Detecting ‘Hawking radiation’ from black holes using today’s telescopes

In 1974, Stephen Hawking famously stated that black holes should emit and absorb particles at the same time. This so-called “Hawking radiation” has not been observed before, but now a research group from Europe has found that Hawking radiation should be observable with existing telescopes that are able to detect particles of light with very high energy.

When two massive black holes collide and merge, or when a neutron star and a black hole do, they emit gravitational waves, ripples in the fabric of spacetime that travel outward. Some of these waves will flood Earth millions or billions of years later. These waves were predicted by Einstein in 1916 and first directly observed by LIGO detectors in 2016. Since then, dozens of gravitational waves from black hole mergers have been detected.

These mergers also emit a number of “black hole chunks”, smaller black holes on the order of an asteroid’s mass, created in the resulting extremely strong gravitational field around the merger due to so-called “non-linear” high-velocity effects in general. relativity. These nonlinearities arise because of the inherently complex solutions to Einstein’s equations, as the warped feedback of spacetime and matter reacts on each other to create new spacetime and matter.

This complexity also generates gamma-ray bursts of extremely energetic photons. These bursts have similar characteristics, with a time delay from merging on the order of their evaporation time. A bite mass of 20 kilotons has an evaporation lifetime of 16 years, but this number can change drastically because the evaporation time is proportional to the cube of the bite mass.

Heavier chunks will initially provide a steady gamma-ray burst signal, characterized by reduced particle energy proportional to the Hawking temperature. The Hawking temperature is inversely proportional to the mass of the black hole.

Using numerical calculations using a public, open-source code called BlackHawk, which calculates Hawking evaporation spectra for any distribution of black holes, the research team showed that Hawking radiation from bits of black holes produces bursts of gamma rays that have a characteristic fingerprint. The work is published on arXiv prepress server.

The detection of such events that have multiple signals – gravitational waves, electromagnetic radiation, neutrino emissions – is called multimessenger astronomy in the astrophysical community and is part of the observation programs at the LIGO gravitational wave detectors in the USA, VIRGO in Italy and in Japan, the KAGRA gravitational wave telescope.

Visible signals from black hole evaporation always involve photons above the TeV range (a trillion electron volts, about 0.2 microjoules; for example, the CERN Large Hadron Collider in Europe, the largest particle accelerator on the planet, collides protons head-on with a total energy of 13.6 TeV). This provides a “golden opportunity,” the group writes, for so-called high-energy atmospheric Cherenkov telescopes to detect this Hawking radiation.

These Cherenkov telescopes are ground-based antenna dishes that can detect very energetic photons (gamma rays) in the energy range of 50 GeV (billion electron volts) to 50 TeV. These antennas accomplish this by detecting flashes of Cherenkov radiation, which are produced when gamma rays cascade through Earth’s atmosphere and travel faster than the normal speed of light waves in air.

Recall that light travels slightly slower in air than in a vacuum because air has an index of refraction slightly greater than one. Hawking gamma rays, which cascade through the atmosphere, exceed this slower value and create Cherenkov radiation (also called bremsstrahlung – German Bremsstrahlung). The blue light seen in the pools of water that surround the reaction rods in a nuclear reactor is an example of Cherenkov radiation.

There are now four telescopes that can detect these cascades of Cherenkov radiation – the High Energy Stereoscopic System (HESS) in Namibia, the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) on one of the Canary Islands, the First G-APD Cherenkov Telescope (FACT), also on La Palma Island in the Canary Islands and the Very Energetic Radiation Imaging Telescope System (VERITAS) in Arizona. Although each uses a different technology, all can detect Cerenkov photons in the GeV-TeV energy range.

The detection of such Hawking radiation would also shed light (ahem…) on the production of chunks of black holes, as well as the production of particles at energies higher than can be achieved on Earth, and may hold signs of new physics such as supersymmetry, extra dimensions or the existence of composite particles based on strong power.

“It was a surprise to find that bites of black holes can radiate beyond the detection capabilities of current high-energy Cherenkov telescopes on Earth,” said lead author Giacomo Cacciapaglia from Université Lyon Claude Bernard 1 in Lyon, France. Noting that the direct detection of Hawking radiation from bits of black holes would be the first evidence of the quantum behavior of black holes, he said, “If the proposed signal is observed, we will have to challenge the current knowledge of the nature of black holes” and the production of the bite.

Cacciapaglia said they plan to contact colleagues from the experimental groups and then use the collected data to search for Hawking radiation, which they propose.

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