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In particle theory, a magnetic monopole arises from a topological glitch in the vacuum configuration of gauge fields in a Grand Unified Theory or other gauge unification scenario. The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding Universe.

According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place. This creates a problem, because it predicts that the monopole density today should be about 10¹¹ times the critical density of our Universe, according to the Big Bang model. But so far, physicists have been unable to find even one. Also, the Universe appears to be close to its critical density - for all matter combined.

Non-inflationary Big Bang cosmology suggests that monopoles should be plentiful, and the failure to find magnetic monopoles is one of the main problems that led to the creation of cosmic inflation theory. In inflation, the visible universe was much smaller in the period before inflation, and despite the very short time before inflation, it would have been small enough for the whole visible universe to have been within the horizon, and thus not requiring many monopoles. At the moment, versions of inflation seem to be the most likely cosmological theories.

The idea of magnetic monopoles existing is an appealing one, in light of the very natural and elegant way they would fit into a number of theories that physicists find promising. For example, Paul Dirac's conclusion (related to the Aharonov-Bohm effect) that the existence of magnetic monopoles implies that both electric and magnetic charge are quantized is unquestioned.

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