Detecting ‘Hawking radiation’ from black holes using current telescopes

In 1974, Stephen Hawking famously claimed that black holes must both emit and absorb particles. This so-called ‘Hawking radiation’ has not yet been observed, but now a research group from Europe has discovered that Hawking radiation should be observable by existing telescopes that are able to detect very high-energy light particles.

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

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

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

Heavier pieces will initially produce 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 an open source public code called BlackHawk, which calculates the Hawking evaporation spectra for each distribution of black holes, that the Hawking radiation from the black hole pieces produces gamma-ray bursts that have a characteristic fingerprint . The work has been published on the arXiv preprint server.

Detecting such events, which have multiple signals – gravitational waves, electromagnetic radiation, neutrino emissions – is called multimessenger astronomy in the astrophysical community and is part of the observing programs of the LIGO gravitational wave detectors in the US, VIRGO in Italy and, in Japan, the KAGRA gravitational wave telescope.

Visible signals of black hole evaporation always include photons above the TeV range (one trillion electron volts, about 0.2 microjoules). For example, the CERN Large Hadron Collider in Europe, the largest particle accelerator in the world, 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 Cherenkov telescopes in the atmosphere to detect this Hawking radiation.

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

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

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

Detecting such Hawking radiation would also shed light (ahem…) on the production of black hole chunks, as well as on the production of particles with energies higher than can be achieved on Earth, and could hold signs of new physics such as supersymmetry, extra dimensions, or the existence of composite particles based on the strong force.

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

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

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