arXiv subjects

Sergey G. Rubin

Publications and source records attributed to Sergey G. Rubin.

36 records · Page 2Linked to original sources

arXiv2020

Gravity with Higher Derivatives in D-Dimensions

The aim of this review is to discuss the ways to obtain results based on gravity with higher derivatives in D-dimensional world. We considered the following ways: (1) reduction to scalar tensor gravity, (2) direct solution of the equations of motion, (3) derivation of approximate equations in the presence of a small parameter in the system, and (4) the method of test functions. Some applications are presented to illustrate each method. The unification of two necessary elements of a future theory is also kept in mind - the extra dimensions and the extended form of the gravity.

gr-qc↗

arXiv2020

The interaction of domain walls with fermions in the early Universe

We consider the scalar field solitons and their interaction with the fermions in the early Universe. The analytical form of the reflection coefficient is obtained. The fermion mass is a function of the distance between the fermion and the soliton (wall). The function was approximated by the Woods-Saxon potential.

physics.gen-ph↗

arXiv2020

Inhomogeneous compact extra dimensions and de Sitter cosmology

In the framework of multidimensional $f(R)$ gravity, we study the metrics of compact extra dimensions assuming that our 4D space has the de Sitter metric. Manifolds described by such metrics could be formed at the inflationary and even higher energy scales. It is shown that in the presence of a scalar field, varying in the extra factor space $\mathbb{M}_2$, it is possible to obtain a variety of inhomogeneous metrics in $\mathbb{M}_2$. Each of these metrics leads to a certain value of the 4D cosmological constant $Λ_4$, and in particular, it is possible to obtain $Λ_4 =0$, as is confirmed by numerically obtained solutions. A nontrivial scalar field distribution in the extra dimensions is an important feature of this family of models.

gr-qc↗

arXiv2020

How to make the physical parameters small

We study the evolution of the physical parameter values defined at the sub-planckian energies to values at low energies. The Wilson action is the basis of the research. The presence of the compact extra dimensions has two consequences. The positive point is that the integration over extra dimensions is a promising way to substantially reduce the parameters to be comparable with the observational values. On the other hand, the discreteness of the energy levels of compact extra dimensions complicates the analysis. This difficulty can be overcome with the truncated Green functions.

hep-th↗

arXiv2019

Multidimensional gravity with higher derivatives and inflation

We elaborate on the inflationary model starting from multidimensional Lagrangian and gravity with second-order curvature terms. The effective scalar field is related to the Ricci scalar of extra dimensions. It is shown that the Kretschmann and the Ricci tensor square terms dominate during inflation. The observable values of the spectral index and the tensor-to-scalar ratio are obtained for specific values of the model parameters.

gr-qc↗

arXiv2019

Evolution of sub-spaces at high and low energies

The evolution of sub-spaces in the framework of gravity with higher derivatives is studied. Numerical solutions to exact differential equations are found. It is shown that the initial conditions play crucial role in the space dynamic. Appropriate metrics describing an expanding and a stationary sub-space shed light on the well-known question: why our 3-dim space is large but extra space is small and stable (if exists)? It is assumed that the values of parameters at high energies strongly depend on uncontrolled quantum corrections and, hence, are not equal to their values at low energies. Therefore, there is no way to trace solutions throughout the energy range, and we restrict ourselves to the sub-Planckian and the inflationary energies.

gr-qc↗

arXiv2019

Clusters of primordial black holes

The Primordial Black Holes (PBHs) are gradually involved into consideration as the phenomenon having reliable basis. We discuss here the possibility of their agglomeration into clusters that may have several prominent observable features. The clusters can form due to closed domain walls appearance in the natural and the hybrid inflation with subsequent evolution and gravitational collapse. Early dustlike stages of dominance of heavy metastable dissipative particles, at which star-like objects are formed, can also naturally lead to formation of black hole clusters, remaining in the Universe after decay of particles, from which they have originated. The dynamical evolution of such clusters discussed here is of the crucial importance. Such a model inherits all the advantages of the single PBHs like possible explanation of existence of supermassive black holes (origin of the early quasars), binary BH merges registered by LIGO/Virgo through gravitational waves, contribution to reionization of the Universe, but also has additional benefits. The cluster could alleviate or completely avoid existing constraints on the single PBH abundance making PBHs a real dark matter candidate. The most of existing constraints on (single) PBH density should be re-considered as applied to the clusters. Also unidentified cosmic gamma-ray point-like sources could be (partially) accounted for by them. One can conclude, that it seems really to be much more viable model with respect to the single PBHs.

astro-ph.CO↗

arXiv2019

Inflationary limits on the size of compact extra space

We study restrictions imposed on the parameters of the Kaluza-Klein extra space supplied by the standard inflationary models. It is shown that the size of the extra space cannot be larger than $\sim 10^{-27}$cm and the $D$-dimensional Planck mass should be larger than $\sim 10^{13}$GeV. The validity of these estimates is discussed. We also study the creation of stable excitations of scalar field as the result of the extra metric evolution.

gr-qc↗

arXiv2018

Classical evolution of subspaces

We study evolution of manifolds after their creation at high energies. Several kinds of gravitational Lagrangians with higher derivatives are considered. It is shown analytically and confirmed numerically that an asymptotic growth of the maximally symmetric manifolds depends strongly on their dimensionality. A number of final metrics describing our Universe is quite poor if we limit ourselves with a maximally symmetric extra space. We show that the initial conditions can be a reason of nontrivial solutions (funnels) and study their properties.

gr-qc↗

arXiv2018

Classical transitions with the topological number changing in the early Universe

We consider classical dynamics of two real scalar fields within a model with the potential having a saddle point. The solitons of such model are field configurations that have the form of closed loops in the field space. We study the formation and evolution of these solitons, in particular, the conditions at which they could be formed even when the model potential has only one minimum. These non-trivial field configurations represent domain walls in the three-dimensional physical space. The set of these configurations can be split into disjoint equivalence classes. We provide a simple expression for the winding number of an arbitrary closed loop in the field space and discuss the transitions that change the winding number. We also show that non-trivial field configurations could be responsible for the energy density excess that could evade the CMB constraints but could be important at scales which are responsible for the formation of galaxies and the massive primordial black holes.

hep-th↗

arXiv2018

Inhomogeneous extra space as a tool for the top-down approach

In this paper, the top-down approach for the 6-dimensional space has been elaborated. The connection between the cosmological constant and the extra space metric has been obtained. The metric can be found with the necessary accuracy. It is shown that descent from high energies to the low ones leads to the quantum corrections which influence weakly the metric of extra space.

gr-qc↗

arXiv2017

Transitions between topologically non-trivial configurations

We study formation and evolution of solitons within a model with two real scalar fields with the potential having a saddle point. The set of these configurations can be split into disjoint equivalence classes. We give a simple expression for the winding number of an arbitrary closed loop in the field space and discuss the evolution scenarios that change the winding number. These non-trivial field configurations lead to formation of the domain walls in the three-dimensional physical space.

hep-th↗

arXiv2017

Inhomogeneous compact extra dimensions

We show that an inhomogeneous compact extra space possesses two necessary features --- their existence does not contradict the observable value of the cosmological constant $Λ_4$ in pure $f(R)$ theory, and the extra dimensions are stable relative to the "radion mode" of perturbations, the only mode considered. For a two-dimensional extra space, both analytical and numerical solutions for the metric are found, able to provide a zero or arbitrarily small $Λ_4$. A no-go theorem has also been proved, that maximally symmetric compact extra spaces are inconsistent with 4D Minkowski space in the framework of pure $f(R)$ gravity.

gr-qc↗

arXiv2016

Interpenetrating subspaces as a funnel to extra space

New solution for two interpenetrating universes is found. Higher derivative gravity acting in 6-dimensional space is the basis of the study that allows to obtain stable solution without introducing matter of any sort. Stability of the solution is maintained by a difference between asymptotic behavior at spacial infinities. For an external observer such a funnel looks similar to a spherical wormhole.

gr-qc↗

arXiv2015

The problem of small physical parameters and its possible solution

The mechanism of continuous set of different universes formation is elaborated. It provides tool to solve the problem of observed smallness of physical parameters. Solution of two puzzles - the hierarchy and the cosmological constant problems based on multidimensional gravity is discussed.

gr-qc↗

arXiv2015

Two-dimensional manifold with point-like defects

We study a class of two-dimensional compact extra spaces isomorphic to the sphere $S^2$ in the framework of multidimensional gravitation. We show that there exists a family of stationary metrics that depend on the initial (boundary) conditions. All these geometries have a singular point. We also discuss the possibility for these deformed extra spaces to be considered as dark matter candidates.

gr-qc↗

arXiv2015

Deformed compact extra space as dark matter candidate

We elaborate the possibility for a deformed extra space to be considered as the dark matter candidate. To perform calculations a class of two-dimensional extra metrics was considered in the framework of the multidimensional gravity. It was shown that there exists a family of stationary metrics of the extra space possessing point-like defect. Estimation of cross section of scattering of a particle of the ordinary matter on a spatial domain with deformed extra space is in agreement with the observational constraints.

gr-qc↗

arXiv2015

Scalar field localization on deformed extra space

Field localization on 2-dim extra space is considered in the framework of f(R) gravity. It is shown that an interference of local matter energy distribution and a metric of the extra space form a point-like defect - 4-dim brane. The energy-momentum of the brane depends on initial conditions that could lead to the cosmological $Λ$ term being arbitrarily small.

gr-qc↗