“At first, we thought it was absurd,” said theoretical physicist Asimina Arvanitaki, at the Perimeter Institute for Theoretical Physics, who proposes that black holes can be thought of as nature’s particle accelerators, and how we may be able to discover new particles through detection of the gravitational waves black holes create. “I’m not surprised. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy.”
Dark matter accounts for approximately 85 percent of all the mass density in the known Universe. The elusive substance is “dark”, meaning scientists cannot detect or interact with it in any meaningful way. This week, an experiment conducted at the European Organization for Nuclear Research (CERN), demonstrates a landmark new technique for capturing and measuring the extremely rare decay of a sub-atomic particle. Their results, indicate how precise measurements of this process could hint at new physics, beyond the Standard Model developed in the 1970s.
While the Standard Model, 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, scientists have concluded that it does not to account for everything they observe, including dark matter that makes up the bulk of the material universe. The discovery of the Higgs boson have yet to be followed by hints of other particles that match the properties of dark matter.
Superradiance –Massive Dark Matter Clouds Orbiting Black Holes
Black hole super-radiance is a fascinating process in general relativity and a unique window on ultralight particles beyond the standard model and the Higgs field. Bosons — such as axions and dark photons — with wavelengths comparable to size of astrophysical black holes grow exponentially to form large clouds, spinning down the black hole in the process, and produce monochromatic, continuous gravitational wave radiation.
In the era of gravitational wave astronomy and increasingly sensitive observations of black holes and their properties, superradiance of new light particles is a promising avenue to search for new physics and for new particles with black holes.
In June 18, 2019 The Daily Galaxy posted about New Scientist’s Daniel Cossins interview with Arvanitaki describing the little-known concept of superradiance, where a spinning black hole surrenders a bit 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 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’s 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, the Nobel Prize-winning experiment that detected gravitational waves caused by the ancient collisions of black holes.
LIGO detectors, suggests Arvanitaki, could discern a signal from a cloud of dark-matter particles feeding off the spin of a black hole, that would be more like a steady hum rather than a collision. “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 picture 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 concluded 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 physicists at LIGO to prepare for the detector’s third run, which began in April and immediately started detecting new gravitational waves.
“We may be able to use black holes as particle detectors of a different kind,” said Will East, Arvanitaki’s colleague at Perimeter, who works at the interface of gravitational physics, astrophysics, and cosmology.
Image credit:Hubble Space Telescope image showing the distribution of dark matter (grey) in the giant galaxy cluster Abell