Exploring the String Axiverse with Precision Black Hole Physics

TL;DR

The paper proposes that rotating black holes can serve as precision detectors for ultra-light axions predicted by string theory, via Penrose superradiance that forms a gravitational atom around the black hole. It develops a semi-analytic spectroscopy framework for hydrogenic axion bound states in Kerr spacetime, and analyzes the dynamical evolution of the axion cloud including gravitational-wave emission and non-linear self-interactions such as Bosenova. The authors outline observable signatures—Regge-plane gaps and trajectories, gravitational-wave signals in current and future detectors, and QCD-axion-specific electromagnetic channels—showing that Advanced LIGO could probe QCD axions and LISA/ET could access broader axiverse scenarios. They discuss anthropic considerations for multiple axions, emphasize the need for numerical simulations to refine predictions, and highlight the potential impact on black hole spin evolution and growth histories. Overall, the work motivates targeted searches for axion-induced imprints in black-hole systems as a powerful test of string-inspired new physics.

Abstract

It has recently been suggested that the presence of a plenitude of light axions, an Axiverse, is evidence for the extra dimensions of string theory. We discuss the observational consequences of these axions on astrophysical black holes through the Penrose superradiance process. When an axion Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole "nucleus" forming a gravitational atom in the sky. The occupation number of superradiant atomic levels, fed by the energy and angular momentum of the black hole, grows exponentially. The black hole spins down and an axion Bose-Einstein condensate cloud forms around it. When the attractive axion self-interactions become stronger than the gravitational binding energy, the axion cloud collapses, a phenomenon known in condensed matter physics as "Bosenova". The existence of axions is first diagnosed by gaps in the mass vs spin plot of astrophysical black holes. For young black holes the allowed values of spin are quantized, giving rise to "Regge trajectories" inside the gap region. The axion cloud can also be observed directly either through precision mapping of the near horizon geometry or through gravitational waves coming from the Bosenova explosion, as well as axion transitions and annihilations in the gravitational atom. Our estimates suggest that these signals are detectable in upcoming experiments, such as Advanced LIGO, AGIS, and LISA. Current black hole spin measurements imply an upper bound on the QCD axion decay constant of 2 x 10^17 GeV, while Advanced LIGO can detect signals from a QCD axion cloud with a decay constant as low as the GUT scale. We finally discuss the possibility of observing the gamma-rays associated with the Bosenova explosion and, perhaps, the radio waves from axion-to-photon conversion for the QCD axion.

Figures (15)

• Figure 1: Axionic Black Hole Atom: The spinning black hole "feeds" superradiant states forming an axion Bose-Einstein condensate. The resulting bosonic atom will emit gravitons through axion transitions between levels and annihilations and will emit axions as a consequence of self-interactions in the axion field.
• Figure 2: The part of the black hole and axion parameter space potentially affected by superradiance. For axion and black hole masses in the colored region the time required to create a substantial axion cloud is shorter than the age of the Universe. For masses in a light colored region the superradiance rate is faster than the Eddington accretion rate.
• Figure 3: The regions in the black hole Regge plane affected by superradiance for the QCD axion with $m=3\cdot 10^{-11}$ eV (the upper panel) and for a lighter axion with $m=10^{-17}$ eV. The data points correspond to spin measurements obtained by fitting the thermal continuum X-ray spectra McClintock:2009dn. Old black holes are expected to be found in the shaded region, where they are not affected by superradiance. Young black holes may be found also on the dashed colored lines inside the gap. Different colors correspond to superradiant levels with different orbital quantum numbers $l$.
• Figure 4: The contour plot of constant gravitational wave signal from axion transitions between the $6g$ and the $5g$ levels for a black hole located at 20 Mpc away from the Earth. The projected sensitivity curves of Advanced LIGO advLIGO and Einstein Telescope ET assume $10^4$ seconds of a coherent integration time.
• Figure 5: Superradiance rates obtained using our semi-analytic method (solid lines), non-relativistic approximation (dashed lines) and WKB approximation (dotted line) for a near-extremal black hole, $a/r_g=0.999$. Different colors correspond to superradiant levels with different values of the angular quantum number $l$.