Abstract
Abstract
We study a new source of stochastic gravitational wave background (SGWB) from the final
collapse of a network of topological defects. Typically, the final collapse is considered
negligible for generating gravitational waves (GWs) due to its subdominance compared with the
network's long-term evolution in the scaling regime. However, in some cases, a network can be
driven outside of horizon by inflation and later re-enter horizon. Then, the network's final
collapse after re-entering horizon becomes the dominant GW source and therefore cannot be
neglected. We demonstrate this phenomenon in the context of N
DW = 1 string-wall networks
which naturally arise in axion models, although the framework can be generalized to other types of
topological networks. The final collapse of walls bounded by strings releases GWs. Our calculation
of the corresponding GW spectrum suggests it could be related to the first few bins of the
nano-Hertz SGWB signal possibly detected by various Pulsar Timing Array (PTA) collaborations.
However, it is important to note that such GW spectrum falls within a relatively narrow frequency
range, which may not completely account for the PTA signal that spans more than one order of
magnitude in frequency. Furthermore, with different parameter choices, the resultant GWs
generated in this mechanism could be probed by various GW interferometry experiments.