arXiv subjects

Alexander Zhuk

Publications and source records attributed to Alexander Zhuk.

At least 73 records · Page 4Linked to original sources

arXiv2011

Non-relativistic limit of Randall-Sundrum model: solutions, applications and constraints

In the Randall-Sundrum model with one brane, we found the approximate and exact solutions for gravitational potentials and accelerations of test bodies in these potentials for different geometrical configurations. We applied these formulas for calculation of the gravitational interaction between two spheres and found the approximate and exact expressions for the relative force corrections to the Newton's gravitational force. We demonstrated that the difference between relative force corrections for the approximate and exact cases increases with the parameter $l$ (for the fixed distance $r$ between centers of the spheres). On the other hand, this difference increases with decreasing of the distance between the centers of the spheres (for the fixed curvature scale parameter $l$). We got the upper limit for the curvature scale parameter $l\lesssim 10\, μ$m. For these values of $l$, the difference between the approximate and exact solutions is negligible.

gr-qc↗

arXiv2011

Weak-field limit of f(R)-gravity in three and more spatial dimensions

We investigate a point-like massive source in non-linear f(R) theories in the case of arbitrary number of spatial dimensions D\geq 3. If D>3 then extra dimensions undergo toroidal compactification. We consider a weak-field approximation with Minkowski and de Sitter background solutions. In both these cases point-like massive sources demonstrate good agreement with experimental data only in the case of ordinary three-dimensional (D=3) space. We generalize this result to the case of perfect fluid with dust-like equations of state in the external and internal spaces. This perfect fluid is uniformly smeared over all extra dimensions and enclosed in a three-dimensional sphere. In ordinary three dimensional (D=3) space, our formulas are useful for experimental constraints on parameters of f(R) models.

gr-qc↗

arXiv2011

Latent solitons, black strings, black branes, and equations of state in Kaluza-Klein models

In Kaluza-Klein models with an arbitrary number of toroidal internal spaces, we investigate soliton solutions which describe the gravitational field of a massive compact object. We single out the physically interesting solution corresponding to a point-like mass. For the general solution we obtain equations of state in the external and internal spaces. These equations demonstrate that the point-like mass soliton has dust-like equations of state in all spaces. We also obtain the PPN parameters, which give the possibility to obtain the formulas for perihelion shift, deflection of light and time delay of radar echoes. Additionally, the gravitational experiments lead to a strong restriction on the parameter of the model: $τ= -(2.1\pm 2.3)\times 10^{-5}$. The point-like mass solution contradicts this restriction. The condition $τ=0$ satisfies the experimental limitation and defines a new class of solutions which are indistinguishable from general relativity. We call such solutions latent solitons. Black strings and black branes belong to this class. Moreover, the condition of stability of the internal spaces singles out black strings/branes from the latent solitons and leads uniquely to the black string/brane equations of state $p_i=-ε/2$, in the internal spaces and to the number of the external dimensions $d_0=3$. The investigation of multidimensional static spherically symmetric perfect fluid with dust-like equation of state in the external space confirms the above results.

gr-qc↗

arXiv2010

Kaluza-Klein models: can we construct a viable example?

In Kaluza-Klein models, we investigate soliton solutions of Einstein equation. We obtain the formulas for perihelion shift, deflection of light, time delay of radar echoes and PPN parameters. We find that the solitonic parameter k should be very big: |k|\geq 2.3\times10^4. We define a soliton solution which corresponds to a point-like mass source. In this case the soliton parameter k=2, which is clearly contrary to this restriction. Similar problem with the observations takes place for static spherically symmetric perfect fluid with the dust-like equation of state in all dimensions. The common for both of these models is the same equations of state in our three dimensions and in the extra dimensions. All dimensions are treated at equal footing. To be in agreement with observations, it is necessary to break the symmetry between the external/our and internal spaces. It takes place for black strings which are particular examples of solitons with k\to \infty. For such k, black strings are in concordance with the observations. Moreover, we show that they are the only solitons which are at the same level of agreement with the observations as in general relativity. Black strings can be treated as perfect fluid with dust-like equation of state p_0=0 in the external/our space and very specific equation of state p_1=-(1/2)εin the internal space. The latter equation is due to negative tension in the extra dimension. We also demonstrate that dimension 3 for the external space is a special one. Only in this case we get the latter equation of state. We show that the black string equations of state satisfy the necessary condition of the internal space stabilization. Therefore, black strings are good candidates for a viable model of astrophysical objects (e.g., Sun) if we can provide a satisfactory explanation of negative tension for particles constituting these objects.

gr-qc↗

arXiv2010

Classical tests of multidimensional gravity: negative result

In Kaluza-Klein model with toroidal extra dimensions, we obtain the metric coefficients in a weak-field approximation for delta-shaped matter sources. These metric coefficients are applied to calculate the formulas for frequency shift, perihelion shift, deflection of light and parameterized post-Newtonian (PPN) parameters. In the leading order of approximation, the formula for frequency shift coincides with well-known general relativity expression. However, for perihelion shift, light deflection and PPN parameter $γ$ we obtain formulas $Dπr_g/[(D-2)a(1-e^2)]$, $(D-1)r_g/[(D-2)ρ]$ and $1/(D-2)$ respectively, where $D$ is a total number of spatial dimensions. These expressions demonstrate good agreement with experimental data only in the case of ordinary three-dimensional $(D=3)$ space. This result does not depend on the size of the extra dimensions. Therefore, in the considered multidimensional Kaluza-Klein models the point-like masses cannot produce the gravitational field which corresponds to the classical gravitational tests.

gr-qc↗

arXiv2010

Bouncing inflation in nonlinear $R^2+R^4$ gravitational model

We study a gravitational model with curvature-squared $R^2$ and curvature-quartic $R^4$ nonlinearities. The effective scalar degree of freedom $ϕ$ (scalaron) has a multi-valued potential $U(ϕ)$ consisting of a number of branches. These branches are fitted with each other in the branching and monotonic points. In the case of four-dimensional space-time, we show that the monotonic points are penetrable for scalaron while in the vicinity of the branching points scalaron has the bouncing behavior and cannot cross these points. Moreover, there are branching points where scalaron bounces an infinite number of times with decreasing amplitude and the Universe asymptotically approaches the de Sitter stage. Such accelerating behavior we call bouncing inflation. For this accelerating expansion there is no need for original potential $U(ϕ)$ to have a minimum or to check the slow-roll conditions. A necessary condition for such inflation is the existence of the branching points. This is a new type of inflation. We show that bouncing inflation takes place both in the Einstein and Brans-Dicke frames.

hep-th↗

arXiv2009

Problematic aspect of extra dimensions

We show that in multidimensional Kaluza-Klein models the formula of the perihelion shift is $Dπm'^2c^2r_g^2/[2(D-2)M^2]$ where $D$ is a total number of spatial dimensions. This expression demonstrates good agreement with experimental data only in the case of ordinary three-dimensional $(D=3)$ space. This result does not depend on the size of the extra dimensions. Therefore, considered multidimensional Kaluza-Klein models face a severe problem.

gr-qc↗

arXiv2009

Non-relativistic limit of multidimensional gravity: exact solutions and applications

It is found the exact solution of the Poisson equation for the multidimensional space with topology $M_{3+d}=\mathbb{R}^3\times T^d$. This solution describes smooth transition from the newtonian behavior $1/r_3$ for distances bigger than periods of tori (the extra dimension sizes) to multidimensional behavior $1/r^{1+d}_{3+d}$ in opposite limit. In the case of one extra dimension $d=1$, the gravitational potential is expressed via compact and elegant formula. These exact solutions are applied to some practical problems to get the gravitational potentials for considered configurations. Found potentials are used to calculate the acceleration for point masses and gravitational self-energy.It is proposed models where the test masses are smeared over some (or all) extra dimensions. In 10-dimensional spacetime with 3 smeared extra dimensions, it is shown that the size of 3 rest extra dimensions can be enlarged up to submillimeter for the case of 1TeV fundamental Planck scale $M_{Pl(10)}$. In the models where all extra dimensions are smeared, the gravitational potential exactly coincides with the newtonian one regardless of size of the extra dimensions. Nevertheless, the hierarchy problem can be solved in these models.

gr-qc↗

arXiv2009

Multidimensional gravity in non-relativistic limit

It is found the exact solution of the Poisson equation for the multidimensional space with topology $M_{3+d}=\mathbb{R}^3\times T^d$. This solution describes smooth transition from the newtonian behavior $1/r_3$ for distances bigger than periods of tori (the extra dimension sizes) to multidimensional behavior $1/r^{1+d}_{3+d}$ in opposite limit. In the case of one extra dimension $d=1$, the gravitational potential is expressed via compact and elegant formula. It is shown that the corrections to the gravitational constant in the Cavendish-type experiment can be within the measurement accuracy of Newton's gravitational constant $G_N$. It is proposed models where the test masses are smeared over some (or all) extra dimensions. In 10-dimensional spacetime with 3 smeared extra dimensions, it is shown that the size of 3 rest extra dimensions can be enlarged up to submillimeter for the case of 1TeV fundamental Planck scale $M_{Pl(10)}$. In the models where all extra dimensions are smeared, the gravitational potential exactly coincides with the newtonian one. Nevertheless, the hierarchy problem can be solved in these models.

hep-th↗

arXiv2009

The shape of multidimensional gravity

In the case of one extra dimension, well known Newton's potential $ϕ(r_3)=-G_N m/r_3$ is generalized to compact and elegant formula $ϕ(r_3,ξ)=-(G_N m/r_3)\sinh(2πr_3/a)[\cosh(2πr_3/a)-\cos(2πξ/a)]^{-1}$ if four-dimensional space has topology $\mathbb{R}^3\times T$. Here, $r_3$ is magnitude of three-dimensional radius vector, $ξ$ is extra dimension and $a$ is a period of a torus $T$. This formula is valid for full range of variables $r_3 \in [0,+\infty)$ and $ξ\in [0,a]$ and has known asymptotic behavior: $ϕ\sim 1/r_3$ for $r_3>>a$ and $ϕ\sim 1/r_4^2$ for $r_4=\sqrt{r_3^2+ξ^2}<<a$. Obtained formula is applied to an infinitesimally thin shell, a shell, a sphere and two spheres to show deviations from the newtonian expressions. Usually, these corrections are very small to observe at experiments. Nevertheless, in the case of spatial topology $\mathbb{R}^3\times T^{d}$, experimental data can provide us with a limitation on maximal number of extra dimensions.

gr-qc↗

arXiv2008

On the problem of inflation in nonlinear multidimensional cosmological models

We consider a multidimensional cosmological model with nonlinear quadratic $R^2$ and quartic $R^4$ actions. As a matter source, we include a monopole form field, D-dimensional bare cosmological constant and tensions of branes located in fixed points. In the spirit of the Universal Extra Dimensions models, the Standard Model fields are not localized on branes but can move in the bulk. We define conditions which ensure the stable compactification of the internal space in zero minimum of the effective potentials. Such effective potentials may have rather complicated form with a number of local minima, maxima and saddle points. Then, we investigate inflation in these models. It is shown that $R^2$ and $R^4$ models can have up to 10 and 22 e-foldings, respectively. These values are not sufficient to solve the homogeneity and isotropy problem but big enough to explain the recent CMB data. Additionally, $R^4$ model can provide conditions for eternal topological inflation. However, the main drawback of the given inflationary models consists in a value of spectral index $n_s$ which is less than observable now $n_s\approx 1$. For example, in the case of $R^4$ model we find $n_s \approx 0.61$.

hep-th↗

arXiv2007

Extra dimensions and Lorentz invariance violation

We consider effective model where photons interact with scalar field corresponding to conformal excitations of the internal space (geometrical moduli/gravexcitons). We demonstrate that this interaction results in a modified dispersion relation for photons, and consequently, the photon group velocity depends on the energy implying the propagation time delay effect. We suggest to use the experimental bounds of the time delay of gamma ray bursts (GRBs) photons propagation as an additional constrain for the gravexciton parameters.

hep-ph↗

arXiv2007

1/R multidimensional gravity with form-fields: stabilization of extra dimensions, cosmic acceleration and domain walls

We study multidimensional gravitational models with scalar curvature nonlinearity of the type 1/R and with form-fields (fluxes) as a matter source. It is assumed that the higher dimensional space-time undergoes Freund-Rubin-like spontaneous compactification to a warped product manifold. It is shown that for certain parameter regions the model allows for a freezing stabilization of the internal space near the positive minimum of the effective potential which plays the role of the positive cosmological constant. This cosmological constant provides the observable late-time accelerating expansion of the Universe if parameters of the model is fine tuned. Additionally, the effective potential has the saddle point. It results in domain walls in the Universe. We show that these domain walls do not undergo inflation.

hep-th↗

arXiv2006

AdS non-linear curvature-squared and curvature-quartic multidimensional (D=8) gravitational models with stabilized extra dimensions

We investigate $D$-dimensional gravitational model with curvature-quadratic and curvature-quartic correction terms: $R+R^2+R^4$. It is assumed that the corresponding higher dimensional spacetime manifold undergos a spontaneous compactification to a manifold with warped product structure. Special attention is paid to the stability of the extra-dimensional factor space for a model with critical dimension D=8. It is shown that for certain parameter regions the model allows for a freezing stabilization of this space. The effective four-dimensional cosmological constant is negative and the external four-dimensional spacetime is asymptotically AdS.

hep-th↗

arXiv2006

Sp-brane accelerating cosmologies

We investigate time dependent solutions (S-brane solutions) for product manifolds consisting of factor spaces where only one of them is non-Ricci-flat. Our model contains minimally coupled free scalar field as a matter source. We discuss a possibility of generating late time acceleration of the Universe. The analysis is performed in conformally related Brans-Dicke and Einstein frames. Dynamical behavior of our Universe is described by its scale factor. Since the scale factors of our Universe are described by different variables in both frames, they can have different dynamics. Indeed, we show that with our S-brane ansatz in the Brans-Dicke frame the stages of accelerating expansion exist for all types of the external space (flat, spherical and hyperbolic). However, applying the same ansatz for the metric in the Einstein frame, we find that a model with flat external space and hyperbolic compactification of the internal space is the only one with the stage of the accelerating expansion. Scalar field can prevent this acceleration. It is shown that the case of hyperbolic external space in Brans-Dicke frame is the only model which can satisfy experimental bounds for the fine structure constant variations. We obtain a class of models where a pare of dynamical internal spaces have fixed total volume. It results in fixed fine structure constant. However, these models are unstable and external space is non-accelerating.

hep-th↗

arXiv2005

AdS and stabilized extra dimensions in multidimensional gravitational models with nonlinear scalar curvature terms 1/R and R^4

We study multidimensional gravitational models with scalar curvature nonlinearities of the type 1/R and R^4. It is assumed that the corresponding higher dimensional spacetime manifolds undergo a spontaneous compactification to manifolds with warped product structure. Special attention is paid to the stability of the extra-dimensional factor spaces. It is shown that for certain parameter regions the systems allow for a freezing stabilization of these spaces. In particular, we find for the 1/R model that configurations with stabilized extra dimensions do not provide a late-time acceleration (they are AdS), whereas the solution branch which allows for accelerated expansion (the dS branch) is incompatible with stabilized factor spaces. In the case of the R^4 model, we obtain that the stability region in parameter space depends on the total dimension D=dim(M) of the higher dimensional spacetime M. For D>8 the stability region consists of a single (absolutely stable) sector which is shielded from a conformal singularity (and an antigravity sector beyond it) by a potential barrier of infinite height and width. This sector is smoothly connected with the stability region of a curvature-linear model. For D<8 an additional (metastable) sector exists which is separated from the conformal singularity by a potential barrier of finite height and width so that systems in this sector are prone to collapse into the conformal singularity. This second sector is not smoothly connected with the first (absolutely stable) one. Several limiting cases and the possibility for inflation are discussed for the R^4 model.

hep-th↗

arXiv2004

Phenomenology of brane-world cosmological models

We present a brief review of brane-world models - models in which our observable Universe with its standard matter fields is assumed as localized on a domain wall (three-brane) in a higher dimensional surrounding (bulk) spacetime. Models of this type arise naturally in M-theory and have been intensively studied during the last years. We pay particular attention to the covariant projection approach, the Cardassian scenario, to induced gravity models, self-tuning models and the Ekpyrotic scenario. A brief discussion is given of their basic properties and their connection with conventional FRW cosmology.

gr-qc↗

arXiv2004

Remarks on dimensional reduction of multidimensional cosmological models

Multidimensional cosmological models with factorizable geometry and their dimensional reduction to effective four-dimensional theories are analyzed on sensitivity to different scalings. It is shown that a non-correct gauging of the effective four-dimensional gravitational constant within the dimensional reduction results in a non-correct rescaling of the cosmological constant and the gravexciton/radion masses. The relationship between the effective gravitational constants of theories with different dimensions is discussed for setups where the lower dimensional theory results via dimensional reduction from the higher dimensional one and where the compactified space components vary dynamically.

gr-qc↗