One of the most profound messages Stephen Hawking left for humanity is that nothing lasts forever – and scientists might finally be ready to prove it.
This idea was conveyed by what was arguably Hawking’s most important work: the hypothesis that black holes “leak” thermal radiation, evaporating in the process and ending their existence by final explosion. This radiation would eventually become known as “Hawking radiation,” after the great scientist. But to this day, it is a concept that remains undetectable and purely hypothetical. But now, some scientists think they may have found a way to finally change that; perhaps we will soon be well on our way to cementing Hawking’s influence as fact.
The team suggests that when larger black holes collide and merge catastrophically, tiny hot “chunks” could be launched into space – and that could be the key.
It is important to note that Hawking had stated that the smaller the black hole, the faster it will scatter Hawking radiation. Thus, supermassive black holes with masses millions or billions of times that of the Sun would theoretically take longer than the expected lifetime of the cosmos to completely “escape.” In other words, how could we even detect leaks over such a long term? Well, maybe we can’t – but when it comes to these chunks of asteroid-mass black holes, dubbed “Bocconcini di Buchi Neri” in Italian, we might be in luck.
Tiny black holes like these could evaporate and explode on a time scale actually observable by humans. Additionally, the end of these black holes’ lives should be marked by a characteristic signal, the team says, which indicates their deflation and death via the escape of Hawking radiation.
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“Hawking predicted that black holes evaporate by emitting particles”, Francesco Sannino, a scientist behind the proposal and a theoretical physicist at the University of Southern Denmark told Space.com. “We decided to study this and the observational impact of producing many pieces of black holes, or ‘Bocconcini di Buchi Neri,’ which we imagined forming during a catastrophic event such as the merger of two black holes astrophysics.”
Little black holes can’t keep their cool
The origin of the Hawking radiation dates back to a 1974 letter by Stephen Hawking titled “Black Hole Explosions? » which was published in Nature. The letter arose when Hawking considered the implications of quantum physics for the formalism of black holes, a phenomenon arising from Albert Einstein’s theory of general relativity. This was interesting because quantum theory and general relativity are two theories that notoriously resist unification, even today.
Hawking radiation has remained worryingly undetected for 50 years now for two possible reasons: first, most black holes might not emit this thermal radiation at all, and second, if they do, it might not be detectable. Additionally, in general, black holes are very strange objects to begin with and therefore complex to study.
“The mind-blowing thing is that black holes have temperatures inversely proportional to their mass. That means 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 regions of space, you’ll find temperatures around minus 454 degrees Fahrenheit (minus 270 degrees Celsius). This is due to a uniform field of radiation left just after the Big Bang, called the cosmic microwave background or CMB. This field is also often called a “cosmic fossil”, due to its advanced age. Additionally, 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 cooler than the CMB,” Sannino said. “Therefore, only smaller black holes should emit Hawking radiation that could potentially be observed.”
Research author Giacomo Cacciapaglia of the French National Center for Scientific Research told Space.com that because the vast majority of black holes in the current universe are of astrophysical origin, with masses exceeding that many times from the sun, they cannot emit observable Hawking radiation. .
“Only black holes lighter than the Moon can emit Hawking radiation. We propose that this type of black hole can be produced and ejected during a black hole merger and start radiating right after its production,” added Cacciapaglia . “Pieces of black holes would be produced in large numbers near a black hole merger.”
However, these black holes are too small to create effects to view them directly, as the Event Horizon Telescope did for supermassive black holes by focusing on the bright material around them.
The team suggests that there is a unique signature that could be used to indicate the existence of these black hole pieces. This would appear as a powerful burst of high-energy radiation called a gamma-ray burst occurring in the same region of the sky where a black hole merger was detected.
The researchers said these Bocconcini di Buchi Neri black holes would emit Hawking radiation faster and faster as they lost mass, accelerating their explosive demise. Those with masses of around 20,000 tonnes would take around 16 years to evaporate, while examples of chunk black holes with masses of at least 100,000 kilotons could last up to hundreds of years.
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 much energy that represents, Sannino explained that the Large Hadron Collider (LHC) at CERN in Europe, the largest particle accelerator on the planet, collides head-on with protons with total energy of 13.6 TeV.
However, researchers have an idea of how to detect these pieces of black holes as they evaporate. First, black hole mergers could be detected via the emission of gravitational waves, which are tiny ripples in space-time predicted by Einstein, emitted when objects collide.
Astronomers could then follow up on these mergers with gamma-ray telescopes, such as the high-altitude Cherenkov Gamma-ray Observatory in water, capable of detecting photons with energies between 100 gigaelectronvolts (GeV) and 100 TeV.
The team recognizes that there is a long way to go before the existence of small black holes can be confirmed, and therefore a long way to go before Hawking radiation can be validated once and for all.
“As this is a new idea, there is a lot of work to be done. We plan to better model the emission of Hawking radiation at high energies beyond the TeV scale, where our knowledge in particle physics become less certain, and this will involve experimental collaborations in the search for these unique signatures within their data set,” concluded Cacciapaglia. “Over a longer period, we plan to study in detail the production of chunks during catastrophic astrophysical events like black hole mergers.”
The team’s research is available as a pre-print article on the arXiv repository.