Phase Transitions and Magnetic Monopole Production in the Very

In grand unified models, the abundance of superheavy magnetic monopoles in the universe can be suppressed if (1) the phase transition which creates the monopoles occurs after much supercooling; and (2) immediately after the phase transition, the effective monopole mass is large compared to the temperature. These requirements impose constraints on the history of the early universe. The Georgi-Glashow SU5 group probably breaks to SU4 x U1 before it reaches SU3 x SU2 X U1. Submitted to Physical Review Letters. x This work was supported in part by the Department of Energy under Contract Number DE-AC03-76SF00515 and in part by the National Science Foundation.