5.3 Monopole problem

The final problem that inflation was designed to solve is the absence of observed magnetic monopoles (isolated sources of magnetic field), which is a prediction of some grand unified theories. This absence is known as the monopole problem.

Monopoles have never been observed, and it is not known whether they exist. It is possible that the theories that predict them are not correct. However, if they are created at the extremely high energies present in the very early Universe, then – by expanding the scale of the Universe by a factor of, say, 20–30 orders of magnitude – their space density becomes so diluted that they become sufficiently rare for it to be highly improbable that we would have detected one.