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# Black holes go bosenova: researchers investigate effects of superradiance on runaway bosonic field growth around black holes

John Wheeler, one of the preeminent physicist of the 20^{th} century whose work advanced the fields of general relativity, quantum mechanics, and the union of both in the study of black holes – once remarked that “a black hole has no hair”. John Wheeler had a knack for coining such memorable and somewhat quizzical statements that would leave an indelible impact on physics. Indeed, this idea became known as the “no-hair theorem”, which is to say that black holes, like elementary particles, have three properties: mass, spin, and charge. According to conventional theory, once a black hole is formed, whatever properties might have been attributed to the “stuff” that formed it – for which hair is a metaphor -- are completely erased and reduced to mass, angular momentum, and charge.

As the study and elucidation of black hole physics has advanced, the no-hair theorem has come under increasing scrutiny. In the 1970's physicist Stephen Hawking looked at the effect of the strong gravitational curvature of a black hole on the quantum field structure near the event horizon and determined that it should cause the radiation of particles from the vacuum. The emitted particles would carry energy away from the black hole, and hence slowly evaporate its mass. What ensued was one of the greatest debates in physics, does this radiated vacuum energy, somehow entangled with the trans-horizon area, carry away information of what is inside the black hole? If not, what happens to all that information that once characterized the matter that created the black hole? Quantum theory stipulates that information must be conserved, so is information leaking out of the black hole as well as mass? In other words, does the black hole have hair?

Physicist Nassim Haramein offered a solution to this seeming paradox with the elucidation of an entirely new idea termed the Black Whole – where the conventional dynamics of a black hole are balanced with that of a white hole, so that there is a continual equilibrium flow of information across the event horizon. Just as quantum vacuum fluctuations have been shown to carry mass-energy away from black holes, it has also been shown how particular couplings of vacuum structure can contribute to black hole growth. While the mainstream seemed to leave Haramein's solution unacknowledged, it would later be independently seized upon in the work of Leonard Susskind, professor of theoretical physics at Stanford University, to resolve the AMPS firewall paradox. See our article Firewalls or Cool Horizons for more.

With the detection of gravitational waves, the question is moving from purely theoretical work to actual empirical evaluation. Analysis of the signal of gravitational waves from the merger of black holes may contain data on whether the event horizon is smooth or turbulent, which is to say whether or not black holes have hair.

In a new study using simulations of black hole dynamics, William East from the Perimeter Institute for Theoretical Physics, Canada, and Frans Pretorius from Princeton University have evaluated the possibility of an exotic phenomenon around black holes in which they may spontaneously grow “long hair”. Through an effect related to the superfluid property of space, also known as wave dark matter, the angular momentum of black holes might couple with bosonic fields and exponentially amplify the energy through superradiance. Such an effect could transfer up to 9% of a black hole’s mass to the ergosphere region outside of the event horizon, comparable to the Penrose process, theorized by Roger Penrose whereby energy can be extracted from the angular momentum of black holes. Note that in this scenario the black hole essentially forms a gravitational atom, where axions or other quanta of bosonic fields play the role of electrons in the outer orbital and the black hole is the nucleus.

Calculations show that the run-away amplification of the bosonic field around a black hole may result in a truly remarkable phenomenon, known as a black hole bomb, analogous to the bosenova observed in experiments of Bose-Einstein condensates. Essentially, the bosonic field around the black hole grows with so much energy (from non-linear interactions) that eventually it becomes unstable, collapses in on itself, and explodes – reminiscent of the effect of stellar collapse leading to a supernova, hence the play on words ‘bosenova’. What’s exciting about this latest theoretical suggestion is that it is fully testable – if black holes are creating non-linear Bose-Einstein condensates around them that eventually go nova then the light from such explosions can be detected and analyzed. Successful identification of such an event can potentially provide observational data to help resolve the no-hair debate, and hence a major outstanding question in the quantum structure of black holes.

However, East and Pretorius’ work seems to suggest that the so-called black hole bomb may not actually be explosive – that instead of being a non-linear, or exponential runaway process a black hole will “grow hair” in a smooth and controlled manner, resulting in a black hole embedded in a massive bosonic BEC field, orbiting at exactly the same angular frequency as the black hole horizon. This leaves the detection of such effects to the decoding of gravitational waves, which may contain telltale signs of quantum structure around black holes – invaluable experimental data to advance unified physics.

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