  Embedded defects are predicted in a host of particle physics theories, in
particular in the standard electroweak theory. They can be stabilized by
interactions with the cosmological plasma, but will decay once the plasma falls
out of equilibrium, emitting a substantial fraction of non-thermal photons. If
the decay happens after a redshift of about $10^6$, these photons will give
rise to spectral distortions of the Cosmic Microwave Background, which leads to
strong constraints on the underlying particle physics theory. Such constraints
apply to any model with decaying defects, and in particular to theories
predicting decaying vortons, thereby leading to constraints stronger than the
dark matter limit.
