Abstract
In this contribution, we discuss the cosmological scenario where unstable domain walls are formed in the early universe and their late-time annihilation produces a significant amount of gravitational waves. After describing cosmological constraints on long-lived domain walls, we estimate the typical amplitude and frequency of gravitational waves observed today. We also review possible extensions of the standard model of particle physics that predict the formation of unstable domain walls and can be probed by observation of relic gravitational waves. It is shown that recent results of pulser timing arrays and direct detection experiments partially exclude the relevant parameter space, and that a much wider parameter space can be covered by the next generation of gravitational wave observatories.
Highlights
The progress of direct observations [1,2] of gravitational waves (GWs) is bringing about drastic developments in astrophysics and cosmology
The domain walls can be annihilated if we introduce an additional term that explicitly breaks Zn symmetry [121,122] or if we assume that the Zn symmetry is anomalous for quantum chromodynamics (QCD) [55,116]
Various well-motivated particle physics models predict the formation of unstable domain walls in the early universe, and it is possible to probe such models by observing GWs produced by them
Summary
The progress of direct observations [1,2] of gravitational waves (GWs) is bringing about drastic developments in astrophysics and cosmology. Their unstability might be guaranteed if the discrete symmetry is only approximate and explicitly broken by a small parameter in the theory In such a scenario, a significant amount of GWs can be produced during the process of collisions and annihilations of domain walls, and they may remain as a stochastic GW background in the present universe. A significant amount of GWs can be produced during the process of collisions and annihilations of domain walls, and they may remain as a stochastic GW background in the present universe Observations of such relic GWs will enable us to trace the events in the very early universe and provide a new way of investigating physics at very high energies.
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