A black hole is capable of engulfing everything that crosses its path and does not let anything escape from its interior . Therefore, it might seem paradoxical to talk about the emission of particles from a black hole, but this has been studied by a collaboration of scientists from the universities of Amsterdam and Harvard. They have not discovered anything new, but have proposed a new mechanism by which completely new particles could form in the vicinity of a black hole and how we could use gravitational waves to study and understand them.
It has been known for some years now that a black hole can lose mass and even “evaporate”, vanish. The best-known process that steals mass from a black hole is that of Hawking radiation , which is a process by which the black hole can spontaneously emit particles . This happens because in the vicinity of the black hole, the curvature of spacetime is such that a large number of “virtual” particles can be created spontaneously.
These virtual particles, predicted by quantum field theory, would be constantly creating and annihilating themselves. However, close to a black hole it can happen that, after being created, one of the particles falls into the event horizon (the region of no return of the black hole) and the other is ejected (if it had enough energy). For all practical purposes, this will cause the black hole to lose the mass corresponding to the energy it has managed to escape .
If a black hole does not accumulate mass at a sufficient rate, it can eventually disappear due to Hawking radiation. However, a black hole the mass of the Sun would take several times the age of the universe to completely evaporate , so this could only be seen in very low-mass black holes.
Daniel Baumann ‘s team has studied the process of superradiance , which can be understood without resorting to quantum physics. This process is one of many others that can extract energy from a rotating black hole . Another, perhaps more famous, would be the one proposed by Roger Penrose by disintegration of particles within the region known as the ergosphere. Superradiance is the phenomenon that occurs when a wave (in principle of any type, but especially gravitational) strikes the black hole and is reflected and absorbed . What we would expect to happen is simply that this initial wave would be divided into two components, one for the reflected wave and one for the part absorbed by the black hole, so that the energy at the beginning would have been distributed among the final components.
When we have a rotating black hole, it can happen that the absorbed wave has “negative energy” , that is, it has the effect of reducing that rotation , so that, by conservation of energy, the reflected wave will acquire more energy than the original wave. . Basically, by impacting the wave, it would be managing to slow down the rotation and be amplified in the process . Although we have said that this phenomenon does not require quantum physics to be explained, we can go to one of its most studied systems, the atom , to find an analog that allows us to understand all this in other terms .
This superradiance can be understood as a stimulation of the black hole to emit particles, in the same way that an atom can be stimulated with light to emit more light. Well, the article by Baumann et al proposes a process similar to that of the photoelectric effect , by which an atom would emit electrons after receiving sufficiently energetic light (as in solar panels or in elevator detectors), but for the black hole . When a massive object (black hole, neutron star, etc.) orbits around a black hole, the cloud of particles formed after superradiance is capable of absorbing the orbital energy of the companion star . That is, it could affect its orbit, reducing it and gaining energy in the process. By absorbing this energy, part of the cloud would be emitted and ejected (as was the case with the electron in the atom).
This process could greatly alter the evolution of the binary system , reducing the time needed for the two objects to collide. When the second object interacts with the cloud of particles emitted in the superradiance , its orbit would be reduced in jumps , instead of slowly. In addition, the process of emission of this cloud of particles will be amplified for certain orbital distances and will depend enormously on the particle content of the cloud , so that studying in detail the gravitational waves emitted by the pair of stars while they orbit and end up colliding could give us invaluable information about the existence of new particles . This result may prove key in our search for dark matter as well as physics beyond the Standard Model of particle physics.
Reference:
D,.Baumann et al, 2022, Sharp Signals of Boson Clouds in Black Hole Binary Inspirals. Physical Review Letters, 128 (22) DOI: 10.1103/PhysRevLett.128.221102
