arXiv subjects

Cagdas Ulus Agca

Publications and source records attributed to Cagdas Ulus Agca.

4 recordsLinked to original sources

arXiv2026

Geometrically Significant Surfaces of Black Holes from a Single Scalar

Black hole spacetimes contain several geometrically distinguished hypersurfaces, including event and Cauchy horizons, stationary-limit surfaces, and curvature singularities. These structures are usually identified by different geometric or causal criteria. We show that, for the Kerr--Newman black hole, the membrane-paradigm pressure of a stretched horizon admits an analytically continued scalar representative whose fully factorized form has zeros, poles, and singular factors aligned with these standard Kerr--Newman loci. Its zeros occur at the outer and inner horizons, its poles occur at the outer and inner stationary-limit surfaces, its higher-order divergence contains the ring-singularity factor, and its large-$r$ behavior records the asymptotic decay of the scalar. The construction is not proposed as a new invariant characterization of the spacetime, nor as a physical extension of the membrane fluid into the black-hole interior. Rather, it gives a compact membrane-pressure-based diagnostic whose analytic structure reorganizes several familiar surfaces of the Kerr--Newman geometry. We also note a secondary, purely algebraic analogy with generalized multi-component van der Waals-type equations of state.

gr-qc↗

arXiv2025

Fluid-membrane Descriptions of Various Black Holes

The membrane paradigm of black holes is an effective theory that replaces the event horizon with a fictitious yet tangible fluid. It has provided us with valuable insights, especially in understanding the environment of black holes. The paradigm establishes a fluid/gravity correspondence that allows the computation of the thermal properties of the black hole in terms of the transport coefficients of the fluid. Recently, we showed that this is a van der Waals-type fluid for Kerr and especially for Johannsen-Psaltis black holes. Here, we use the paradigm to study the properties of various black holes in different dimensions to test the predictive capacity of effective theory. Among these, for the BTZ black holes, the paradigm gives a physical bulk viscosity, unlike the generic examples, for which the paradigm works with a negative bulk viscosity. For Lorentz-violating black holes, we demonstrate that the parameter for Lorentz violation is seen as a hair under the paradigm, which shifts the ergoregion where the fluid pressure diverges. It might have a consequence for black hole jets. For asymptotically safe quantum-corrected black holes, the paradigm detects the final state of evaporation, i.e., a remnant with a correct value for the mass that still has an event horizon. Finally, we check the paradigm with stringy black holes that have dilatonic and axionic charges, and the fluid produces the known results.

gr-qc↗

arXiv2025

A holographic description on half planes and wedges for N = 1 SUSY BF theory in 2D

BF theory is a topological field theory that appears in different parts of theoretical physics and one of its important uses is in lower dimensional holography settings. While it can be defined as a dimensional reduction of 3D CS theory, it is also equivalent to JT gravity. Moreover, further holographic settings relate BF theory to a particle on group theory. Here, we reconsider this "simplest holography" construction as SL(2,R) invariant particle on group theory and extend the web of dualities diagram in terms of a holographic description of the 2D N=1 BF theory on half-plane and find its 1D particle on group description as N=1B supermultiplet superconformal quantum mechanics. Moreover, we provide wedge space holography-type construction to achieve codimension 2 holography and show the web of dualities diagram also closes diagonally for BF and CS theories. For BF theory it leads to a complex 1D particle on group mechanics on the face of the wedge, a 0D quantum mechanics on the boundary theory of the faces, and its supersymmetric extension is a supermultiplet containing complex boson and complex scalar of the Landau-Ginzburg type. Upon this construction boundary theory also realizes a global U_j(1) invariance which extends the total symmetry group. To have a more complete picture we also included wedge space holography for 3D CS theory as an appendix and showed its codimension 2 holography corresponds to a 1D particle on group we found earlier.

hep-th↗

arXiv2024

Membrane Paradigm Approach to the Johannsen-Psaltis Black Hole

The Johannsen-Psaltis (JP) black hole is a phenomenologically viable metric obtained by judiciously deforming the Kerr black hole such that the metric is asymptotically flat and causal and is consistent with the weak field, post-Newtonian tests of gravity; however, it has additional hairs besides its mass and angular momentum. As it deviates from the Kerr black hole in the strong coupling regime, it is a useful metric to test the Kerr hypothesis that states that all astrophysical black holes are of the Kerr type. Here we give a membrane description of this black hole that effectively amounts to replacing the observable part of the black hole with a fluid with thermal properties. A timelike membrane, a stretched horizon local in time is assumed to exist. This membrane is expected to approximate the null event horizon that is highly non-local in time. We derive the energy-momentum tensor of the fluid and all the transport coefficients using the action formulation to the membrane as advocated by Parikh and Wilczek. In the fluid description, one observes that the finiteness of the transport coefficients constrains the additional hairs of the Johannsen-Psaltis black hole. Analytically continuing the pressure of the fluid to all values of the radial coordinate $r$, one obtains interesting Van der Waals-type behavior of the pressure of the fluid which diverges at the radius of the outer ergosphere, lending support, from the membrane paradigm's perspective to the claim that relativistic astrophysical jets are produced by the ergoregion of the black hole.

gr-qc↗