Dimensional reduction in the early universe: Where have the massive particles gone

In many models based on reduction from greater than four dimensions, there are absolutely stable particles with masses of order R-1 (R is the compactification scale). If the temperature of the universe were ever close to R-1, these massive states would have been present and some would have survived annihilation. We calculate the present mass density due to these particles and find both the five-dimensional model and some versions of N = 8 supergravity to be unacceptable. We discuss some possible solutions to this problem. (orig.)