[Paper Review] Universal Gravitational Wave Signatures of Cosmological Solitons

Kaloian D. Lozanov, Misao Sasaki+1 more|arXiv (Cornell University)|Apr 13, 2023

Cosmology and Gravitation Theories8 citations

TL;DR

The paper shows that cosmological solitons generically induce a universal background of gravitational waves via soliton isocurvature perturbations, extending GW signals to lower frequencies, with axion-like particle oscillons as a concrete example.

ABSTRACT

Cosmological solitonic objects such as monopoles, cosmic strings, domain walls, oscillons and Q-balls often appear in theories of the early Universe. We demonstrate that such scenarios are generically accompanied by a novel production source of gravitational waves stemming from soliton isocurvature perturbations. The resulting induced universal gravitational waves (UGWs) reside at lower frequencies compared to gravitational waves typically associated with soliton formation. We show that UGWs from axion-like particle (ALP) oscillons, originating from ALP misalignment, extend the frequency range of produced gravitational waves by more than two orders of magnitude regardless of the ALP mass and decay constant and can be observable in upcoming gravitational wave experiments. UGWs open a new route for gravitational wave signatures in broad classes of cosmological theories.

Motivation & Objective

• Motivate and characterize cosmological solitons (monopoles, strings, domain walls, oscillons, Q-balls) as a source of early-Universe physics.
• Demonstrate that soliton isocurvature perturbations induce a second-order gravitational-wave background (UGWs) with a broader, lower-frequency spectrum.
• Show that UGWs are a universal feature across soliton scenarios and are observable in upcoming GW experiments.
• Illustrate the framework with axion-like particle oscillons as a concrete, phenomenologically relevant example.

Proposed method

• Model the perturbed FLRW metric with scalar and tensor perturbations and assume a radiation-dominated background with sub-dominant soliton matter.
• Derive linear evolution equations for curvature perturbations sourced by soliton isocurvature via S = delta rho_phi / rho_phi − (3/4) delta rho_r / rho_r and solve for Phi and S on subhorizon scales.
• Use second-order perturbation theory to compute induced GWs from curvature perturbations, adopting a Gaussian isocurvature power spectrum P_S(k) and a finite initial time x_i = k tau_i.
• Compute the present-day GW spectrum Omega_GW,0(k) from Omega_GW,c(k) with standard redshift/dof factors and radiation content.

Experimental results

Research questions

• RQ1 Do soliton isocurvature perturbations universally induce a second-order GW background across soliton scenarios?
• RQ2 How does the induced GW spectrum extend to lower frequencies compared to GWs from soliton formation?
• RQ3 What are the implications for axion-like particle oscillon scenarios and their observability in future GW experiments?
• RQ4 How do the initial conditions and UV cutoffs in P_S(k) affect the predicted UGWs?
• RQ5 Can UGWs serve as a broad-signature probe of early-Universe soliton formation across different cosmological models?

Key findings

• Solition-induced UGWs arise generically from soliton isocurvature perturbations and extend to much lower frequencies than GWs from soliton formation.
• In ALP oscillon scenarios, UGWs broaden the GW spectrum by more than two orders of magnitude toward low frequencies, independent of the ALP mass and decay constant.
• The GW signal strength depends on the energy fraction in oscillons and the initial isocurvature amplitude constrained by CMB bounds on isocurvature perturbations.
• The formalism accounts for subhorizon generation of isocurvature and includes the finite initial time x_i, providing a universal mechanism for UGWs across soliton types.
• UGWs offer a new observational channel to probe broad classes of cosmological theories featuring soliton formation, beyond traditional high-frequency GW signals.
• For ALPs, the framework links parameters like H_inf, F, and m to observable GW signatures in upcoming experiments.