Annu. Rev. Astron. Astrophys. 1991. 29: 325-362
Copyright © 1991 by . All rights reserved

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2.2.3. MONOPOLES The spontaneous breakdown of symmetry in the GUT scenario causes the initial larger symmetry group of the unified interactions to decompose into smaller ones, which include the symmetry group of the electromagnetic theory. A general theorem tells us that whatever the initial symmetry group, the process of symmetry breaking inevitably generates physical solutions that describe magnetic monopoles. The energy of a monopole is about 1016 GeV, corresponding to a mass of about 1.8 x 10-8 g. Assuming that no more than one monopole is created in one horizon-size sphere, the mass density of monopoles at the GUTs epoch works out to about 1.7 x 1065 gcm-3.This is diluted by expansion through a factor of about 4 x 1026 to a present mass density of 10-15 g cm-3 - far in excess of the critical density. With a density of this order, the universe would have contracted to a = 0 in time less than 105 years! Since monopoles are stable objects, they cannot be destroyed. Thus the problem of excess monopole density seems insurmountable within the standard model.