[Paper Review] Gravitational Waves from Supercool Axions

Luigi Delle Rose, Giuliano Panico+2 more|arXiv (Cornell University)|Dec 12, 2019

Cosmology and Gravitation Theories41 references116 citations

TL;DR

The paper analyzes Peccei–Quinn phase transitions for the QCD axion, showing first-order transitions with strong supercooling that produce stochastic gravitational waves with a peak in the 100–1000 Hz range, potentially detectable by LIGO and future detectors.

ABSTRACT

We study the dynamics of the Peccei-Quinn (PQ) phase transition for the QCD axion. In weakly coupled models the transition is typically second order except in the region of parameters where the PQ symmetry is broken through the Coleman-Weinberg mechanism. In strongly coupled realizations the transition is often first order. We show examples where the phase transition leads to strong supercooling lowering the nucleation temperature and enhancing the stochastic gravitational wave signals. The models predict a frequency peak in the range 100-1000 Hz with an amplitude that is already within the sensitivity of LIGO and can be thoroughly tested with future gravitational wave interferometers.

Motivation & Objective

• Investigate the nature of the Peccei–Quinn phase transition in various axion models.
• Determine when the PQ transition is first order and capable of strong supercooling.
• Predict the resulting gravitational wave signals and their detectability by current and future interferometers.
• Explore weakly coupled (elementary) and strongly coupled (composite) axion scenarios.
• Assess implications for reheating and cosmological constraints after the transition.

Proposed method

• Analyze KSVZ-type and radiative PQ breaking scenarios in elementary axion models.
• Compute finite-temperature effective potentials including Coleman–Weinberg contributions and thermal corrections.
• Evaluate bounce actions S3/T and nucleation temperatures Tn to establish supercooling regimes.
• Derive gravitational-wave spectra from bubble collisions and relate peak frequency to model parameters.
• Investigate RG-improved potentials and gauge- or quartic-dominated dynamics.
• Extend the discussion to composite axions from gauge theories and conformal sectors.

Experimental results

Research questions

• RQ1 Under which conditions is the PQ phase transition first order versus second order across different axion realizations?
• RQ2 How does supercooling influence the nucleation temperature and the resulting gravitational-wave signal?
• RQ3 What are the characteristic GW frequency and amplitude ranges predicted for these scenarios, and are they within LIGO/ET reach?
• RQ4 How do weakly coupled elementary axions compare to composite/strongly coupled axions in terms of phase-transition dynamics and GW production?

Key findings

• The models predict a frequency peak in the gravitational-wave spectrum in the range 100–1000 Hz.
• The predicted GW amplitudes can be within the sensitivity of current LIGO and testable with future detectors like the Einstein Telescope.
• Strong supercooling enhances the GW signal by prolonging the phase transition and increasing bubble collision contributions.
• In weakly coupled KSVZ-like models the PQ transition is typically second order unless radiative/Coleman–Weinberg mechanisms induce first order.
• In composite/strongly coupled axion scenarios the PQ transition can be first order, but nucleation may proceed rapidly with smaller supercooling, leading to a relatively suppressed GW signal.