Abstract
Some models of the Kaluza-Klein theory contain absolutely stable particles with masses of order Planck mass. The Present number density of such particles is severely bounded by the total energy density of the universe. If these particles had existed plentifully in the early universe, most of them should have disappeared through pair annihilations. The annihilation rate depends strongly on the size of the extra space. We investigate the time evolution of the number density of such particles in a 5-dimensional model paying special attention to the time dependence of the size of the extra space.

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