Abstract
The LIGO observatories can potentially detect stochastic gravitational waves arising from phase transitions which happened in the early universe at temperatures around T ∼ 108 GeV. This provides an extraordinary opportunity for discovering the phase transition associated with the breaking of the Peccei-Quinn symmetry, required in QCD axion models. Here we consider the simplest Peccei-Quinn models and study under which conditions a strong first-order phase transition can occur, analyzing its associated gravitational wave signal. To be detectable at LIGO, we show that some supercooling is needed, which can arise either in Coleman-Weinberg-type symmetry breaking or in strongly-coupled models. We also investigate phase transitions that interestingly proceed by first breaking the electroweak symmetry at large scales before tunneling to the Peccei-Quinn breaking vacuum. In this case, the associated gravitational wave signal is more likely to be probed at the proposed Einstein Telescope.
Highlights
To be detectable at LIGO, we show that some supercooling is needed, which can arise either in Coleman-Weinberg-type symmetry breaking or in strongly-coupled models
We start by providing a lightning description of axion physics to set notations, we investigate the occurrence of a first-order phase transition in the simplest PQ constructions
The predicted values of β/H∗ from (3.34) are quite large, 100, which makes it impossible to be seen at LIGO, since bubble collisions would be the main source of GWs in this case, and only Einstein Telescope (ET) could be able to detect this type of phase transition — see figure 2
Summary
We show that if the spontaneous breaking of the PQ symmetry occurred via a first-order cosmological phase transition, this would have left a stochastic GW signal potentially detectable by LIGO as well as future GW observatories In this case the transition proceeds by bubble nucleation and the collisions between the bubbles as well as the motion of the thermal plasma which surrounds them are sufficiently violent. Given the event rates of these mergers, the magnitude of the popcorn in the LIGO frequency band is around h2Ω ∼ 10−9, which enters in the detectability range for ET and marginally so for LIGO at design sensitivity This signal represents a ‘foreground’ for the cosmic GW backgrounds, and it should be subtracted away in order to be able to detect a possible background from cosmological phase transitions. The recent numerical simulations of these networks [27] suggest that the spectrum of this signal falls a bit short to be detectable
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