Detecting “Hawking radiation” from black holes using current telescopes

In 1974, Stephen Hawking claimed that black holes must not only emit particles, but also absorb them. This so-called “Hawking radiation” has not yet been observed, but a European research group has discovered that Hawking radiation should be observable by existing telescopes, capable of detecting very high-energy light particles.

When two massive black holes collide and merge, or a neutron star and a black hole do so, they emit gravitational waves, ripples in the fabric of space-time that propagate outward. Some of these waves break onto Earth millions or billions of years later. These waves were predicted by Einstein in 1916 and first observed directly by LIGO detectors in 2016. Dozens of gravitational waves from black hole mergers have been detected since then.

These mergers also emit a number of “black hole pieces”, smaller black holes with masses on the order of an asteroid, created in the resulting extremely strong gravitational field around the merger due to the effects called “non-linear” at high speed in general. relativity. These nonlinearities arise due to the inherently complex solutions of Einstein’s equations, as distorted spacetime and masses reflect back to each other and both react and create new spacetimes and masses.

This complexity also generates gamma bursts of extremely energetic photons. These bursts have similar characteristics, with a delay from merger of the order of their evaporation time. A piece of mass 20 kilotons has an evaporation life of 16 years, but this number can change dramatically since the evaporation time is proportional to the mass of the piece cubed.

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

The research team showed, through numerical calculations using public open source code called BlackHawk that calculates Hawking evaporation spectra for any black hole distribution, that Hawking radiation from black hole pieces creates gamma-ray bursts that have a distinctive fingerprint. The work is published on the arXiv preprint server.

The detection of such events, which have multiple signals (gravitational waves, electromagnetic radiation, neutrino emissions) is called multimessenger astronomy in the astrophysics community and is part of the LIGO gravitational wave detector observing programs in the United States, VIRGO in Italy and, in Italy. Japan, the KAGRA gravitational wave telescope.

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

These Cherenkov telescopes are ground-based parabolic antennas capable of detecting very energetic photons (gamma rays) in the energy range of 50 GeV (billion electron volts) to 50 TeV. These antennas achieve this by detecting flashes of Cherenkov radiation produced when gamma rays cascade through Earth’s atmosphere, traveling faster than the ordinary speed of light waves in air.

Remember that light travels slightly slower in air than in a vacuum because air has a refractive index slightly greater than one. Hawking gamma radiation cascading down through the atmosphere exceeds this slower value, creating Cerenkov radiation (also called bremsstrahlung – Bremsstrahlung in German). The blue light observed in puddles of water surrounding the reaction rods of a nuclear reactor is an example of Cerenkov radiation.

There are now four telescopes capable of detecting these cascades of Cerenkov 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 Cherenkov Telescope G-APD (FACT), also on the island of La Palma in the Canary Islands, and Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona. Although each uses different technology, they can all detect Cerenkov photons in the GeV-TeV energy range.

The detection of such Hawking radiation would also shed light (ahem…) on the production of pieces of black holes, as well as on the production of particles at energies higher than those achievable on Earth, and could carry signs of new physics such as supersymmetry, additional dimensions or the existence of composite particles based on the strong force.

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

Cacciapaglia said they plan to contact colleagues in the experimental groups and then use the collected data to research the Hawking radiation they propose.

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