Our present theories of particle physics and cosmology, taken together,
suggest that very early in its history, the universe underwent a series of
phase transitions, at which topological defects, similar to those formed in
some condensed matter transitions, may have been created. Such defects, in
particular cosmic strings, may survive long enough to have important
observable effects in the universe today. Predicting these effects requires us
to estimate the initial defect density and the way that defects subsequently
evolve. Very similar problems arise in condensed matter systems, and recently
it has been possible to test some of our ideas about the formation of defects
using experiments with liquid helium-3 (in collaboration with the Low
Temperature Laboratory in Helsinki). I shall review the present status of this
theory.
Phase Transitions and Topological Defects in the Early Universe