Certain four-dimensional gauge models which can arise from a compactification of the ${\mathrm{E}}_8$×${\mathrm{E}}_8$ superstring theory predict the existence of ’t Hooft–Polyakov monopoles. Although typically superheavy, the primordial monopoles first appear when the temperature of the universe reaches the TeV scale. One phenomenologically attractive model with three fermion families predicts the existence of monopoles with mass ∼${10}^{16}$ GeV carrying three quanta of Dirac charge. They may occur in the galaxy at the level of the Parker bound.

• Received 6 November 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.1707