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Description
It has been realized that string theory contains, in addition to fundamental strings, higher-dimensional fundamental objects - 'branes'. The starting point of brane gas cosmology is to consider a hot initial state in which all degrees of freedom, including those of strings and branes, are thermally excited. An important symmetry of string theory, t-duality, can be used to show that the resulting cosmology is nonsingular. If the background space-time geometry is described in terms of dilaton gravity, a mechanism is suggested which only allows three spatial dimensions to become macroscopic.