A network of cosmic strings may produce a stochastic gravitational wave background (SGWB) from the combined emission of closed string loops throughout the cosmic history, and the characteristic scale invariant spectrum can be used to probe the expansion history of the universe. In particular, a short inflationary period caused by a supercooled phase transition would cause a departure from scale invariance, which is similar to the one caused by a non-standard matter-dominated era or from particle production. We point out that in the case of a short inflationary period, the SGWB receives an additional contribution from bubble collisions and plasma-related sources associated to the phase transition itself, which can leave imprints at future GW observatories allowing to disentangle these different physical origins.
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