Browsing by Author "Duztas, Koray"

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Hawking radiation at the zero temperature limit
(Springer, 2024) Duztas, Koray
We show that the thermal radiation derived by Hawking can be smoothly extended to the T=0 limit for Kerr black holes. The emission of the modes with omega>m Omega comes to a halt as the surface gravity vanishes. However, Kerr black holes smoothly continue to radiate both in bosonic and fermionic modes with omega 0, which strictly differentiates it from the non-thermal radiation of soft particles by extremal Kerr black holes. We also note that the thermal radiation at the zero limit, drives the black hole away from extremality in accord with the third law and the cosmic censorship conjecture.
Second Order Variations and the Overspinning Problem in Kerr-Taub-NUT Spacetime
(Amer Physical Soc, 2025) Duztas, Koray
Previously, we attempted to overspin Kerr-Taub-NUT (Kerr-Taub-Newman-Unti-Tamburino) black holes with test fields satisfying the null energy condition. We derived that a nongeneric overspinning could be achieved with strict restrictions on the test field modes and the initial spacetime parameters. Here, we incorporate the effect of the second order variations by evaluating the Sorce-Wald condition for Kerr-Taub-NUT black holes. We calculate the second variation of the area for the subcollection of Kerr-Taub-NUT black holes for which overspinning is possible due to first order perturbations. We carry out an analysis to second order, avoiding the common order of magnitude problems where the second order variations appear in the form of a product with the square of an additional small parameter. We derive that the contribution of the second order variations fix the final state of the spacetime parameters driving the Kerr-Taub-NUT black hole away from extremality. The event horizon is preserved in accord with the cosmic censorship conjecture and the laws of black hole dynamics.
Self Energy in Kerr-Sen Space-Time Incorporated to Overspinning and Overcharging Problems
(Springer Heidelberg, 2025) Duztas, Koray
Previous attempts to overspin or overcharge Kerr-Sen black holes have been incomplete, as they neglect back-reaction effects. In this study, we incorporate the self-energy of perturbations, resulting from the induced increase in the angular velocity and the electrostatic potential of the event horizon, as described in a seminal work by Will. Our analysis demonstrates that Kerr-Sen black holes cannot be overspun or overcharged by perturbations that satisfy the null energy condition, within a comprehensive second-order framework that includes the self-energy of the perturbations.