Scalar fields around black hole binaries in LIGO-Virgo-KAGRA

TL;DR

The paper investigates whether light scalar fields can form environments around merging black holes that measurably modify the gravitational-wave phasing. A fast semi-analytic waveform model, grounded in a nonrelativistic Schrödinger-Poisson treatment and validated against NR simulations, is developed and used in Bayesian analyses of GW data. The study yields the first upper bounds on scalar-field environments in compact binaries, with GW190728 showing tentative evidence for such an environment under superradiance priors that point to a light scalar with mass $m_\phi \sim 10^{-12}\,\mathrm{eV}$. The results highlight potential degeneracies with spin and formation-history priors and pave the way for stronger constraints with future detectors.

Abstract

Light scalar particles arise naturally in many extensions of the Standard Model and are well-motivated dark matter candidates. Gravitational interactions near black holes can trigger the growth of dense scalar configurations that, if sustained during inspiral, alter binary dynamics and imprint signatures on gravitational-wave signals. Detecting such effects would provide a novel probe of fundamental physics and dark matter. Here we develop a semi-analytic waveform model for binaries in scalar environments, validated against numerical relativity simulations, and apply it in a Bayesian analysis of the LIGO-Virgo-KAGRA catalog. Our results set physically meaningful upper bounds on scalar environments around compact binaries. When superradiance priors are included, we find tentative evidence for such an environment in GW190728 with $\ln B_{\mathrm{vac}}^{\mathrm{env}} \approx 3.5$, which would correspond to the existence of a light scalar field with mass $\sim 10^{-12}\,\mathrm{eV}$.

Figures (8)

• Figure 1: Top: Density snapshot of a NR simulation of an equal-mass binary BH with total mass $M=60 M_\odot$ in a scalar field with $\alpha=0.43$ and an asymptotic density $\rho_\phi \approx 8.6 \times 10^5\,g\per\cubic cm$ at the boundaries of a cubic box with $L\approx4\times 10^4\,{\rm km}$. We can see a monopole overdense structure of radius $r_\phi\approx M/\alpha^2 \approx 480 \,{\rm km}$; these develop around the binary in just a couple of orbits. Bottom: Waveform frequency extracted from an NR simulation compared to the maximum-likelihood samples from IMRPhenomXP and our model IMRPhenomXP_Scalar (see \ref{['fig:pe:inj1']}).
• Figure 2: Marginalized posterior distributions from the analyses of an injected NR waveform (same as in \ref{['fig:waveform_comparison']}), using the recovery vacuum model IMRPhenomXP (green) and environment model IMRPhenomXP_Scalar (blue). The red dashed lines indicate the injected values. The vertical dashed-dot lines in 1D histograms represent the 90% credible interval. Unlike the vacuum model, which leads to significant biases, the environment model can accurately recover the system parameters; $\alpha$ is not well measurable and so its posterior is omitted. The Bayes factor in the model comparison is $\ln\mathcal{B}_{\rm vac}^{\rm env}=3.8$.
• Figure 3: Posteriors on scalar field density $\bar{\rho}_\phi$, particle mass $m_\phi$, and scalar-to-BH mass-ratio $M_\phi/m_1$ from the analysis of event GW190728, with $\tau_{\rm d}=10^6\,\rm yrs$. The gray curve corresponds to the analysis with (agnostic) broad priors. The dashed lines show the priors from the superradiance interpretation.
• Figure 4: Same as \ref{['fig:pe:inj1']} (main text) but for a different asymptotic density. The Bayes factor in the model comparison is $\ln\mathcal{B}_{\rm vac}^{\rm env}=-0.4$.
• Figure 5: Half-violin plots of the scalar-field density posteriors for selected GWTC-3 events. The black dashed line marks the 90% upper bound ($\bar{\rho}_\phi\xspace^{90\%}$). Several bounds lie many orders of magnitude below typical superradiant cloud densities. GW190728[0.8]_064510 and GW190814 have few support close to zero density.