arXiv subjects

Shohreh Abdolrahimi

Publications and source records attributed to Shohreh Abdolrahimi.

At least 19 recordsLinked to original sources

arXiv2025

Properties of a deformed Myers-Perry black hole

Considering an exact solution of the five-dimensional Einstein equations in vacuum space, which represents a distorted Myers-Perry black hole with a single angular momentum, we investigate how the distortion affects the horizon surface of this black hole. We illustrate a special case where we have a bumpy deformed black hole horizon that feels the presence of the external sources. However, the ergosphere is oblivious to the presence of external sources.

gr-qc↗

arXiv2025

Distorted black ring with rotation on the two-sphere

We construct a stationary black ring spacetime with rotation only along the $S^2$ direction that is free of conical singularities. We use a solution-generating technique to add external gravitational fields and show these can be used to remove conical singularities from an asymptotically flat black ring with a rotating $S^2$.

gr-qc↗

arXiv2024

Distorted static black holes with a bubble

We construct a family of local static, vacuum five-dimensional solutions with two commuting spatial isometries describing a black hole with a $S^3$ horizon and a 2-cycle `bubble' in the domain of outer communications. The solutions are obtained by adding distortions to an asymptotically flat seed solution. We show that the conical singularities in the undistorted geometry can be removed by an appropriate choice of the distortion.

gr-qc↗

arXiv2022

A Distorted Charged Dilaton Black Ring

We present the distorted five dimensional static black rings in Einstein-Maxwell-dilaton (EMd) gravity by using the sigma model structure of the dimensionally reduced field equations. We investigate how a static and neutral distribution of external matter distorts the black ring. In the case of a distorted five dimensional static black rings in EMd gravity the distortion parameters can be adjusted such that the solution is free of conical singularities in the region of the validity of the solution.

gr-qc↗

arXiv2022

Voyager 3: A Concept Mission to Interstellar Medium

Voyager 3 is a concept mission that sends a space telescope to the interstellar medium in a reasonable amount of time. Voyager 3 would take a direct image of an exoplanet using the solar gravitational lensing at the distance of 550 astronomical unit (AU). The spacecraft would use its suite of scientific instruments to study the environment of the local solar system and interstellar medium and finalize its primary mission by imaging an exoplanet. Two potential architectures are proposed to meet this mission directive, using multiple gravitational assists and sizable electric propulsion burns to achieve the high escape speeds necessary to reach 550 AU. This paper expands on the science mission objectives, trajectory, and the preliminary design of the baseline spacecraft.

physics.space-ph↗

arXiv2020

A Distorted Black Ring

We investigate how a static and neutral distribution of external matter distorts a 5-dimensional static black ring. We obtain a general expression for the distorted metric in terms of the background metric functions and the distortion fields, and find a multipole expansion for the latter. We demonstrate that the gravitational field of these external sources can be adjusted to remove the conical singularity of the undistorted black ring solution. We analyze properties of the distorted black ring for the specific cases of dipole and quadrupole distortions.

gr-qc↗

arXiv2019

Ingoing Eddington-Finkelstein Metric of an Evaporating Black Hole

We present an approximate time-dependent metric in ingoing Eddington-Finkelstein coordinates for an evaporating nonrotating black hole as a first-order perturbation of the Schwarzschild metric, using the linearized back reaction from a realistic approximation to the stress-energy tensor for the Hawking radiation in the Unruh quantum state.

hep-th↗

arXiv2015

Properties of the distorted Kerr black hole

We investigate the properties of the ergoregion and the location of the curvature singularities for the Kerr black hole distorted by the gravitational field of external sources. The particular cases of quadrupole and octupole distortion are studied in detail. We also investigate the scalar curvature invariants of the horizon and compare their behaviour with the case of the isolated Kerr black hole. In a certain region of the parameter space the ergoregion consists of a compact region encompassing the horizon and a disconnected part extending to infinity. The curvature singularities in the domain of outer communication, when they exist, are always located on the boundary of the ergoregion. We present arguments that they do not lie on the compact ergosurface. For quadrupole distortion the compact ergoregion size is negatively correlated with the horizon angular momentum when the external sources are varied. For octupole distortion infinitely many ergoregion configurations can exist for a certain horizon angular momentum. For some special cases we can have $J^2/M^4 > 1$ and yet avoid the naked singularity.

gr-qc↗

arXiv2015

Double Images from a Single Black Hole

In the simulations of the multi-black holes and merging black holes a larger primary image and a secondary smaller image which looks like an eyebrow and the deformation of the shadows have been observed. However, this kind of eyebrow-like structure was considered as unique feature of multi black hole systems. In this paper, we illustrate the new result that in the case of octupole distortions of a Schwarzschild black hole the local observer sees two shadows or two images for this single black hole, i.e., also an eyebrow-like structure. Presence of two images in our case is remarkable, as we have only one black hole, however, the observer sees two dark images of this single black hole.

gr-qc↗

arXiv2015

Hawking Radiation Energy and Entropy from a Bianchi-Smerlak Semiclassical Black Hole

Eugenio Bianchi and Matteo Smerlak have found a relationship between the Hawking radiation energy and von Neumann entropy in a conformal field emitted by a semiclassical two-dimensional black hole. We compare this relationship with what might be expected for unitary evolution of a quantum black hole in four and higher dimensions. If one neglects the expected increase in the radiation entropy over the decrease in the black hole Bekenstein-Hawking A/4 entropy that arises from the scattering of the radiation by the barrier near the black hole, the relation works very well, except near the peak of the radiation von Neumann entropy and near the final evaporation. These discrepancies are calculated and discussed as tiny differences between a semiclassical treatment and a quantum gravity treatment.

hep-th↗

arXiv2015

Distorted Local Shadows

We introduce the notion of a local shadow for a black hole and determine its shape for the particular case of a distorted Schwarzschild black hole. Considering the lowest-order even and odd multiple moments, we compute the relation between the deformations of the shadow of a Schwarzschild black hole and the distortion multiple moments. For the range of values of multiple moments that we consider, the horizon is deformed much less than its corresponding shadow, suggesting the horizon is more `rigid'. Quite unexpectedly we find that a prolate distortion of the horizon gives rise to an oblate distortion of the shadow, and vice-versa.

gr-qc↗

arXiv2014

Myers-Perry Black Hole in an External Gravitational Field

We obtain a new exact solution of the 5D Einstein equations in vacuum describing a distorted Myers-Perry black hole with a single angular momentum. Locally, the solution is interpreted as a black hole distorted by a stationary $U(1)\times U(1)$ symmetric distribution of external matter. Technically, the solution is constructed by applying a 2-fold Bäcklund transformation on a 5D distorted Minkowski spacetime as a seed. The physical quantities of the solution are calculated, and a local Smarr-like relation on the black hole horizon is derived. It possesses the same form as the Smarr-like relation for the asymptotically flat Myers-Perry black hole. It is demonstrated that in contrast to the asymptotically flat Myers-Perry black hole, the ratio of the horizon angular momentum and the mass $J^2/ M^3$ is unbounded, and can grow arbitrarily large. We study the properties of the ergoregion and the horizon surface. The external field does not influence the horizon topology. The horizon geometry however is distorted, and any regular axisymmetric geometry is possible.

gr-qc↗

arXiv2014

Production of black holes and string balls in a two-dimensional split-fermion model

We present cross-sections for the black hole and string ball production in proton-proton collisions in a TeV-scale gravity model with split fermions in two dimensions. The cross-section for black hole and string ball production in the split-fermion model is smaller than in the non-split-fermion model. The drop of the cross-section for the string ball production can be one to two orders of magnitude with the increase of the width of the brane from $L=0$ to 15 $\text{TeV}^{-1}$. The cross-section for string ball production in two-dimensional split fermion model reduces more in comparison to black holes. Black holes are quite hard to be observed at the LHC. In fact, taking into account the current experimental limits on the fundamental Planck scale, black holes cannot be produced at the LHC. Cross-section for string ball production depends significantly on string coupling constant, making it very model dependent. We investigate the range of values of string coupling constant from 0.02 to 0.4. There has been no evidence for production of string balls at $\sqrt{s}=8$ TeV. A two-dimensional split fermion model with a extremely thick brane can account for the absence of signature of string balls for a world with the value of fundamental Planck scale even as low as 1 TeV.

hep-ph↗

arXiv2014

Thermodynamic of Distorted Reissner-Nordström Black Holes in Five-dimensions

In this paper, we study mechanics and thermodynamics of distorted, five-dimensional, electrically charged (non-extremal) black holes on the example of a static and "axisymmetric" black hole distorted by external, electrically neutral matter. Such a black hole is represented by the derived here solution of the Einstein-Maxwell equations which admits an $\mathbb{R}^1\times U(1)\times U(1)$ isometry group. We study the properties of this distorted black hole.

hep-th↗

arXiv2013

Distorted Five-dimensional Electrically Charged Black Holes

We derive and study distorted, five-dimensional, electrically charged, non-extremal black holes on the example of a static and "axisymmetric" black hole distorted by external, electrically neutral matter.The solution satisfies Einstein-Maxwell equations which admits an $\mathbb{R}^1\times U(1)\times U(1)$ isometry group. The inner horizon remains regular if the distortion fields are finite and smooth at the outer horizon. There exists a certain duality transformation between the inner and the outer horizon surfaces which links surface gravity, electrostatic potential, and space-time curvature invariants calculated at the black hole horizons. The product of the inner and outer horizon areas depends only on the black hole's electric charge and the geometric mean of the areas is the upper (lower) limit for the inner (outer) horizon area. The horizon areas, electrostatic potential, and surface gravity satisfy the Smarr formula. We formulated the zeroth and the first laws of mechanics and thermodynamics of the distorted black hole and found a correspondence between the global and local forms of the first law. To illustrate the effect of distortion we consider the dipole-monopole and quadrupole-quadrupole distortion fields. The relative change in the Kretschamnn scalar due to the distortion is greater at the outer horizon than at the inner one. Calculating the maximal proper time of free fall from the outer to the inner horizons we show that the distortion can noticeably change the black hole interior. The change depends on type and strength of distortion fields. In particular, due to the types of distortion fields considered here the black hole horizons can either come arbitrarily close to or move far from each other.

gr-qc↗

arXiv2013

Velocity Effects on an Accelerated Unruh-DeWitt Detector

We analyze the response of an Unruh-DeWitt detector moving along an unbounded spatial trajectory in a two-dimensional spatial plane with constant independent magnitudes of both the four-acceleration and of a timelike proper time derivative of the four-accelration. In a Fermi-Walker frame moving with the detector, the direction of the acceleration rotates at a constant rate around a great circle. This is the motion of a charge in a uniform electric field when in the frame of the charge there is both an electric and a magnetic field. We compare the response of this detector to a detector moving with constant velocity in a thermal bath of the corresponding temperature for non-relativistic velocities, and in two regimes: ultraviolet and infrared. In infrared regime, the detector in the Minkowski space-time moving along the spatially two-dimensional trajectory should move with a higher speed to keep up with the same excitation rate of the inertial detector in a thermal bath. In ultraviolet regime, the dominant modification in the response of this detector compared to the black body spectrum of Unruh radiation is the same as the dominant modification perceived by a detector moving with constant velocity in a thermal bath.

gr-qc↗

arXiv2012

Spectral Methods in General Relativity and Large Randall-Sundrum II Black Holes

Using a novel numerical spectral method, we have found solutions for large static Randall-Sundrum II (RSII) black holes by perturbing a numerical AdS_5-CFT_4 solution to the Einstein equation with a negative cosmological constant Lambda that is asymptotically conformal to the Schwarzschild metric. We used a numerical spectral method independent of the Ricci-DeTurck-flow method used by Figueras, Lucietti, and Wiseman for a similar numerical solution. We have compared our black-hole solution to the one Figueras and Wiseman have derived by perturbing their numerical AdS_5-CFT_4 solution, showing that our solution agrees closely with theirs. We have also deduced the new results that to first order in 1/(-ΛM^2), the Hawking temperature and entropy of an RSII static black hole have the same values as the Schwarzschild metric with the same mass, but the horizon area is increased by about 4.7/(-Λ).

hep-th↗

arXiv2012

Large Randall-Sundrum II Black Holes

Using a novel numerical spectral method, we have constructed an AdS_5-CFT_4 solution to the Einstein equation with a negative cosmological constant Lambda that is asymptotically conformal to the Schwarzschild metric. This method is independent of the Ricci-DeTurck-flow method used by Figueras, Lucietti, and Wiseman. We have perturbed the solution to get large static black hole solutions to the Randall-Sundrum II (RSII) braneworld model. Our solution agrees closely with that of Figueras et al. and also allows us to deduce the new results that to first order in 1/(-Lambda M^2), the Hawking temperature and entropy of an RSII static black hole have the same values as the Schwarzschild metric with the same mass, but the horizon area is increased by about 4.7/(-Lambda).

hep-th↗