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Gokhan Alkac

Publications and source records attributed to Gokhan Alkac.

At least 19 recordsLinked to original sources
arXiv2026

The Smarr Formula is Gauss's Law: A Kerr-Schild Single-Copy Perspective

In the Kerr-Schild double copy, static and spherically symmetric black hole solutions of general relativity are mapped to purely electric solutions of Maxwell's theory in flat spacetime. We demonstrate that, for these configurations, the thermodynamic Smarr formula is structurally identical to the single-copy Gauss's law. Extending this to asymptotically anti-de Sitter spacetimes, we prove that the thermodynamic pressure-volume term naturally emerges from a gauge-theoretic background subtraction. This relationship establishes a novel connection between the classical double copy and black hole thermodynamics.

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arXiv2026

Weyl double copy in Lifshitz spacetimes

Lifshitz black hole solutions pose particular challenges for reconciling the two main formulations of the classical double copy: the Kerr-Schild double copy and the Weyl double copy. Recent work has suggested that consistency between the two can be restored, in certain cases, only by adopting a regularization prescription in the Weyl double copy. In this paper, we test this prescription on three examples from the literature, each with a distinct novel feature, and show that the prescription remains valid in all cases.

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arXiv2026

Regular AdS$_3$ black holes from regularized Gauss-Bonnet coupling

We obtain a three-dimensional bi-vector-tensor theory of the generalized Proca class by regularizing the Gauss-Bonnet invariant within the Weyl geometry. We show that the theory admits a regular AdS$_3$ black hole solution with primary hairs. Introducing a deformation in the theory, a different regular AdS$_3$ black hole solution is obtained. Charged generalizations of these solutions are given by coupling to Born-Infeld electrodynamics.

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arXiv2025

AdS$_3$ black holes with primary Proca hair from a regularized Gauss-Bonnet coupling

We construct a consistent three-dimensional Einstein-Gauss-Bonnet theory as a vector-tensor theory within the generalized Proca class by employing a regularization procedure based on the Weyl geometry, which was introduced recently in \link{2504.13084}. We then obtain an asymptotically AdS$_3$, static, and circularly symmetric black hole solution with primary Proca hair. Afterward, we investigate the effect of the scalar-tensor Gauss-Bonnet coupling constructed previously by different regularization schemes. We further generalize these solutions by incorporating an electric charge. As special cases, we find a regular black hole solution in addition to charged and uncharged stealth BTZ black hole solutions.

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arXiv2025

Simulating Hawking radiation in quantum many-body systems: deviations from the thermal spectrum

We investigate a recently proposed one-to-one correspondence between quantum field theories in two-dimensional curved spacetime and quantum many-body systems, which enables the simulation of Hawking radiation in static background spacetimes. In particular, we demonstrate that deviations from the thermal spectrum, as predicted by the well-known tunneling method, can be observed in many-body simulations.

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arXiv2025

On the consistency of non-commutative geometry inspired Reissner-Nordström black hole solution

We revisit the non-commutative geometry inspired Reissner-Nordström black hole solution obtained by smearing the point sources with a Gaussian distribution. We show that while the form of the metric function and the physical properties derived from that remain valid, not all the components of Einstein equations are satisfied. We construct an improved energy-momentum tensor that consistently satisfies Einstein equations and show that it leads to a different prediction for the region where the strong energy condition is violated. For certain choice of parameters, our proposal predicts the violation of the energy condition outside the Cauchy horizon, which might be important for observational signatures.

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arXiv2025

Classical double copy in the black hole mini-superspace

We give a novel formulation of classical double copy in the mini-superspace of static, spherically symmetric black holes where the map between the solutions of general relativity and Maxwell's theory can be realized in Boyer-Lindsquit coordinates. By employing the reduced action principle, we show that the double copy structure can be generalized to Lovelock gravities and quasi-topological gravities. However, the gravitational solutions are mapped to the purely electric solutions of Maxwell's theory with a difference: instead of a direct match between the Kerr-Schild scalar in the gravity side and the scalar potential in the gauge theory side, the scalar potential becomes a polynomial in the Kerr-Schild scalar, giving rise to a generalization of the Kerr-Schild double copy. We calculate the Regge-Teitelboim surface charges and prove that the mass of the black hole solution is identified with the electric charge corresponding to the Coulomb part of the gauge theory solution in the same way it does in the case of general relativity.

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arXiv2025

Microscopic entropy of static black holes in 3d Lovelock gravities

We give a microscopic derivation of the semi-classical entropy of static black holes in 3d Lovelock gravities, which are certain 3d Horndeski theories that were recently discovered from higher-dimensional Lovelock gravities via various methods and admit novel black hole solutions analogous to higher-dimensional ones. Assuming the ground state is described by the soliton obtained from the black hole solution by performing a double Wick rotation, we reproduce the semi-classical entropy from Cardy formula without central charges for the asymptotic growth of the number of states. We explain in detail how the mass of the soliton, which is needed in the Cardy formula, can be computed in the mini-superspace formalism.

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arXiv2024

Scaling symmetry, Smarr relation, and the extended first law in lower-dimensional Lovelock gravity

Recently, it was discovered that lower-dimensional versions of Lovelock gravity exist as scalar-tensor theories that are examples of Horndeski gravity. We study the thermodynamics of the static black hole solutions in these theories up to cubic order through Euclidean methods. Considering solutions with spherical, planar and hyperbolic event horizons ($k=+1, 0, -1$), we show that the universality of the thermodynamics for planar black holes ($k=0$) and the extended 1st law that include the variation of the couplings together with their associated potentials hold also in lower dimensions. We find that in $D=4, 6$ where the 2nd- and the 3rd-order Lovelock Lagrangians are boundary terms respectively, the Smarr relation is modified since the entropy is not a homogenous function in these dimensions. We also present a derivation of the Smarr relation and its modified version based on the global scaling properties of the reduced action that is used to obtain the solutions consistently. Unlike the other hairy black hole solutions that have been analyzed before, despite the terms in the reduced action that break the scaling symmetry, the derivation still follows from a conserved Noether charge.

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arXiv2024

Regularized Weyl double copy

We propose a regularization procedure in the sourced Weyl double copy, a spinorial version of the classical double copy, such that it matches much more general results in the Kerr-Schild version. In the regularized Weyl double copy, the anti-de Sitter (AdS) and the Lifshitz black holes, which form the basis of the study of strongly coupled gauge theories at finite temperature through the AdS/CFT correspondence and its non-relativistic generalization, become treatable. We believe that this might pave the way for finding out a relation between the classical double copy and holography.

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arXiv2023

3D Lovelock Gravity and the Holographic c-Theorem

We show that the scalar-tensor theory that arises in a rigorous $D \to 3$ limit of Lovelock gravity up to cubic order admits a holographic c-theorem and verify that the value of the c-function at the UV fixed point matches with the Weyl anomaly coefficient (up to an irrelevant factor). The constructed c-function is the same as one that follows from the "naive limit", which is obtained by scaling the couplings by a factor of $\frac{1}{D-3}$, and then setting $D=3$.

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arXiv2023

Lower-dimensional Limits of Cubic Lovelock Gravity

In this paper, we obtain the lower-dimensional limits $(p=2,3,4,5,6)$ of cubic Lovelock gravity through a regularized Kaluza-Klein reduction. By taking a flat internal space for simplicity, we also study the static black hole solutions in the resulting theories. We show that the solutions match with the ones obtained from the "naive limit" of $D$-dimensional equation for the metric function, which is obtained by first scaling the relevant couplings by a factor of $\frac{1}{D-p}$ and then taking the limit $D\to p$, with one important exception: In 4D, one obtains the expected solution only for the black hole with a planar horizon.

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arXiv2022

Generalized Black Holes in 3D Kerr-Schild Double Copy

The double copy of the Coulomb solution in three dimensions is a non-vacuum solution that can be obtained through different matter couplings. It is the static black hole solution of Einstein-Maxwell theory or general relativity minimally coupled to a free scalar field (with one ghost sign in the action in both cases). We consider generalizations of these matter couplings by paying particular attention to the regularity of the static black solution on the gravity side and the corresponding single copy electric field in the gauge theory. We show that i) Einstein-Born-Infeld theory yields a singular double copy, which admits stable orbits for certain choices of parameters, with a regular single copy electric field. ii) Black hole solutions constructed in arXiv:2104.10172 by coupling to the scalar field exemplify mostly regular double copies with regular single copy electric fields and also admit stable orbits. Additionally, we use these solutions to investigate the connection between horizons on the gravity side and electric fields on the gauge theory side, which was previously observed in four dimensions.

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arXiv2021

Kerr-Schild Double Copy of the Coulomb Solution in Three Dimensions

While the Kerr-Schild double copy of the Coulomb solution in dimensions higher than three is the Schwarzschild black hole, it is known that it should be a non-vacuum solution in three dimensions. We show that the static black hole solution of Einstein-Maxwell theory (with one ghost sign in the action) is the double copy with the correct Newtonian limit, which provides an improvement over the previous construction with a free scalar field that does not vanish at infinity. By considering a negative cosmological constant, we also study the charged Bañados-Teitelboim-Zanelli black hole and find that the single copy gauge field is the Coulomb solution modified by a term which describes an electric field linearly increasing with the radial coordinate, which is the usual behaviour of the Schwarzschild-AdS black hole in higher dimensions when written around a flat background metric.

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arXiv2021

The Kerr-Schild Double Copy in Lifshitz Spacetime

The Kerr-Schild double copy is a map between exact solutions of general relativity and Maxwell's theory, where the nonlinear nature of general relativity is circumvented by considering solutions in the Kerr-Schild form. In this paper, we give a general formulation, where no simplifying assumption about the background metric is made, and show that the gauge theory source is affected by a curvature term that characterizes the deviation of the background spacetime from a constant curvature spacetime. We demonstrate this effect explicitly by studying gravitational solutions with non-zero cosmological constant. We show that, when the background is flat, the constant charge density filling all space in the gauge theory that has been observed in previous works is a consequence of this curvature term. As an example of a solution with a curved background, we study the Lifshitz black hole with two different matter couplings. The curvature of the background, i.e., the Lifshitz spacetime, again yields a constant charge density; however, unlike the previous examples, it is canceled by the contribution from the matter fields. For one of the matter couplings, there remains no additional non-localized source term, providing an example for a non-vacuum gravity solution corresponding to a vacuum gauge theory solution in arbitrary dimensions.

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arXiv2020

More on the Classical Double Copy in Three Spacetime Dimensions

It is well-known that General Relativity (GR) in three spacetime dimensions (3D) has no well-defined Newtonian limit. Recently, a static solution mimicking the behaviour of the expected Newtonian potential has been found in arXiv:1904.11001 by studying the classical double copy of a point charge in gauge theory. This is the first example where the vacuum solution in the gauge theory leads to a non-vacuum solution on the gravity side. The resulting energy-momentum tensor was attributed to a free scalar ghost field; however, alternatively, the source can be seen as one resulting from a spacelike perfect fluid. In this paper, we first give an alternative derivation of the solution where there is no need to perform a generalized gauge transformation to obtain a quadratic Lagrangian without propagating ghost fields. Then, we present a stationary version of the solution and show that the scalar field interpretation of the source does not survive in this case, leaving the spacelike fluid as the only possibility. We give the gauge theory single copy of our solution and comment on the implications of our results on the validity of the classical double copy in 3D. The effect of the cosmological constant is also discussed.

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