Hawking radiation: researchers finally think they know how to detect and confirm it

In 1974, the physicist Stephen Hawking, while working on the thermodynamics of black holes , highlighted a theoretical process taking place on the edge of the event horizon: the radiation of Hawking . In the environment of a black hole , the gravitational field is so intense that it separates the pairs of particle-antiparticle resulting from the quantum fluctuations of the vacuum, one particle being absorbed and the other re-emitted. Possessing the electromagnetic characteristic of black body radiation, this radiation is still entirely theoretical. However, the gravitational data from the GW170817 event could contain indications of its presence.

The existence of this radiation would mean that black holes evaporate slowly, resolving the information paradox. But, just like gravitational waves until just a few years ago, it is too weak to be detected by our current instruments.

However, two cosmologists have recently shown that gravitational waves could contain echoes of Hawking radiation. the study was published in the Journal of Cosmology and Astroparticle Physics.

Echoes of Hawking radiation in gravitational waves

The analogs of black holes developed in the laboratory seem to suggest that the Hawking radiation is real . But gravitational waves could play a role in the process. Because if Hawking's radiation is real, there should be a quantum “blur” around the outside of the event horizon of a black hole. And this “quantum fog” should produce an echo in the generated gravitational waves.

Spatiotemporal representation of the echoes of the gravitational waves of a membrane on the stretched horizon, following the gravitational collapse coming from a fusion of binary neutron stars . Credits: Jahed Abedi and Niayesh Afshordi

“Scientists have not been able to determine experimentally whether a material escapes from black holes until the very recent detection of gravitational waves. If the quantum blur responsible for the Hawking radiation exists around the black holes, the gravitational waves could bounce on it, which would create smaller gravitational wave signals after the main gravitational collision event, similar to repeated echoes.” Says Niayesh Ashfordi, astrophysicist at the University of Waterloo.

Potential echo cues to take with caution

This is what Afshordi and his colleague, the cosmologist Jahed Abedi of the Max Planck Institute for Gravitational Physics in Germany, think they could have detected in the gravitational data. Their results, they say, correspond to simulated echoes predicted by fuzzy black hole models emitting Hawking radiation. But there are some precautions to take.

Representation of the amplitude-frequency of the first echo peak 1 second after the fusion of the two Hawkings radiation stars (GW170817). The blue area between 63 Hz and 92 Hz is the frequency range in which an echo peak was most likely to occur. Credits: Jahed Abedi and Niayesh Afshordi

For one thing, an analysis last year of the gravitational wave data from GW150914 found no evidence of Hawking radiation. Additionally, another study from last year included a concerted analysis of all gravitational wave signals collected to date, looking for evidence of gravitational wave echoes and, by extension, Hawking radiation. The authors found no statistically significant evidence of echoes.

But it is quite possible, in fact, that our instruments are still not sensitive enough to detect Hawking radiation. And Afshordi recognizes that the signal detected by the team could in reality simply be a spurious noise in the data. The way to find out would be to look for similar signals in other gravitational wave data sets.


Echoes from the Abyss: A highly spinning black hole remnant for the binary neutron star merger GW170817

Jahed Abedi, Niayesh Afshordi


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