  In superconductors, and in other systems with a local U(1) gauge invariance,
there are two mechanisms that form topological defects in phase transitions. In
addition to the standard Kibble mechanism, thermal fluctuations of the magnetic
field also lead to defect formation. This mechanism is specific to local gauge
theories, predicts a qualitatively different spatial defect distribution and is
the dominant source for topological defects in slow transitions. I review the
arguments that lead to these conclusions and discuss possibilities of testing
the scenario in superconductor experiments.
