arXiv subjects

Tamar Simhon

Publications and source records attributed to Tamar Simhon.

6 recordsLinked to original sources

arXiv2026

Defrosting the Born-Infeld dyonic frozen star with tachyon matter: spectrum of oscillations

The frozen star is a model for the interior of an astrophysical black hole that is externally indistinguishable from the singular geometry of a Schwarzschild black hole, even though it contains no singularities nor trapped surfaces. The frozen star corresponds to a macroscopic ``BIon'', a solution of a certain type of Born-Infeld Lagrangian coupled to Einstein gravity, which consists of rigid flux tubes sourced by an electric charge localized at its center and a uniform distribution of opposite charge along its outer surface. This configuration is a solution of the effective action that describes the end point of tachyon condensation in string. The rigidity of the flux tubes implies that the star is ultrastable under linear perturbations of the geometry and matter. The star can be effectively deformed, or ``defrosted'', allowing internal pulsations, as expected from regular, horizonless astrophysical black holes. To describe the defrosted star, we need to include a perturbative amount of magnetic-monopole charge and tachyon kinetic energy. This recasts the star as a macroscopic ``DIon'', whose flux tubes can stretch and contract, having both electric and magnetic charges localized at its center and distributed uniformly along its outer surface. Previously, the non-radial oscillations of the defrosted star were analyzed to leading order in a perturbative ``defrosting'' parameter $\gamma$, by describing the energy-momentum tensor as that of a fluid. Here, we derive the same spectrum in a much simpler way, using the Born-Infeld Lagrangian. Thus, we verify that the sound velocities scale as $\gamma$ and the parametrically long lifetimes scale as $1/\gamma^2$. Because the perturbation equations are derived from a Lagrangian, their consistency is guaranteed for arbitrary deviations away from spherical symmetry, facilitating numerical studies on frozen stars away from equilibrium.

gr-qc↗

arXiv2025

Formation of Frozen Stars from collapsing matter by tunneling

The frozen star is a type of black hole mimicker: An ultracompact object whose exterior geometry resembles that of a general-relativistic black hole but differs in its matter composition and in the regularity of its interior geometry. It is sourced by a spherically symmetric collection of open-string flux tubes, which posses an extremely anisotropic energy-momentum-stress tensor with maximally negative radial pressure. The frozen star represents an effective classical description of the highly quantum, closed-string polymer model. A key challenge for any model of a black hole mimicker is to explain how such objects can form from a collapsing body of matter. We started to address this important problem in a previous article by adapting the Euclidean-action method of Gibbons and Hawking to show that the transition into a frozen star is likely. Here, we improve on our previous results by showing that the transition probability for a collapsing shell of matter to tunnel quantum mechanically into a frozen star is unity, up to negligible corrections. Our conclusion is that such a transition is therefore inevitable.

gr-qc↗

arXiv2024

Kerr Black hole mimickers sourced by a string fluid

We present rotating solutions of Einstein's gravity coupled to an effective Born-Infeld theory that describes the end of open-string tachyon condensation after the decay of an unstable $D$-brane or a brane-antibrane system. The geometry of these solutions is that of the rotating frozen star. The solutions are stationary, non-singular and ultracompact, and their exterior geometry is identical, for all practical purposes, to that of the Kerr solution. The Born-Infeld matter consists of radial electric-flux tubes that emanate from, or end at, the ellipsoidal core of the star. Each end of the flux tubes carries an electric charge, so that the electric field can be viewed as being sourced by an ellipsoidal charge distribution of positive and negative charges near the center of the star. Meanwhile, the star's outer layer is equal and oppositely charged, resulting in a vanishing electric field in the external spacetime. We show that these rotating solutions are ultrastable against radial perturbations, just like their static frozen star counterparts. They are also effectively immune to ergoregion instabilities, as we discuss.

hep-th↗

arXiv2023

Thermodynamics of frozen stars

The frozen star is a recent proposal for a non-singular solution of Einstein's equations that describes an ultracompact object which closely resembles a black hole from an external perspective. The frozen star is also meant to be an alternative, classical description of an earlier proposal, the highly quantum polymer model. Here, we show that the thermodynamic properties of frozen stars closely resemble those of black holes: Frozen stars radiate thermally, with a temperature and an entropy that are perturbatively close to those of black holes of the same mass. Their entropy is calculated using the Euclidean-action method of Gibbons and Hawking. We then discuss their dynamical formation by estimating the probability for a collapsing shell of "normal" matter to transition, quantum mechanically, into a frozen star. This calculation followed from a reinterpretation of a transitional region between the Euclidean frozen star and its Schwarzschild exterior as a Euclidean instanton that mediates a phase transition from the Minkowski interior of an incipient Schwarzschild black hole to a microstate of the frozen star interior. It is shown that, up to negligible corrections, the probability of this transition is $e^{-A/4}$, with $A$ being the star's surface area. Taking into account that the dimension of the phase space is $e^{+A/4}$, we conclude that the total probability for the formation of the frozen star is of order unity. The duration of this transition is estimated, which we then use to argue, relying on an analogy to previous results, about the scaling of the magnitude of the off-diagonal corrections to the number operator for the Hawking-like particles. Such scaling was shown to imply that the corresponding Page curve indeed starts to go down at about the Page time, as required by unitarity.

gr-qc↗

arXiv2023

Black holes as frozen stars: Regular interior geometry

We have proposed a model geometry for the interior of a regular black hole mimicker, the frozen star, whose most startling feature is that each spherical shell in its interior is a surface of infinite redshift. The geometry is a solution of the Einstein equations which is sourced by an exotic matter with maximally negative radial pressure. The frozen star geometry was previously presented in singular coordinates for which $-g_{tt}$ and $g^{rr}$ vanish in the bulk and connect smoothly to the Schwarzschild exterior. Additionally, the geometry was mildly singular in the center of the star. Here, we present regular coordinates for the entirety of the frozen star. Each zero in the metric is replaced with a small, dimensionless parameter $\varepsilon$; the same parameter in both $-g_{tt}$ and $g^{rr}$ so as to maintain maximally negative radial pressure. We also regularize the geometry, energy density and pressure in the center of the star in a smooth way. Our initial analysis uses Schwarzschild-like coordinates and applies the Killing equations to show that an infalling, point-like object will move very slowly, effectively sticking to the surface of the star and never coming out. If one nevertheless follows the trajectory of the object into the interior of the star, it moves along an almost-radial trajectory until it comes within a small distance from the star's center. Once there, if the object has any amount of angular momentum at all, it will be reflected outwards by a potential barrier onto a different almost-radial trajectory. Finally, using Kruskal-like coordinates, we consider the causal structure of the regularized frozen star and discuss its $\varepsilon\to 0$ limit, for which the geometry degenerates and becomes effectively two dimensional.

gr-qc↗

arXiv2021

Black holes as frozen stars

We have recently proposed a model for a regular black hole, or an ultra-compact object, that is premised on having maximally negative radial pressure throughout the entirety of the object's interior. This model can be viewed as that of a highly entropic configuration of fundamental, closed strings near the Hagedorn temperature, but from the perspective of an observer who is ignorant about the role of quantum physics in counteracting against gravitational collapse. The advantage of this classical perspective is that one can use Einstein's equations to define a classical geometry and investigate its stability. Here, we complete the model by studying an important aspect of this framework that has so far been overlooked: The geometry and composition of the outermost layer of the ultra-compact object, which interpolates between the bulk geometry of the object and the standard Schwarzschild vacuum solution in its exterior region. By imposing a well-defined set of matching conditions, we find a metric that describes this transitional layer and show that it satisfies all the basic requirements; including the stability of the object when subjected to small perturbations about the background solution. In fact, we are able to show that, at linearized order, all geometrical and matter fluctuations are perfectly frozen in the transitional layer, just as they are known to be in the bulk of the object's interior.

gr-qc↗