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Description
Cosmic string networks constitute an important potential signature of new physics. For both theoretical and phenomenological reasons, it can be interesting to consider networks that are metastable instead of absolutely stable, such that the strings decay away at some point in cosmic history. The metastability of these networks is known to result in a characteristic cutoff in the stochastic gravitational wave signal they produce. I will first review the usual setup in which the network is destroyed via quantum tunnelling, and then discuss how in generic models the network is expected to break at much earlier times, either via finite-temperature effects or by attaching to pre-existing monopoles during network percolation. This “early-time” breaking significantly modifies the parameter space for which such a network can act as a source for the NANOgrav signal. I will also revisit quantum-tunnelling induced breaking; numerical simulations suggest that this occurs mainly at rare high tension points on the strings, yielding a larger rate than usually assumed.