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DÜZTAŞ, Koray

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Now showing 1 - 7 of 7
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    Testing cosmic censorship conjecture for extremal and near-extremal (2 + 1)-dimensional MTZ black holes
    (IOP Publishing, 2020-09-03) Düztaş, Koray; Jamil, M.; Shaymatov, S.; Ahmedov, B.; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    We test the validity of the weak cosmic censorship conjecture for the (2 + 1)-dimensional charged anti-de Sitter black hole solution, which was derived by Martinezet al(MTZ). We first construct a thought experiment by throwing test charged particles on an extremal MTZ black hole. We derive that extremal (2 + 1) dimensional black holes can be overcharged by test particles, unlike their analogues in (3 + 1) and higher dimensions. Nearly-extremal MTZ black holes can also be overcharged, by a judicious choice of energy and charge for the test particles when the second order effects are ignored. We also incorporate the second order effects for nearly extremal MTZ black holes by adapting the method developed by Sorce and Wald. However it turns out that the second order effects cannot compensate for the overcharging of MTZ black holes.
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    ArticlePublicationOpen Access
    Overspinning Kerr-MOG black holes by test fields and the third law of black hole dynamics
    (Springer Nature, 2020-01-11) Düztaş, Koray; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    We evaluate the validity of the weak form of the cosmic censorship conjecture and the third law of black hole dynamics for Kerr-MOG black holes interacting with test scalar fields. Ignoring backreaction effects, we first show that both extremal and nearly extremal Kerr-MOG black holes can be overspun into naked singularities by test fields with a frequency slightly above the superradiance limit. In addition, nearly extremal Kerr-MOG black holes can be continuously driven to extremality by test fields. Next, we employ backreaction effects based on the argument that the angular velocity of the event horizon increases before the absorption of the test field. Incorporating the backreaction effects, we derive that the weak form of the cosmic censorship and the third law are both valid for Kerr-MOG black holes with a modification parameter alpha less than or similar to 0.03, which includes the Kerr case with alpha=0.
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    ArticlePublicationOpen Access
    String analog of Reissner–Nordström black holes cannot be overcharged
    (World Scientific Publishing Co Pte Lt, 2019-09-28) Düztaş, Koray; Jamil, M.; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    In this work, we attempt to overcharge extremal and nearly extremal charged black holes in string theory, known as the Garfinkle-Horowitz-Strominger solution. We first show that extremal black holes cannot be overcharged analogous to the case of Reissner-Nordstrom (RN) black holes. Contrary to their analog in general relativity, nearly extremal black holes can neither be overcharged beyond extremality, nor can they be driven to extremality by the interaction with test particles. Therefore, the analysis in this work also implies that the third law of black hole thermodynamics holds for the relevant charged black holes in string theory perturbed by test particles. This can be interpreted as a stronger version of the third law since one can drop out the continuity proviso for the relevant process.
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    ArticlePublicationOpen Access
    Kerr–Newman black holes can be generically overspun
    (Springer Nature, 2019-04-08) Düztaş, Koray; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    We construct thought experiments involving the perturbations of Kerr-Newman black holes by neutral test fields to evaluate the validity of the weak form of the cosmic censorship conjecture. We first show that neglecting backreaction effects, extremal Kerr-Newman black holes which satisfy the condition (J(2)/M-4) < (1/3) can be overspun by scalar fields. This result, which could not be discerned in the previous analyses to first order, is prone to be fixed by employing backreaction effects. However the perturbation of Kerr-Newman black holes by neutrino fields leads to a generic overspinning of the black hole due to the absence of a lower limit for the frequency of the incident wave to ensure that it is absorbed by the black hole. For this case, the destruction of the event horizon cannot be fixed by any form of backreaction effects. This result should not be interpreted as a counter-example to any of the previous results which were based on the assumption that the null energy condition is satisfied. We clarify and justify our arguments with numerical examples.
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    Overcharging dilaton black holes in (2+1) dimensions to extremality and beyond
    (World Scientific Publishing Co Pte Lt, 2020-12) Düztaş, Koray; Jamil, M.; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    We test whether static charged dilaton black holes in (2 + 1) dimensions can be turned into naked singularities by sending in test particles from infinity. We derive that overcharging is possible and generic for both extremal and nearly extremal black holes. Our analysis also implies that nearly extremal charged dilaton black holes can be continuously driven to extremality and beyond, unlike nearly extremal Banados-Teitelboim-Zanelli, Kerr and Reissner-Nordstrom black holes which are overspun or overcharged by a discrete jump. Thus, the weak form of the cosmic censorship conjecture and the third law of black hole thermodynamics are both violated in the interaction of charged dilaton black holes in (2 + 1) dimensions, with test particles. We also derive that there exist no points, where the heat capacity vanishes or diverges in the transition from black holes to naked singularities. The phase transitions that could potentially prevent the formation of naked singularities do not occur.
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    ArticlePublicationOpen Access
    Cosmic censorship, massless fermionic test fields, and absorption probabilities
    (Springer, 2021-12-24) Düztaş, Koray; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    In the conventional approach, fermionic test fields lead to a generic overspinning of black holes resulting in the formation of naked singularities. The absorption of the fermionic test fields with arbitrarily low frequencies is allowed for which the contribution to the angular momentum parameter of the space-time diverges. Recently we have suggested a more subtle treatment of the problem considering the fact that only the fraction of the test fields that is absorbed by the black hole contributes to the space-time parameters. Here, we re-consider the interaction of massless spin (1/2) fields with Kerr and Kerr-Newman black holes, adapting this new approach. We show that the drastic divergence problem disappears when one incorporates the absorption probabilities. Still, there exists a range of parameters for the test fields that can lead to overspinning. We employ backreaction effects due to the self-energy of the test fields which fixes the overspinning problem for fields with relatively large amplitudes, and renders it non-generic for smaller amplitudes. This non-generic overspinning appears likely to be fixed by alternative semi-classical and quantum effects.
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    ArticlePublicationOpen Access
    The variational method, backreactions, and the absorption probability in Wald type problems
    (Springer, 2021-01-19) Düztaş, Koray; Natural and Mathematical Sciences; DÜZTAŞ, Koray
    We argue that the variational method in Wald type thought experiments, involves order of magnitude problems when one imposes the fact that δM is inherently a first order quantity itself. One observes that the contribution of the second order perturbations is actually of the fourth order. Therefore backreactions have to be explicitly calculated. Here, we re-consider the overspinning problem for Kerr–Newman black holes interacting with test fields. We calculate the backreaction effects due to the induced increase in the angular velocity of the event horizon, which brings a partial solution to the overspinning problem. To bring an ultimate solution, we argue that the absorption probability should be taken into account in Wald type problems where black holes interact with test fields. This fundamentally alters the course of the analysis of the thought experiments. Due to the fact that a small fraction of the challenging modes is absorbed by the black holes, overspinning is prevented for both nearly extremal and extremal cases. Some extreme cases are easily fixed by backreaction effects. The arguments do not apply to the generic overspinning by fermionic fields for which the absorption probability is positive definite.