Black holes are areas of space with incredibly powerful gravity that are impenetrable to all matter and electromagnetic waves. The delicate physical details of these fascinating cosmic bodies have been the subject of innumerable study projects, yet they have not yet been fully understood.

Recent theoretical research on the extremal Kerr black hole class, which consists of stationary, uncharged black holes with converging inner and outer horizons, was conducted by scientists at the Universities of California-Santa Barbara, Warsaw, and Cambridge. Their research, which was released in Physical Review Letters, demonstrates how the special properties of these black holes may make them the ideal “amplifiers” of novel, undiscovered physics.

According to Maciej Kolanowski, one of the researchers who conducted the study, “This research has its origin in a previous project started during my visit to UC Santa Barbara.” “I began talking about extremely cold black holes with Jorge Santos at Cambridge and Gary Horowitz at UCSB. We soon discovered that contrary to what was initially thought, generic extremal black holes actually look extremely different.

Kolanowski, Horowitz, and Santos demonstrated in their earlier study that extremal black holes are subject to infinite tidal forces in the presence of a cosmological constant. In other words, if a live thing fell into the black hole, gravity would smash it before it could even approach the black hole’s center. However, the researchers demonstrated that this effect disappears if the cosmological constant is zero, as it is in many astrophysical settings.

Grant Remmen stated that the idea for the current study “sparked at UC Santa Barbara’s weekly Gravity Lunch.” “After hearing Horowitz speak about his research on black hole boundary singularities, I struck up a conversation with him to find out if there were any alternative causes for such phenomena. I had an idea thanks to my earlier work on effective field theories (EFTs), specifically the creation of physics models with quantum corrections. I questioned Horowitz about the possibility of singularities on the horizons of extreme black holes being caused by the higher-derivative terms in a gravitational EFT (i.e., quantum corrections to the Einstein equations).

Horowitz and Remmen began working together with Kolanowski and Santos to test this theory through a series of computations after Remmen communicated his hypothesis with them. The researchers took into account Einstein’s gravity coupled to its leading quantum corrections in their computations.

The Riemann tensor, a mathematical construct that describes the curvature of spacetime, is linear in the Einstein equations, according to Remmen. The primary Einstein corrections in three spatial dimensions are terms with cubic (third power) and quartic (fourth power) curvatures. Such terms are referred to as “higher-derivative terms” since curvature is a gauge of derivatives of the spacetime geometry. We evaluated how these higher-derivative terms would affect fast-rotating black holes.