Abstract
We explore the inflationary dynamics leading to formation of closed domain walls in course of evolution of an axion like particle (ALP) field whose Peccei-Quinn-like phase transition occurred well before inflationary epoch. Evolving after inflation, the domain walls may leave their imprint in stochastic gravitational waves background, in the frequency range accessible for the pulsar timing array measurements. We derive the characteristic strain power spectrum produced by the distribution of the closed domain walls and relate it with the recently reported NANOGrav signal excess. We found that the slope of the frequency dependence of the characteristic strain spectrum generated by the domain walls is very well centered inside the range of the slopes in the signal reported by the NANOGrav. Analyzing the inflationary dynamic of the ALP field, in consistency with the isocurvature constraint, we reveal those combinations of the parameters where the signal from the inflationary induced ALPs domain walls could saturate the amplitude of the NANOGrav excess. The evolution of big enough closed domain walls may end up in wormholes with the walls escaping into baby universes. We study the conditions when closed walls escape into baby universes and could leave a detectable imprint in the stochastic gravitational waves background.
Highlights
We explore the inflationary dynamics leading to formation of closed domain walls in course of evolution of an axion like particle (ALP) field whose Peccei-Quinn-like phase transition occurred well before inflationary epoch
Analyzing the inflationary dynamic of the ALP field, in consistency with the isocurvature constraint, we reveal those combinations of the parameters where the signal from the inflationary induced ALPs domain walls could saturate the amplitude of the NANOGrav excess
We explored the impact of the evolution of closed domain walls, created in course of the dynamic of the ALP field spectating the inflation, on the stochastic gravitational waves (GWs) background in the frequency range accessible for the pulsars timing array (PTA) measurements
Summary
By their seminal papers [1,2], offering the most attractive solution of the strong CP problem in QCD, Roberto Peccei and Helen Quinn, gave rise to an extensive line of actively developing research, the axion cosmology [3,4,5,6,7,8,9], generalized, in the mean time, in the cosmology of axionlike particles (ALPs) [10,11,12,13,14]. The deviations can be so large that, in some domains, the ALP field is already localized in a position to start oscillations about its different minimum, once it is liberated from the Hubble friction In this case the neighboring domain with the ALP field oscillating about different minima must be interpolated by a closed domain wall, with stress energy density defined by the scale of PQ-like symmetry breaking and the mass of the ALP [32,33]. The abundance of such walls of a given size is defined by the initial misalignment angle θ0, and the ratio of the PQ-like scale to the inflationary Hubble rate.
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