Black holes have long captivated the curiosity of scientists and researchers, serving as enigmatic cosmic bodies with gravity so intense that nothing, not even light, can escape their grasp. While they have been extensively studied, there is still much to uncover about the intricate nature of black holes. Recently, a team of researchers from the University of California–Santa Barbara, the University of Warsaw, and the University of Cambridge conducted a theoretical study on extremal Kerr black holes. These black holes, which are uncharged and stationary, possess unique characteristics that could potentially make them amplifiers of new and unknown physics.
The origin of the research lies in a previous project initiated during a visit to UC Santa Barbara. Maciej Kolanowski, one of the researchers involved in the study, explains that the initial discussions with Gary Horowitz (UCSB) and Jorge Santos (University of Cambridge) revealed that generic extremal black holes differ significantly from previous beliefs.
In a previous paper, Kolanowski, Horowitz, and Santos explored the effects of a cosmological constant on extremal black holes and discovered that these black holes experience infinite tidal forces. Essentially, if a living being were to fall into such a black hole, the gravitational forces would crush them long before they neared the center. However, the team also found that in scenarios where the cosmological constant is assumed to be zero, as in many astrophysical scenarios, this effect disappears.
This revelation sparked a conversation between Grant Remmen and Horowitz, leading to collaboration with Kolanowski and Santos. They set out to test the idea that higher-derivative terms in a gravitational effective field theory (EFT), also known as quantum corrections to the Einstein equations, could result in singularities on the horizon of extreme black holes.
Extremal black holes have the maximum possible rate of rotation, with their horizons moving at the speed of light. Through their calculations, the researchers discovered that the inclusion of higher-derivative EFT corrections makes the horizons of extremal black holes singular, subject to infinite tidal forces. This finding diverges from typical black holes, where finite tidal forces only reach infinity at the center.
Remmen explains that the value of the coefficients in these EFT terms, which represent the “dial settings” in the laws of physics, are influenced by the particles and couplings present at high energies and short distances. Consequently, the coefficients serve as indicators of new physics. The team also found that the strength of the tidal divergence at the horizon and the potential occurrence of tidal singularities are heavily dependent on these EFT coefficients.
Their calculations suggest that the spacetime geometry near the horizon of extremal black holes is sensitive to new physics at higher energies. Surprisingly, this sensitivity to new physics occurs even for values of the coefficients generated by the Standard Model of particle physics. Remmen emphasizes that these results challenge the notion of “decoupling” between different distance scales typically observed in physics. In the case of rapidly spinning black holes, the low-energy EFT breaks down at the horizon.
Overall, this team’s calculations highlight the potential of extremal Kerr black holes for probing new physical phenomena. While the horizon of these black holes can be significantly large, it was not expected to exhibit infinite tidal forces in the EFT. However, their findings demonstrate that this is indeed the case. Remmen expresses curiosity about the possibility of resolving these singularities through ultraviolet physics, raising the question of whether the horizon’s sensitivity to new physics extends to the Planck scale or if it smooths out at the EFT-associated short-distance scale.
The research conducted by these scientists sheds light on the remarkable nature of extremal Kerr black holes. Their unique characteristics and sensitivity to new physics present opportunities for further exploration and understanding of the universe’s intricacies. As scientists continue to delve into the complexities of black holes, new revelations are expected to emerge, expanding our knowledge of these cosmic phenomena. Through collaborative efforts and theoretical studies like this, we inch closer to unraveling the mysteries of the universe.
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