Few things are more mind bending than black holes, gravitational waves, and the nearly massless hypothetical particles called axions, which could be the mysterious dark matter whose gravity holds galaxies together. Now, a team of theoretical physicists has tied all three together in a surprising way. If the axion exists and has the right mass, they argue, then a spinning black hole should produce a vast cloud of the particles, which should, in turn, produce gravitational waves akin to those discovered a year ago by the Laser Interferometer Gravitational-Wave Observatory (LIGO). If the idea is correct, LIGO might be able to detect axions, albeit indirectly.
“It's an awesome idea,” says Tracy Slatyer, a particle astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge, who was not involved in the work. “The [LIGO] data is going to be there, and it would be amazing if we saw something.” Benjamin Safdi, a theoretical particle physicist at MIT, is also enthusiastic. “This is really the best idea we have to look for particles in this mass range,” he says.
A black hole is the intense gravitational field left behind when a massive star burns out and collapses to a point. Within a certain distance of that point—which defines the black hole’s “event horizon”—gravity grows so strong that not even light can escape. In September 2015, LIGO detected a burst of ripples in space called gravitational waves that emanated from the merging of two black holes.
The axion—if it exists—is an uncharged particle perhaps a billionth as massive as the electron or lighter. Dreamed up in the 1970s, it helps explain a curious mathematical symmetry in the theory of particles called quarks and gluons that make up protons and neutrons. Axions floating around might also be the dark matter that’s thought to make up 85% of all matter in the universe. Particle physicists are searching for axions in experiments that try to convert them into photons using magnetic fields.
But it may be possible to detect axions by studying black holes with LIGO and its twin detectors in Louisiana and Washington states, argue Asimina Arvanitaki and Masha Baryakhtar, theorists at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and their colleagues.
If its mass is in the right range, then an axion stuck in orbit around a black hole should be subject to a process called superradiance that occurs in many situations and causes photons to multiply in a certain type of laser. If an axion strays near, but doesn’t cross, a black hole’s event horizon, then the black hole’s spin will give the axion a boost in energy. And because the axion is a quantum particle with some properties like those of the photon, that boost will create more axions, which will, in turn, interact with the black hole in the same way. The runaway process should thus generate vast numbers of the particles.
But for this to take place, a key condition has to be met. A quantum particle like the axion can also act like a wave, with lighter particles having longer wavelengths. For superradiance to kick in, the axion’s wavelength must be as long as the black hole is wide. So the axion’s mass must be extremely light: between 1/10,000,000 and 1/10,000 the range probed in current laboratory experiments. The axions wouldn’t just emerge willy-nilly, either, but would crowd into huge quantum waves like the orbitals of the electrons in an atom. As fantastical as that sounds, the basic physics of superradiance is well established, Safdi says.
The axion cloud might reveal itself in multiple ways, Baryakhtar says. Most promising, axions colliding in the cloud should annihilate one another to produce gravitons, the particles thought to make up gravitational waves just as photons make up light. Emerging from orderly quantum clouds, the gravitons would form continuous waves with a frequency set by the axion’s mass. LIGO would be able to spot thousands of such sources per year, Baryakhtar and colleagues estimate in a paper published 8 February in Physical Review D—although tracking those continuous signals may be harder than detecting bursts from colliding black holes. Spotting multiple same-frequency sources would be a “smoking gun” for axions, Slatyer says.
The axion clouds could produce indirect signals, too. In principle, a black hole can spin at near light speed. However, generating axions would sap a black hole’s angular momentum and slow it. As a result, LIGO should observe that the spins of colliding black holes never reach that ultimate speed, but top out well below it, Baryakhtar says. Detecting that limit on spin would be challenging, as LIGO can measure a colliding black hole’s spin with only 25% precision.
Safdi cautions that the analysis assumes that LIGO will see lots of black-hole mergers and will perform as expected. And if LIGO doesn’t see the signals, it won’t rule out the axion, he says. Still, he says, “This is probably the most promising paper I’ve seen so far on the new physics we might probe with gravitational waves.”