Gravitational waves from quasi-stable strings

We estimate the stochastic gravitational wave spectrum emitted from a network of cosmic strings in which the latter are effectively stable against breaking by monopole pair creation. The monopoles are produced at a higher scale from an earlier symmetry breaking and experience significant inflation before reentering the horizon. This gives rise to monopole-antimonopole pairs connected by string segments and the string loop formation essentially ceases. As a consequence, the lower frequency portion of the gravitational wave spectrum is suppressed relative to the no-inflation case with stable strings, which evades the stringent PPTA bound on the dimensionless string tension Gμ. We display the modified spectrum, accessible in the ongoing and future experiments, for Gμ values in the range 10^-10-10^-15. We show how this 'quasi-stable' string network is realized in realistic grand unified theories.