One of the most profound messages Stephen Hawking has left to humanity is that nothing lasts forever - and that scientists may finally be willing to prove it.
This idea was conveyed by what was perhaps Hawking's most important work: the hypothesis that black holes "leak" thermal radiation, vaporizing in the process and ending their existence with a final explosion. This radiation would eventually become known as "Hawking radiation" after the great scientist. However, to this day it is a concept that remains unnoticed and purely hypothetical. But now some scientists think they may have found a way to finally change that; we may soon be on our way to confirming Hawking radiation as fact.
The team suggests that when larger black holes collide and merge catastrophically, small and hot 'chunks' of black holes can be launched into space - and that could be the key.
Importantly, Hawking had said that the smaller the black hole, the faster it would leak Hawking radiation. So supermassive black holes with masses millions or billions of times that of the Sun would theoretically take longer than the predicted lifetime of the cosmos to completely "leak". In other words, how would we even detect such a massive leak in the long term? Well, maybe we can't - but when it comes to these bits of asteroid-mass black hole, called "Bocconcini di Buchi Neri" in Italian, we might be in luck.
Small black holes like this could vaporize and explode on a time scale that is indeed observable by humans. Furthermore, the end of life of these black holes should be marked by a characteristic signal, the team says, indicating that they are deflating and dying due to the leakage of Hawking radiation.
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"Hawking predicted that black holes evaporate by emitting particles," says Francesco Sannino, a scientist behind this proposal and a theoretical physicist at the University of Southern Denmark, told Space.com. "We wanted to study this, as well as the observational impact of the production of many black hole pieces, or 'Bocconcini di Buchi Neri,' which we thought would be created during a catastrophic event such as the merger of two astrophysical black holes."
Small black holes can't keep their cool
The origins of Hawking radiation date back to a 1974 letter written by Stephen Hawking entitled "Black hole explosions?" that appeared in Nature. The letter came about as Hawking considered the implications of quantum physics on the formalism of black holes, phenomena arising from Albert Einstein's general theory of relativity. This was interesting because quantum theory and general relativity are two theories that notoriously resist unification, even today.
Hawking radiation has remained disturbing and unnoticed for fifty years for two possible reasons: first, most black holes may not emit this thermal radiation at all, and second, if they do, it may not be detectable. Moreover, black holes are generally very strange objects and therefore complex to study.
"What's mind-boggling is that black holes have temperatures that are inversely proportional to their mass. This means that the more massive they are, the colder they are, and the less massive they are, the hotter they are," Sannino said.
Even in the emptiest parts of space, you'll find temperatures around minus 454 degrees Fahrenheit (minus 270 degrees Celsius). This is due to a uniform radiation field left over from just after the Big Bang, the 'cosmic microwave background' or 'CMB'. This field is also often called a 'cosmic fossil', because of how old it is. Furthermore, according to the second law of thermodynamics, heat should not be able to flow from a colder body to a warmer body.
"Black holes heavier than a few solar masses are stable because they are colder than the CMB," Sannino said. "Therefore, only smaller black holes are expected to emit Hawking radiation that could potentially be observed."
Study author Giacomo Cacciapaglia of the French National Center for Scientific Research told Space.com that because the vast majority of black holes in the contemporary universe are of astrophysical origin, with masses several times greater than that of the Sun, they have no observable can emit Hawking radiation. .
"Only black holes lighter than the moon can emit Hawking radiation. We propose that these types of black holes could be produced and ejected during a black hole merger and begin to radiate immediately after production," Cacciapaglia added. "Blocks of black holes would be produced in large numbers near a black hole merger."
However, these black holes are too small to create effects that allow them to be directly imaged, as the Event Horizon Telescope has done for supermassive black holes by focusing on the glowing material surrounding them.
The team suggests that a unique signature exists that could be used to indicate the existence of these patches of black holes. This would happen in the form of a powerful explosion of high-energy radiation, called a gamma-ray burst, taking place in the same part of the sky where a black hole merger has been detected.
The researchers said these Bocconcini di Buchi Neri black holes would emit Hawking radiation at an increasingly rapid rate as they lose mass, hastening their explosive demise. Those with masses of around 20,000 tons would take an estimated sixteen years to evaporate, while examples of chunks of black holes with masses of at least 100,000 kilotons could potentially take hundreds of years.
The evaporation and destruction of the pieces would produce photons exceeding the energy range of a trillion electron volts (TeV). To get an idea of how energetic that is, Sannino said CERN's Large Hadron Collider (LHC) in Europe, the largest particle accelerator in the world, collides protons head-on with a total energy of 13.6 TeV.
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However, the researchers do have an idea of how to detect these pieces of black holes as they evaporate. First, black hole mergers could be detected through the emission of gravitational waves, which are tiny ripples in spacetime predicted by Einstein that are emitted when the objects collide.
Astronomers could then monitor these mergers with gamma-ray telescopes, such as the High-Altitude Water Cherenkov gamma-ray observatory, which can observe photons with energies between 100 Gigaelectron volts (GeV) and 100 TeV.
The team acknowledges that there is still a long way to go before the existence of chunks of black holes can be confirmed, and thus a long way to go before we can validate Hawking radiation once and for all.
"Because this is a new idea, there is still a lot of work to be done. We plan to better model Hawking radiation emission at high energies beyond the TeV scale, where our knowledge of particle physics becomes less certain, and this will involve experimental collaborations in the search for these unique signatures within their dataset," concludes Cacciapaglia. "On a longer timeline, we plan to investigate in detail the production of chunks during catastrophic astrophysical events such as black hole mergers."
The team's research is available as a pre-print paper on the arXiv repository.
