Black holes are regions of space characterized by extremely strong gravity, preventing all matter and electromagnetic waves from escaping. These fascinating cosmic bodies are the subject of countless studies, but the nuances of their complex physics remain unexplored. Researchers from the University of California, Santa Barbara, the University of Warsaw, and the University of Cambridge recently conducted a theoretical study focusing on a type of black hole called extreme Kerr black holes, which are Stationary electric charge has overlapping inner and outer layers. outer horizon. Their paper, published in the journal Physical Review Letters, shows that the unique characteristics of these black holes could make them ideal “amplifiers” for new and unknown physics. “This research has its origins in a previous project that was initiated during my visit to UC Santa Barbara,” Maciej Kolanowski, one of the researchers who led the study, told Phys.org. “I started discussing very cold (so called, extremal) black holes with Gary Horowitz (UCSB) and Jorge Santos (at Cambridge). Soon we realized that in fact, generic extremal black holes look very different than it was previously believed.”

In their previous paper, Kolanowski, Horowitz and Santos showed that in the presence of the cosmological constant, extreme black holes are influenced by infinite tidal forces. This means that if a living organism falls into a black hole, it will be crushed by gravity before it gets close to the center of the black hole. However, the team showed that if the cosmological constant is zero, as is the case in many astrophysical scenarios, this effect disappears. “The spark for the current paper was born at UC Santa Barbara’s weekly Gravity Lunch,” Grant Remmen explains. “When speaking with Horowitz after a conference on his work on singularities on black hole horizons, I asked him whether other effects could give rise to such phenomena. My previous work on effective field theory (EFT), including developing physical models with quantum corrections, gave me an idea. In speaking with Horowitz, I wondered whether the higher derivative terms of the gravitational EFT (i.e. the quantum corrections to Einstein’s equations) might themselves lead to singularities at the hole horizon black or not.

After Remmen 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 considered Einstein’s gravity combined with its key quantum corrections. “Einstein’s equations are linear in the Riemann tensor, a mathematical object that describes the curvature of space-time,” Remmen explains. “In all three dimensions of space, Einstein’s main corrections were the cubic (third power) and quartic (fourth power) terms for curvature. Since curvature is a measure of the derivative of spacetime geometry, these terms are called “higher terms”. We calculated the impact of these higher derivative terms on rapidly spinning black holes. »

Extremely massive black holes rotate at the maximum possible speed corresponding to the movement of the horizon at the speed of light. The researchers’ calculations show that higher derivative EFT corrections for extreme black holes make their horizons single, with infinite tidal forces. This is in stark contrast to typical black holes, which have finite tidal forces and only become infinite at the center of the black hole. “Surprisingly, the EFT corrections make the singularity jump from the center of the black hole to the horizon, where you wouldn’t expect it,” Remmen said. “The value of the coefficient preceding a given EFT term – the “dial setting” in the laws of physics – is determined by the couplings and types of particles present at high energies and short distances. In this sense, the EFT coefficients are very sensitive to new physical properties. » Kolanowski, Horowitz, Remmen and Santos also found that the strength of tidal divergence at the horizon of extreme black holes and the likelihood of a tidal singularity strongly depend on the EFT coefficient. Their calculations therefore suggest that the near-horizon space-time geometry of these black holes is sensitive to new physics at higher energies.

“Interestingly, this unexpected singularity is present for the values ​​of these EFT coefficients generated by the Standard Model of particle physics,” Remmen said. “Our results are surprising, because they imply that the description of low-energy physics can break down in situations where one would not expect this to happen. In physics, there is generally a sense of “separation” between different distance scales. For example, you don’t need to know details about water molecules to describe waves using hydrodynamics. However, for rapidly spinning black holes, this is exactly what is happening: low-energy EFTs are appearing on the horizon. Overall, the calculations performed by this research group show the promise of extremely powerful Kerr black holes for probing new physical phenomena. Although the horizon of these black holes may be very wide, it is not predicted to have infinite curvature (i.e. infinite tidal force) in the EFT. Their results showed this to be the case.

Source: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.091402