Black Hole Mergers and the QCD Axion at Advanced LIGO

TL;DR

The paper investigates using Advanced LIGO observations of black-hole mergers to probe ultralight bosons, focusing on the QCD axion with mass $m_a \sim 10^{-14}$–$10^{-10}$ eV via black-hole superradiance. It combines a statistical approach that looks for mass-dependent spin gaps in the BH population with direct searches for monochromatic GWs from axion annihilations or level transitions, including post-merger evolution. The authors predict a detectable SR signature in the spin distribution for $m_a$ in the range $\sim 2\times10^{-13}$–$5\times10^{-12}$ eV after tens of events and show that annihilation signals could yield observable, long-lived GWs out to hundreds of Mpc, potentially up to $\sim 10^4$ sources in a full-sky search. Overall, the work provides a concrete, dual-path strategy to test for QCD axions and other ultralight bosons with current and next-generation GW detectors, complementing existing astrophysical constraints and guiding future observational campaigns.

Abstract

In the next few years Advanced LIGO (aLIGO) may see gravitational waves (GWs) from thousands of black hole (BH) mergers. This marks the beginning of a new precision tool for physics. Here we show how to search for new physics beyond the standard model using this tool, in particular the QCD axion in the mass range ma ~ 10^-14 to 10^-10 eV. Axions (or any bosons) in this mass range cause rapidly rotating BHs to shed their spin into a large cloud of axions in atomic Bohr orbits around the BH, through the effect of superradiance (SR). This results in a gap in the mass vs. spin distribution of BHs when the BH size is comparable to the axion's Compton wavelength. By measuring the spin and mass of the merging objects observed at LIGO, we could verify the presence and shape of the gap in the BH distribution produced by the axion. The axion cloud can also be discovered through the GWs it radiates via axion annihilations or level transitions. A blind monochromatic GW search may reveal up to 10^5 BHs radiating through axion annihilations, at distinct frequencies within ~3% of $2 ma. Axion transitions probe heavier axions and may be observable in future GW observatories. The merger events are perfect candidates for a targeted GW search. If the final BH has high spin, a SR cloud may grow and emit monochromatic GWs from axion annihilations. We may observe the SR evolution in real time.

Figures (4)

• Figure 1: Expected detectable sources in a blind monochromatic GW search, with sensitivity of current aLIGO (dashed), design aLIGO (solid), Voyager (wide-dashed) and Cosmic Explorer (dot-dashed) LIGOWhitePaper for realistic mass and spin distributions, and BH formation rates (Arvanitaki:2014wva). The shaded bands correspond to the range between pessimistic and optimistic BH distributions with design aLIGO (distributions as in Arvanitaki:2014wva, with the most narrow BH mass distribution removed as it is disfavored by the observation of GW150914). The coherent integration time is 2 days and total time 1 yr. The annihilation rate has been updated using the latest superradiance simulations Yoshino:2013ofa. Axion masses in the grayed-out region are disfavored by BH spin measurements Arvanitaki:2014wva; the most optimistic distributions are disfavored by previous null LIGO searches Aasi:2012fwAbbott:2016uddTheLIGOScientific:2016uns.
• Figure 2: Expected distribution of intrinsic (top) and measured (bottom) spins and masses of merging BHs in the absence (left) and the presence (right) of an axion of mass $6\times10^{-13}$ eV, normalized to 1000 events detected at aLIGO. We assume $\sigma_M/M\sim10\%$ measurement error in the mass and $\sigma_{a_*}\sim0.25$ error in the spin Vitale:2014mka*VitaleVitale:2016avz. We have assumed that all BBHs formed at a distance such that they take $10^{10}$ years to merge. The theoretical curves shown are boundaries of the regions where SR had at most $10^{10}$ years to spin down the BHs, and the effect of the companion BH does not significantly affect the SR rate.
• Figure 3: Number of observed events required to show that the BH spin distribution varies with BH mass, assuming the presence of an axion of mass $\mu_a$. Spin measurement errors of $\sigma_{a_*} = 0.25$ are assumed. Blue (red) curves correspond to BHs taking $10^{10}\mathrm{yrs}$ ($10^7\mathrm{yrs}$) from formation to merger. The solid curves shows the median number of events required to reject the separable-distribution hypothesis at $2\sigma$. The upper/lower dashed curves show the upper/lower quartiles, respectively. The test statistic used is the Kolmogorov-Smirnov distance between the spin distributions outside and inside a given BH mass range, maximized over choice of mass range. Shaded region is as in Fig. \ref{['fig:blindsearch']}.
• Figure 4: Expected annual annihilation events for aLIGO and future observatories from products of BH-NS mergers (magenta) or BBH mergers of equal mass (blue). We assume the binary formation mechanism does not allow for superradiance. We take $a_*=0$, $M=1.4M_{\odot}$ for the NS and a power-law mass distribution and flat spin distribution of the merging BHs. The bands represent the merger rate uncertainty given the observed BBHs Abbott:2016nhfTheLIGOScientific:2016pea and simulations for BH-NS (V4l&V2l in Belczynski:2015tba). We assume a coherent integration time of 10 days for BBH and 1 year (or up to the duration of the signal) for BH-NS. Shaded region is as in Fig. \ref{['fig:blindsearch']}.