Do Black Holes Spawn 'Crumbles'? – Colliding Black Hole Fragments Could Provide Evidence of Hawking Radiation

Cosmic debris: When black holes collide, they can release fragments of the gravitational giants—a kind of black hole “crumbs.” They could emit enough Hawking radiation in the gamma-ray range to be detectable even with current detectors, astrophysicists report. If they’re right, it would be the easiest way to directly demonstrate for the first time the radiation from black holes predicted by physicist Stephen Hawking.

In 1974, British physicist Stephen Hawking hypothesized that black holes also emit a kind of radiation. Hawking radiation, now named after him, arises because pairs of virtual particles and their antiparticles are constantly being created at the event horizon by quantum fluctuations. If one is on one side and the other on the other side of this “boundary of no return,” they do not cancel each other out and one of the particles—and the information it contains—can escape.

According to Hawking's theory, the smaller and smaller the black hole, the stronger this radiation. At the same time, Hawking radiation causes black holes to lose energy, and eventually they literally explode in an explosion. But the problem is that until now it has not been possible to directly detect this radiation emitted by black holes. Similar effects have only been observed indirectly through laboratory experiments.

Crater fragments after impact

But now astrophysicists led by Giacomo Cacciapalia of the University of Lyon have discovered a way to demonstrate Hawking radiation from black holes through astronomical observations. The starting point is the collision of black holes, which have already been detected dozens of times by the gravitational wave detectors LIGO and Virgo and Co. This merger of two gravitational giants shakes spacetime, but according to one theory, it could also release tiny “hole fragments.”

“When two black holes merge into one, regions of spacetime with strong nonlinear gravity create black hole fragments just outside the event horizon,” the researchers explain. These fragments, also known by the Italian term “Bocconcini di Buchi Neri,” correspond to small black holes. “Such black hole fragments should be produced in large numbers in the vicinity of mergers,” explains Cacciapalia.

These fragments could explain why the energy budget of merging black holes typically lacks a few solar masses of energy, as they exist in these invisible “black crumbs,” the team reported.

Release of gamma rays and neutrinos

The highlight: Because the “baby holes” formed in the collisions are relatively small, they emit powerful Hawking radiation. In model simulations, Cacciapalia and his colleagues determined that this radiation should be detectable as gamma radiation and high-energy neutrinos in the teraelectronvolt energy range. That’s within the range of the most energetic cosmic particles ever detected.

But black hole fragments can be detected by another method, astrophysicists report. Because the smaller the “black fragments,” the shorter their lifetime. According to the researchers’ calculations, small fragments with a mass of 20,000 tons explode after about 500 seconds, while heavier fragments can take decades to centuries. “Shortly before the end, the energy of the radiation and particles emitted increases and reaches a kind of explosion,” Cacciapalia and his colleagues explain.

Can they be detected by today's observatories?

But this means: “Since the visible signals of black hole annihilation also include photons in the gamma-ray range, they provide an opportunity for high-energy observatories such as HESS, HWAC and LHAASO,” the team wrote. These detectors are designed to capture particularly high-energy cosmic rays and particles. The final explosion of the destroyed “black hole clump” should therefore appear in these telescopes as short bursts of gamma rays in the range of several teraelectronvolts.

In fact, LHAASO recently published a catalogue of such multi-electronvolt events, some of which cannot be attributed to any known source. But this could be precisely the radiation that comes from the annihilation of fragments of black hole collisions: “If these fragments had masses greater than a million tons, they would have taken years to decades to finally be annihilated,” Cacciapalia and his team explain.

This explains why these events cannot yet be attributed to black hole collisions detected using gravitational waves: the LIGO/Virgo catalog simply does not go back far enough.

Direct evidence of Hawking radiation soon?

But that could change in the near future, as the astrophysicists explain. “We have shown that it is possible to observe Hawking radiation that can be emitted by black hole fragments after collisions,” say Cacciapalia and his colleagues. “Gamma-ray bursts are unique footprints of black hole fragments that are no larger than an asteroid.”

This will be the moment when such “crumbs” from black holes and at the same time Hawking radiation, which was theoretically postulated 50 years ago, can be directly demonstrated for the first time. (Preprint, arXiv, doi: 10.48550/arXiv.2405.12880)

Source: arXiv

July 3, 2024 – Nadia Podpregar