arXiv subjects

S. V. Sushkov

Publications and source records attributed to S. V. Sushkov.

13 recordsLinked to original sources

arXiv2026

Cosmological perturbations in the theory of gravity with non-minimal derivative coupling. I. Modes of perturbations

We consider perturbations in the isotropic and homogeneous cosmological model with the spatially flat Friedmann-Lemaitre-Robertson-Walker metric in the framework of the theory of gravity with non-minimal derivative coupling. The Lagrangian of the theory contains the coupling term $ηG^{μν}\nabla_μϕ\nabla_νϕ$ and represents the particular example of a general Horndeski Lagrangian, which results in second-order field equations. It is known that the non-minimal derivative coupling crucially changes scenarios of the Universe evolution on early times. In particular, the $η$-term is dominating on early times and leads to a primary quasi-de Sitter (inflationary) stage which needs no fine-tuned potential. On late times the influence of non-minimal derivative coupling on the Universe evolution completely disappears, and this naturally leads to the transition to the standard cosmological evolution (post-inflationary stage). We have derived a complete set of equations which describe an evolution of scalar, vector and tensor modes of perturbations. All modes are analyzed analytically in two asymptotic cases, and then we construct exact numerical solutions which describe an entire evolution of the modes. We show that all modes, including vector ones, are amplified in the quasi-de Sitter (inflationary) stage, and such the behavior is cardinally distinct from that in Friedmann cosmology.

gr-qc↗

arXiv2025

Neutron stars in the theory of gravity with nonminimal derivative coupling and realistic equations of state

We numerically construct compact stars in the scalar-tensor theory of gravity with non-minimal derivative coupling of a scalar field to the curvature and nonzero cosmological constant. There are two free parameters in this model of gravity: the non-minimal derivative coupling parameter $\ell$ and the cosmological constant parameter $ξ$. We study the relationship between the model parameters and characteristic of the neutron star, what allowed us to limit the permissible range of $ξ$ and $\ell$. In particular, in the case $ξ=-1$ the external geometry of the neutron star coincides with the Schwarzschild anti-de Sitter geometry, while the internal geometry of the star differs from the case of the standard gravity theory. Plenty realistic equations of state of neutron star matter were considered. In general the neutron star model in the theory of gravity with a non-minimal derivative coupling does not contradict astronomical data and is viable.

gr-qc↗

arXiv2025

A `singular' bounce in the theory of gravity with non-minimal derivative coupling

We explore bounce scenarios in the framework of homogeneous and isotropic cosmological models with arbitrary spatial curvature in the theory of gravity with non-minimal derivative coupling. As expected, we find that there are no turning points and/or bounces in cosmological models with negative or zero spatial curvature. At the same time, both a turning point and a bounce can exist in the model with positive spatial curvature. In particular, the bounce is happened at $τ=τ_*$ when $a(τ_*)=a_{min} =(3ζΩ_2)^{1/2}$, where $τ=H_0 t$ is a dimensionless cosmic time. It is important fact that the value $a_{min}$ depends {\em only} on $ζ$ and $Ω_2$, and does {\em not} depend on $Ω_0$, $Ω_3$ and $Ω_4$. We find that near the bounce $a(τ)\approx a_{min}(1+Δτ^2/18ζ)$ and $h(τ)\approx Δτ/9ζ$, where $Δτ=τ-τ_*$. Thus, the scale factor $a(τ)$, the Hubble parameter $h(τ)$, and all corresponding geometrical invariants have a regular behavior near the bounce. As well the values characterizing matter energy densities, such as $ρ_m\sim a^{-3}$ and $ρ_r\sim a^{-4}$, are regular near the bounce. Nevertheless, though the spacetime geometry and energy densities remain to be regular near the bounce, the scalar field has a singular behavior there. Namely, $ϕ'\propto 1/Δτ^2 \to\infty$ as $Δτ\to 0$. As a result, we conclude that the complete dynamical system describing the cosmological evolution in theory of gravity with non-minimal derivative coupling is singular near the bounce. On our knowledge, such the scenario, when the spacetime geometry and matter energy densities remain to be regular at approaching the universe evolution to the moment of bounce, while the behavior of scalar field becomes singular, was unknown before. For this reason, we term this scenario as a {\em `singular' bounce}.

gr-qc↗

arXiv2011

Rotating thin-shell wormhole from glued Kerr spacetimes

We construct a model of a rotating wormhole made by cutting and pasting two Kerr spacetimes. As a result, we obtain a rotating thin-shell wormhole with exotic matter at the throat. Two candidates for the exotic matter are considered: (i) a perfect fluid; (ii) an anisotropic fluid. We show that a perfect fluid is unable to support a rotating thin-shall wormhole. On the contrary, the anisotropic fluid with the negative energy density can be a source for such a geometry.

gr-qc↗

arXiv2010

Vacuum polarization of a massive scalar field in a wormhole spacetime

We calculate the vacuum average value of the field square and the stress-energy tensor of a massive scalar field, with non-minimal coupling $ξ$ to the curvature in the short-throat flat-space wormhole background. The obtained results are not suitable for an analytical analysis and for this reason we provide a numerical analysis for different values of the coupling constant, $ξ$. It was shown that the vacuum polarization cannot self-consistently support the wormhole. Otherwise, the energy-momentum tensor does not violate the null energy condition near the throat.

gr-qc↗

arXiv2009

Wormholes supported by chiral fields

We consider static, spherically symmetric solutions of general relativity with a nonlinear sigma model (NSM) as a source, i.e., a set of scalar fields $Φ= (Φ^1,...,Φ^n)$ (so-called chiral fields) parametrizing a target space with a metric $h_{ab}(Φ)$. For NSM with zero potential $V(Φ)$, it is shown that the space-time geometry is the same as with a single scalar field but depends on $h_{ab}$. If the matrix $h_{ab}$ is positive-definite, we obtain the Fisher metric, originally found for a canonical scalar field with positive kinetic energy; otherwise we obtain metrics corresponding to a phantom scalar field, including singular and nonsingular horizons (of infinite area) and wormholes. In particular, the Schwarzschild metric can correspond to a nontrivial chiral field configuration, which in this case has zero stress-energy. Some explicit examples of chiral field configurations are considered. Some qualitative properties of NSM configurations with nonzero potentials are pointed out.

gr-qc↗

arXiv2008

Composite wormholes in vacuum Jordan-Brans-Dicke theory

New classes composite vacuum wormhole solutions of Jordan-Brans-Dicke gravitation is presented and analysed. It is shown that such solution holds for both, a bridge between separated Schwarzschild and Brans Universes and for a bridge connecting two Schwarzschild asymptotically flat regions joined by Brans throat. We have also noticed that there are some new possible candidates for wormhole spacetimes.

gr-qc↗

arXiv2008

Slowly rotating scalar field wormholes: the second order approximation

We discuss rotating wormholes in general relativity with a scalar field with negative kinetic energy. To solve the problem, we use the assumption about slow rotation. The role of a small dimensionless parameter plays the ratio of the linear velocity of rotation of the wormhole's throat and the velocity of light. We construct the rotating wormhole solution in the second order approximation with respect to the small parameter. The analysis shows that the asymptotical mass of the rotating wormhole is greater than that of the non-rotating one, and the NEC violation in the rotating wormhole spacetime is weaker than that in the non-rotating one.

gr-qc↗

arXiv2007

Slowly rotating wormholes: the first order approximation

We discuss a solution describing a rotating wormhole in the theory of gravity with a scalar field with negative kinetic energy. To solve the problem we use the assumption about slow rotation. The role of a small dimensionless parameter plays the ratio of the linear velocity of rotation of the wormhole's throat and the velocity of light. The rotating wormhole solution is constructed in the framework of the first order approximation with respect to the small parameter. We analyze the obtained solution and study the motion of test particles and the propagation of light in the spacetime of rotating wormhole.

gr-qc↗

arXiv2005

Wormholes supported by a phantom energy

We extend the notion of phantom energy--which is generally accepted for homogeneously distributed matter with $w<-1$ in the universe--on inhomogeneous spherically symmetric spacetime configurations. A spherically symmetric distribution of phantom energy is shown to be able to support the existence of static wormholes. We find an exact solution describing a static spherically symmetric wormhole with phantom energy and show that a spatial distribution of the phantom energy is mainly restricted by the vicinity of the wormhole's throat. The maximal size of the spherical region, surrounding the throat and containing the most part of the phantom energy, depends on the equation-of-state parameter $w$ and cannot exceed some upper limit.

gr-qc↗

arXiv2004

Cosmological evolution of a ghost scalar field

We consider a scalar field with a negative kinetic term minimally coupled to gravity. We obtain an exact non-static spherically symmetric solution which describes a wormhole in cosmological setting. The wormhole is shown to connect two homogeneous spatially flat universes expanding with acceleration. Depending on the wormhole's mass parameter $m$ the acceleration can be constant (the de Sitter case) or infinitely growing.

gr-qc↗

arXiv2002

< phi^2 > for a scalar field in 2D black holes: a new uniform approximation

We study nonconformal quantum scalar fields and averages of their local observables (such as ^{ren} and T_{ab}^{ren}) in a spacetime of a 2-dimensional black hole. In order to get an analytical approximation for these expressions the WKB approximation is often used. We demonstrate that at the horizon WKB approximation is violated for a nonconformal field, that is when the field mass or/and the parameter of non-minimal coupling do not vanish. We propose a new "uniform approximation" which solves this problem. We use this approximation to obtain an improved analytical approximation for ^{ren} in the 2-dimensional black hole geometry. We compare the obtained results with numerical calculations.

hep-th↗

arXiv2002

Wormholes supported by the kink-like configuration of a scalar field

We study the problem of existence of static spherically symmetric wormholes supported by the kink-like configuration of a scalar field. With this aim we consider a self-consistent, real, nonlinear, nonminimally coupled scalar field $ϕ$ in general relativity with the symmetry-breaking potential $V(ϕ)$ possessing two minima. We classify all possible field configurations ruling out those of them for which wormhole solutions are impossible. Field configurations admitting wormholes are investigated numerically. Such the configurations represent a spherical domain wall localized near the wormhole throat.

gr-qc↗