“We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” says Asimina Arvanitaki, The Aristarchus chair in theoretical physics at the Perimeter Institute for Theoretical Physics in Waterloo, Canada about solving the mystery of dark matter.
“At first, we thought it was absurd,” Arvanitaki told New Scientist’s Daniel Cossins . “I’m not surprised. How else could you respond to the idea that black holes (Milky Way’s supermassive black hole above) generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together?”
Although our best picture of matter and its workings is a magnificent achievement, describing as it does all known particles and three of the four fundamental forces in a neat set of equations, it is far from perfect. All of the fundamental particles that have been identified to date fit neatly into a theory called the “standard model.” The final piece of that theory fell into place in 2012 when physicists working with the Large Hadron Collider, the world’s largest particle accelerator, near Geneva, Switzerland, confirmed the existence of the Higgs boson.
The Standard Model, writes Arvanitaki, developed more than 30 years ago, successfully describes phenomena from subatomic to galactic scales and have been experimentally tested to a precision of twelve decimals. Nevertheless, scientists have concluded that standard model is insufficient to account for everything they observe, including dark matter, a mysterious substance that astronomers have determined makes up the bulk of the material universe. Hopes that the discovery of the Higgs would be followed by hints of other particles that match the properties of dark matter have so far been dead ends.
Black Hole Particle Detectors
Instead, “We may be able to use black holes as particle detectors of a different kind,” said Will East, Arvanitaki’s colleague at Perimeter.
Many particle theorists were convinced the LHC would yield evidence for supersymmetry, an elegant idea that would solve pretty much everything, including the mystery of dark matter, by introducing a heavy twin for every known particle. But the search has turned up empty, leading some to question the way we craft such conjectures – and others to demand an even bigger collider.
So Arvanitaki, while a graduate student at Stanford University in California, started thinking about entities proposed by physics that couldn’t possibly be revealed by an LHC., which led her to axions, hypothetical particles suggested in the 1970s to solve a mystery known as the charge-parity problem.
Ultralight, they have no electrical charge but generate an entirely new force, which makes them good candidate for dark matter. The trouble was, the force they carry would interact so weakly with other particles that they would never show up at the LHC.
“That might explain why axions fell out of favor,” writes Cossins, “that and the fact that heavyweight particles emerged naturally from supersymmetry, encouraging everyone to build elaborate underground detectors to try to flush them out. ”
In 2010, however, Arvanitaki and her colleagues resurrected the possibility of axions by throwing string theory into the mix., which posits that the six dimensions beyond those we know must fit into unimaginably small spaces, or else we would have seen them. This extra-dimensional structure, according to Arvanitaki, gives rise to all manner of ultralight axions – what she calls a “string axiverse.”
Superradiance and Axions
“Which brings us back to planet-sized particles,” Cossins writes: “when Arvanitaki was writing up the string axiverse idea, a visiting colleague asked if she had heard of black hole superradiance. She hadn’t. And yet once she had spent a year wrapping her head around the idea, she came to realise it could give us a unique opportunity to spot axions, if indeed they exist. ”
In the little-known concept of superradiance, a spinning black hole surrenders a modicum of its rotational energy to a particle that is orbiting just outside it. Only particles with certain properties will be efficient at stealing energy from the black hole in this way. Calculations have shown those particles would then multiply to form a massive cloud surrounding the black hole that continues to siphon away energy until the cloud and the black hole are spinning at the same rate.
The motion of the cloud would generate gravitational waves – slight distortions to the vacuum of spacetime that ripple outward and that could, in principle, be detected on Earth. Since the kind of particles needed to generate the effect are not found in the standard model, any such signal would be evidence for a new kind of particle, in Arvanitaki conjecture, the axion, a hypothetical particle that has long been suggested as a dark-matter candidate that has gained traction because of the success of LIGO, a Nobel Prize-winning experiment that in recent years has managed to detect gravitational waves caused by the distant collisions of black holes.
In principle, LIGO could discern the signal from a cloud of dark-matter particles feeding off the spin of a black hole, though that would be more like a steady hum rather than a sudden crash.
“So now the [axion] waves scatter from the spinning black hole, but then keep bouncing back and forth, and eventually the amplification becomes exponential,” says Arvanitaki. In this version of superradiance, a cloud of gazillions of axions would be created, which would arrange themselves in an orderly fashion, she adds, “a lot like those pictures of atomic orbitals, only on a massive scale”.
The counter-intuitive problem says Arvanitaki, “is to make these ‘black-hole atoms’, the axion wavelength must be as long as the black hole is wide. Except that isn’t a problem here, as wavelength is inversely proportional to mass, and with axions we are talking about extremely light particles.”
What Arvanitaki and her colleagues have recently figured out is that these axion clouds could reveal themselves in gravitational waves, the faint ripples in space-time first picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, which means “you don’t need black holes smashing together. Axions colliding in the cloud should annihilate one another to produce gravitons, the particles thought to comprise gravitational waves.
“Essentially then, axions and black holes combine to dramatic effect, to produce what Arvanitaki describes as “gravitational beacons” that shine out in every direction.
Arvanitaki has been working with researchers from LIGO to prepare for the detector’s third run, which began in April and immediately started detecting gravitational waves.
Keith Riles, a physicist at the University of Michigan and a member of the LIGO team, who in 2018 proposed that gravitational wave detectors might be able to detect dark matter, if dark matter is composed of a particular kind of particle called a “dark photon.” Riles said that the extensive calculations needed to extract such a telltale signal would make it impossible to search in all directions of the sky at once. “The issue we would run into is that there are not enough computers on the planet to do the job,”
A more feasible option would be a targeted search using data from LIGO that corresponds to specific directions in the sky where black holes are already known to exist. Riles added that the possibility of discovering a new particle in this way made the idea worth pursuing.
“Even if it’s a long shot, it would be such a fantastic discovery that we have to pay attention to it,” he said.