  Topological defects are common in many everyday systems. In general, they
appear if a symmetry is broken at a rapid phase transition. In this article, I
explain why it is believed that they should have also been formed in the early
universe and how that would have happened. If topological defects are found,
this will provide a way to study observationally the first fractions of a
second after the Big Bang, but their apparent absence can also tell us many
things about the early universe.
