3.3. The monopole problem (and other relics)

Modern particle theories predict a variety of `unwanted relics', which would violate observations. These include

• Magnetic monopoles.
• Domain walls.
• Supersymmetric particles such as the gravitino.
• `Moduli' fields associated with superstrings.

Typically, the problem is that these are expected to be created very early in the Universe's history, during the radiation era. But because they are diluted by the expansion more slowly than radiation (eg as a^-3 instead of a^-4) it is very easy for them to become the dominant material in the Universe, in contradiction to observations. One has to dispose of them without harming the conventional matter in the Universe.