  Cosmic strings are linear concentrations of energy that form whenever phase
transitions in the early universe break axial symmetries as originally shown by
Kibble. They are the result of frustrated order in the quantum fields
responsible for elementary particles and their interactions. For about two
decades, motivation for their study was provided by the possibility that they
could be behind the density inhomogeneities that led to the observed
large-scale structures in the universe. Precision observations, particularly of
the cosmic microwave background radiation, have limited strings to a
sub-dominant role in structure formation.
  More recently, interest has been revived with the realization that there may
be strong links between field theory cosmic strings and fundamental strings.
The latter are the supposed ultimate building blocks of matter, and in their
original context of superstring theory were thought to be microscopic. However,
in its modern version---sometimes referred to as M-theory---it is possible and
perhaps even mandatory to have macroscopic (cosmological-sized) fundamental
strings...
