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2.6. The Monopole Problem

There is no way the Monopole problem can be regarded as merely a feature. In 1979 Preskill [4] showed how quite generally a significant number of magnetic monopoles would be produced during the cosmic phase transition required by Grand Unification. Being non-relativistic (rho propto a-3), they would rapidly come to dominate over the ordinary matter (which was then relativistic, rho propto a-4). Thus it appeared that a Universe that is nearly 100% magnetic monopoles today was a robust prediction from GUT's, and thus the idea of Grand Unification was easily ruled out.