Thursday, June 29, 2017

Spinning Black Holes Could Create Clouds of Mass

Nothing, not even light, can come out of a black hole. At least, that’s the conventional wisdom, and it’s certainly true that—once the event horizon is crossed—there’s no going back. But for rotating black holes, there’s a region outside the event horizon where strange and extraordinary things can happen, and these extraordinary possibilities are the focus of a new paper in the American Physical Society journal Physical Review Letters.

The study reports simulations of a phenomenon called superradiance, where waves and particles passing in the vicinity of a spinning black hole can extract some of its rotational energy. The authors propose that hypothetical ultralight particles, with masses far lower than that of a neutrino, could get caught in orbit around such a black hole, sapping away some of its angular momentum and being accelerated in the process. Because energy, like the black hole's rotational energy, can give rise to matter, this phenomenon—termed a superradiant instability—converts the black hole’s angular momentum into a massive cloud of these ultra-light particles.

A still image from the researchers' simulations, showing the cloud of ultralight particles (yellow-orange)
 accumulating around a black hole as it spins down.
Image Credit: W.E. East & F. Pretorius

The reason these particles would have to be so much lighter than anything we've ever seen has to do with a quantity called the Compton wavelength. While electrons, protons, neutrinos, and other bits of matter usually behave like particles, they have wavelike properties as well—and just like with photons, the energy of the particles is related to their wavelength. The longer an electromagnetic wave is, the less energy it carries, and it's the same for massive particles; for instance, protons have a shorter Compton wavelength than electrons, because protons have more mass-energy.

For a particle to get caught in this special type of resonant, self-amplifying orbit around a spinning black hole, it has to have a Compton wavelength roughly equal to the size of the event horizon. Even the smallest black holes are at least 15 miles across, which means that each particle would have to carry an extremely small amount of mass-energy; for comparison, the Compton wavelength of an electron at rest is something like two trillionths of a meter.

Each individual particle would have an extremely small amount of energy, but the researchers’ simulations showed that, for particles with the right mass around a black hole spinning with close to its maximum angular momentum, almost 10% of the black hole’s initial effective mass could be extracted into the surrounding cloud. The process only stops when the black hole has spun down to the point where its rotation matches the rate at which the particles orbit it.

Although it's unclear how such a massive and energetic cloud of ultralight particles would interact with ordinary matter, the study's authors predict that we may be able to detect them via their gravitational wave signature. If a black hole that plays host to one of these clouds is involved in a collision that's detected by LIGO or some future gravitational wave detector, the cloud's presence might be visible in the gravitational wave signal produced by the merger.

Another possibility would be the direct detection of gravitational waves from this oscillating cloud of particles as they orbit the black hole. Gravitational waves are only produced by asymmetrical arrangements of mass in motion, so a spherical mass rotating wouldn't produce a strong signal. Neither does a geometric arrangement like the rings of Saturn. But the moon orbiting the earth, for example, does. (Richard Feynman's "Sticky Bead" thought experiment is a great tool for developing an intuition on this.) According to the new article, some scenarios could produce a highly coherent cloud of these particles—meaning they would orbit the black hole in phase, oscillating as a large clump that should release a noticeable gravitational wave signal (especially given that these clouds could theoretically contain up to ~10% of a black hole's initial effective mass).

The paper may have implications for our study of the supermassive black holes that lie at the center of nearly every galaxy, and might serve to draw a link between them and the swaths of dark matter that seem to envelop us. Although such ultralight particles are purely hypothetical for the moment, they could share many of the properties of dark matter, which means that looking for evidence of clouds like this is one possible way to test for the existence of certain dark matter candidates.

In fact, this finding combined with the observation of fast-spinning black holes has already helped rule out certain possibilities. Astronomers have observed black holes rotating at speeds close to their maximum angular velocity, which means they're clearly not susceptible to this kind of instability, or else they'd have spun out their energy into a massive cloud and slowed down. This means that, if we see a black hole spinning as fast as possible, ultralight particles with a Compton wavelength similar to that black hole's size must not exist.

While the cloud seemed to remain stable over time in the researchers’ simulations, other possibilities exist—one of which is a bosenova—a fusion of the words boson and supernova (as well as a pun on the musical style of bossa nova). In a bosenova scenario, the massive cloud would be violently ejected from the vicinity of the black hole all at once after reaching a certain critical point.

The paper's lead author, Dr. William East of the Perimeter Institute, has videos of the study's simulations, along with a number of visually impressive videos from his other work, available on his webpage.

Stephen Skolnick

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