Current cosmological observations strongly favor
that today's energy
density of the Universe is almost equal to the critical density that makes
space-time flat. This result rules out domain walls and
monopoles since their energy densities do not get diluted fast enough
to avoid the overclosure of the Universe. On the other hand, cosmic string
energy density evolves as ~ 
(which is the same as radiation type
energy) and will not overturn today's Universe. Simulation shows that
long strings with a cosmological scale survive up to the present time. Thus,
cosmic strings could serve as an observational window for GUT symmetry
breaking, superstring theory, or both.
