arXiv subjects

Milena V. Skvortsova

Publications and source records attributed to Milena V. Skvortsova.

7 recordsLinked to original sources

arXiv2026

5D black holes and mirror (topological) stars from nonlinear electrodynamics: Existence and stability

We consider static, spherically symmetric solutions of 5D general relativity with magnetic fields governed by nonlinear electrodynamics (NED) with the Lagrangian $L(\cal F)$, ${\cal F} = F_{AB} F^{AB}$, and show that generic solutions describe either 5D black holes (also called black strings due to a circular extra dimension) or so-called mirror stars with perfectly reflecting boundary surfaces (also called topological stars), not to be confused with 4D configurations of mirror matter considered in particle physics. Two particular examples of such solutions have been obtained, admitting analytic expressions for the metric coefficients and $L(\cal F)$, and their stability under radial (monopole) perturbations is studied. While the whole obtained family of black hole solutions turns out to be stable, mirror star solutions prove to be stable only in a certain range in the parameter space. We thus extend to the Einstein-NED system the results previously obtained for Einstein-Maxwell fields.

gr-qc↗

arXiv2025

On the stability of exceptional Brans-Dicke wormholes

In our previous papers we have analyzed the stability of vacuum and electrovacuum static, spherically symmetric space-times in the framework of the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories (STT) of gravity. In the present paper, we continue this study by examining the stability of exceptional solutions of the Brans-Dicke theory with the coupling constant $\omega =0$ that were not covered in the previous studies. Such solutions describe neutral or charged wormholes and involve a conformal continuation: the standard conformal transformation maps the whole Einstein-frame manifold ${\mathbb M}_E$ to only a part of the Jordan-frame manifold ${\mathbb M}_J$, which has to be continued beyond the emerging regular boundary S, and the new region maps to another manifold ${\mathbb M}_{E-}$. The metric in ${\mathbb M}_J$ is symmetric with respect to S only if the charge $q$ is zero. Our stability study concerns radial (monopole) perturbations, and it is shown that the wormhole is stable if $q \ne 0$ and unstable only in the symmetric case $q=0$

gr-qc↗

arXiv2025

Stability ranges of magnetic black holes and mirror (topological) stars in 5D gravity

We discuss static, spherically symmetric solutions to the 5D Einstein-Maxwell equations (belonging to wide classes of multidimensional solutions known at least from the 1990s) and select among them those which must observationally look like local objects whose surface reflects back particles or signals getting there, the so-called mirror stars (also called ``topological stars'' by some authors). Their significant parameters are the Schwarzschild mass $m$ and the magnetic charge $q$, such that $q^2 > 3m^2$, while the radius of their mirror surface is $r_b = 2q^2/(3m) > 2m$. We also discuss their black hole counterparts for which $q^2 \leq 3m^2$. For both these objects, we study spherically symmetric time-dependent perturbations and determine the stability regions in their parameter spaces. Thus, mirror stars turn out to be stable only at $r_b < r_b^{\rm crit} \approx 4.004\,m$, while the black holes prove to be stable in the whole range of their parameters. We calculate the fundamental frequencies and decay rates of black hole perturbations using the WKB and time domain methods. Our stability results disagree with some of those previously announced in the literature.

gr-qc↗

arXiv2025

Alexei Starobinsky and wormhole physics

Alexei Starobinsky is most famous for his great contribution to cosmology, but he has considerable achievements in other branches of gravitational physics and astrophysics, such as the theory of compact objects including black holes and wormholes. In this note, we give a brief review of Alexei's papers devoted to wormhole physics. They mostly concern such issues of common interest as the necessary conditions for wormhole existence in general relativity and its extensions as well as generic properties of some kinds of wormholes. We also extend one of the no-go theorems on thin-shell wormholes to a wider choice of their symmetry and matter content.

gr-qc↗

arXiv2025

Magnetic mirror stars in five dimensions

We discuss a class of solutions of multidimensional gravity which are formally related to black-hole solutions but can observationally look like compact stars whose surface reflects back all particles or signals getting there. Some particular examples of such solutions are presented and studied, including those with a magnetic field in Maxwell or nonlinear electrodynamics (NED) in five dimensions. For NED as a possible source for magnetic mirror stars, we formulate a methodology of solving the 5D Einstein-NED equations and point out the conditions under which there always exist mirror star solutions. We also note that some of the Einstein-Maxwell solutions under consideration are discussed in the literature and called ``topological stars'' due to the circular topology of the fifth dimension.

gr-qc↗

arXiv2024

On the stability of electrovacuum space-times in scalar-tensor gravity

We study the behavior of static, spherically symmetric solutions to the field equations of scalar-tensor theories (STT) of gravity belonging to the Bergmann-Wagoner-Nordtvedt class, in the presence of an electric and/or magnetic charge. This class of theories includes the Brans-Dicke, Barker and Schwinger STT as well as nonminimally coupled scalar fields with an arbitrary parameter $\xi$. The study is restricted to canonical (nonphantom) versions of the theories and scalar fields without a self-interaction potential. Only radial (monopole) perturbations are considered as the most likely ones to cause an instability. The static background solutions contain naked singularities, but we formulate the boundary conditions in such a way that would preserve their meaning if a singularity is smoothed, for example, due to quantum gravity effects. These boundary conditions look more physical than those used by other authors. Since the solutions of all STT under study are related by conformal transformations, the stability problem for all of them reduces to the same wave equation, but the boundary conditions for perturbations (and sometimes the boundaries themselves) are different in different STT, which affects the stability results. The stability or instability conclusions are obtained for different branches of solutions in the theories under consideration and are presented in a table form.

gr-qc↗

arXiv2023

On the stability of spherically symmetric space-times in scalar-tensor gravity

We study the linear stability of vacuum static, spherically symmetric solutions to the gravitational field equations of the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories (STT) of gravity, restricting ourselves to nonphantom theories, massless scalar fields and configurations with positive Schwarzschild mass. We consider only small radial (monopole) perturbations as the ones most likely to cause an instability. The problem reduces to the same Schroedinger-like master equation as is known for perturbations of Fisher's solution of general relativity (GR), but the corresponding boundary conditions that affect the final result of the study depend on the choice of the STT and a particular solution within it. The stability or instability conclusions are obtained for the Brans-Dicke, Barker and Schwinger STT as well as for GR nonminimally coupled to a scalar field with an arbitrary parameter $\xi$.

gr-qc↗