Is There a Monopole Problem ?

We show that there exists a range of parameters in SU(5) theory for which the GUT symmetry remains broken at high temperature, thus avoiding the phase transition that gives rise to the overproduction of monopoles. The thermal production of monopoles can be naturally suppressed, keeping their number density below the cosmological limits. A. Introduction It has been known for a long time that the existence of magnetic monopoles (a single one would suffice) would lead to the quantization of electromagnetic charge. In grand unified theories based on a simple group (or their products), the electromagnetic charge is necessarily quantized and thus the magnetic monopoles are the necessary outcome of the theory. This, what should be a blessing, is however precisely what makes these theories incompatible with standard cosmology. [email protected]. Present address: CERN, CH-1211 Geneva 23, Switzerland. [email protected]. [email protected]