Black holes are regions of space that have extremely strong gravity, preventing all matter and electromagnetic waves from escaping. These fascinating cosmic objects have been the focus of countless research studies, yet their complex physical nuances have yet to be fully revealed.
Researchers at the University of California-Santa Barbara, the University of Warsaw, and the University of Cambridge recently conducted a theoretical study focusing on a class of black holes known as extreme Kerr black holes, which are stationary, uncharged black holes with simultaneous inner and outer horizons. Their paper published in Physical review lettersIt shows that the unique properties of these black holes could make them ideal “amplifiers” for new, unknown physics.
“This research has its origins in A Previous project “It started while I was visiting UC Santa Barbara,” Maciej Kolanowski, one of the researchers who conducted the study, told Phys.org. “I began discussing ultracold (so-called extreme) black holes with Gary Horowitz (UCSB) and Jorge Santos (at Cambridge). We soon realized that general extreme black holes actually look very different from what was previously thought.”
In their previous paper, Kolanowski, Horowitz, and Santos showed that in the presence of a cosmological constant, extreme black holes are affected by infinite tidal forces. This means that if living organisms fell into the black hole, they would be crushed by gravity before they moved even close to the center of the black hole. However, the team showed that if the cosmological constant is zero, as assumed in many astrophysical scenarios, this effect disappears.
“The spark for the current paper was sparked at a weekly Gravity Luncheon organized by UC Santa Barbara,” Grant Remen explained. “While chatting with Horowitz after talking about his work on black hole horizon singularities, I wondered whether other effects could lead to such phenomena. My previous work in effective field theories (EFTs), especially developing physical models with quantum corrections, gave me the idea While talking with Horowitz, I wondered whether higher-derivative terms in the gravitational electron transform (i.e., quantum corrections to Einstein’s equations) could themselves lead to singularities in the horizons of extreme black holes.
After Remen shared his idea with Horowitz, they began collaborating with Kolanowski and Santos, aiming to test the idea through a series of calculations. In their calculations, the researchers took into account Einstein’s gravity combined with its pioneering quantum corrections.
“Einstein’s equations are linearized in the Riemann tensor, which is a mathematical object that describes the curvature of space-time,” Riemen explained. ‘In three space dimensions, Einstein’s main corrections are the cubic (third power) and quadratic (fourth power) terms in curvature. Since curvature is a measure of the derivatives of the geometry of spacetime, these terms are called ‘higher derivative’ terms.’ “We calculated the effect of these higher-derivative terms on rapidly rotating black holes.”
Extreme black holes spin at the maximum possible rate corresponding to a horizon moving at the speed of light. The researchers’ calculations showed that the higher-derivative EFT corrections of extreme black holes make their horizons singular, with infinite tidal forces. This is in stark contrast to typical black holes, which have finite tidal forces that become infinite only at the center of the black hole.
“Surprisingly, the EFT corrections make the singularity jump all the way from the center of the black hole to the horizon, where you wouldn’t expect it to be,” Remen said. “The value of the coefficient in front of a specific EFT term – the ‘dial settings’ in the laws of physics – is dictated by the couplings and the types of particles present at high energies and short distances. In this sense, EFTs are sensitive to new physics.”
Kolanowski, Horwitz, Riemen, and Santos also found that the strength of tidal divergence at the horizon of extreme black holes, and the probability of a tidal singularity occurring, depends strongly on EFT parameters. Thus, the results of their calculations indicate that the geometry of spacetime near the horizon of these black holes is sensitive to new physics at higher energies.
“Interestingly, this unexpected singularity is present in the values of the electron transfer coefficients (EFT) generated by the Standard Model of particle physics,” Remen said.
“Our results are surprising, because they imply that the low-energy description of physics can break down in a situation where you wouldn’t expect it to happen. In physics, there is usually a sense of ‘separation’ between different distance scales.” For example, you don’t need To know the details of water molecules to describe waves using hydrodynamics. However, for rapidly rotating black holes, this is exactly what happens: low-energy EFT collapses on the horizon.”
Overall, the calculations performed by this team of researchers indicate the promise of extreme Kerr black holes for exploring new physical phenomena. While the horizons of these black holes could be very large, they were not expected to have infinitely large curvature (i.e. infinite tidal forces) in EFT. Their results show that this is so.
“In future work, we are interested in exploring whether the singularities can be resolved by ultraviolet physics,” Reimen added. “The pressing question is whether the horizon’s sensitivity to new physics continues up to the Planck scale, or whether the horizon ‘flattens out’ on the short-distance scale associated with EFT. We are also looking for other situations in which short-distance effects might appear unexpectedly at short-distance scales. big.”
Gary T. Horowitz et al., Extreme black holes as amplifiers for new physics, Physical review letters (2023). doi: 10.1103/PhysRevLett.131.091402
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