ch aracterized by the formation of very long loops, winding a number of t imes around the torus. These loops correspond to the winding string mo des. As the energy density is increased, all the extra energy goes int o these long strings. We then study the lifetime of winding modes as a function of the space dimension. We find that in the low-energy densi ty regime, long winding strings decay only if the space dimension of t he toroidal universe is equal to 3. This finding supports the proposed cosmological scenario by Brandenberger and Vafa, which attempts to ex plain the space dimension and to avoid the initial singularity by mean s of string theory.