Exploring the String Axiverse with Precision Black Hole Physics (1004.3558v2)

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.

• The paper demonstrates that axion superradiance forms dense clouds around spinning black holes, leading to measurable spin-down and gaps in the mass-spin spectrum.
• It reveals that gravitational wave signals from axion transitions offer a potential indirect signature for detecting ultralight axions in the 10⁻⁹ to 10⁻²¹ eV range.
• The study outlines novel observational channels, including axion-photon interactions and episodic Bosenova collapses, that could advance our understanding of quantum gravity.

Insights Into Black Hole Superradiance from the Axiverse

The research presented in the paper investigates the potential existence and observational consequences of an Axiverse, a hypothesized plenitude of light axions predicted by string theory, on the dynamics of astrophysical black holes through superradiance processes. Light axions, with masses in the range $10^{-9}\div 10^{-21}$ eV, could manifest as bound states around rotating black holes, leading to observable astrophysical phenomena such as black hole spin-down, gravitational wave emissions, and axion-photon conversions.

Theoretical Framework and Observational Probes

The core mechanism explored is the superradiance instability, where the energy and angular momentum are extracted from rotating black holes when a light bosonic field, such as an axion, binds to it. This results in the formation of a dense axion cloud, analogous to a Bose-Einstein condensate (BEC), in resonance with the black hole's ergosphere. The authors explore the axion's potential role as a sensitive probe for ultralight particles given their intriguing interaction with astronomical black holes.

The research further elaborates on black hole "Regge trajectories" plotted in the mass versus spin parameter space, showing characteristic gaps and quantized trajectories induced by axion superradiance. These gaps are due to axions capturing spin energy, causing black holes to "spin down," thus altering their state observable through mass-spin relationships. Ambitious measurements of black hole spins and masses can reveal such gaps, offering indirect proof of axion existence.

Potential Observational Signatures

1. Spin-Dependent Black Hole Observations: The identification of gaps in the black hole Regge plane provides empirical clues. These gaps mark where rapidly spinning black holes have had their angular momentum drained by axions.
2. Gravitational Wave Signals: Detectable gravitational wave signals arising from axion transitions and annihilations within the cloud form additional observational footprints, notably within the sensitive band of instruments like Advanced LIGO and other next-generation gravitational wave observatories.
3. Axion-Photon Interactions: While the axion-photon conversions induced by the QCD axion coupling to electromagnetism present a novel astrophysical channel, their detection poses challenges due to low conversion efficiencies and radio frequencies obstructed by Earth's ionosphere, necessitating the use of space-based observatories.
4. Effects of Bosenova Explosions: A remarkable prediction includes episodic Bosenova collapses caused by axion self-interactions overpowering gravitational binding. Observations might capture these in the aftermath of transitions between superradiant black hole states, possibly accompanied by bursts of high-energy photons or unusual gamma-ray signatures.

Implications and Future Directions

The paper paves a compelling road for using astrophysical black holes as natural laboratories, proposing that the observed absence of rapidly rotating black holes—or confirmation of axion-induced anomalies in black hole spin distributions—could resolve puzzles surrounding dark matter and lead to breakthroughs in fundamental physics. If axions are indeed an imprint of a more complex landscape of string vacua, the detected mass and decay constant ranges for axions can elucidate grand unification physics and early Universe conditions.

Furthermore, the scalability of axion mass to black hole size suggests a revisit of black hole mass distribution data across the universe to reveal potential axion fields. Long-term, targeted observations will resolve theoretical uncertainties, advancing our understanding of the quantum-gravitational interplay, potentially challenging existing views on black hole growth and galaxy evolution.

In conclusion, this paper establishes a substantive paradigm wherein precision astronomical data inform core particle physics, with intriguing consequences across a spectrum of scientific disciplines. As instruments evolve, further empirical scrutiny will either consolidate or restrain the presented hypotheses, possibly unveiling new layers to the axion conundrum and the depths of quantum gravity.