arXiv subjects

A. V. Yurov

Publications and source records attributed to A. V. Yurov.

At least 19 recordsLinked to original sources

arXiv2026

Singularity softening and avoidance by the action of thermal radiation in a generalized entropic cosmology

Some relevant aspects of a new form of generalized entropic cosmology, recently introduced by Nojiri, Odintsov and Faraoni, are considered. The setup is a logarithmic equation of state for a viscous dark fluid coupled with dark matter, in the ordinary Friedmann-Lemaître-Robertson-Walker flat universe. The influence of thermal effects, caused by Hawking radiation, near the singularity, are carefully investigated. In particular, their role on the formation and specific type of the Big Rip expected to occur within a finite time. It is shown that a scenario arises, where a qualitative change towards the good direction, in the type of the singularity formed, does occur. On top of that, another very interesting scenario is obtained, where the singularity vanishes completely.

gr-qc↗

arXiv2024

Holographic bounce cosmological models induced by viscous dark fluid from a generalized non-singular entropy function

Bounce cosmological models containing a dark viscous fluid in a spatially flat Friedmann-Robertson-Walker (FRW) universe are considered. The universe evolution is described in terms of generalized equation of state (EoS) parameters, in presence of the bulk viscosity. Entropic cosmology plays a key role in the discussion, and the matter bounce behavior is described based on a non-singular, generalized entropy function, recently proposed by Odintsov and Paul. Three different forms for the scale factor are investigated: an exponential, a power-law, and a double-exponential function, respectively. Appropriate bounce cosmological models are formulated, via the relevant parameters of the modified EoS, and analytical expressions for the corresponding infrared cut-off are obtained, via the particle horizon. Results are displayed in holographic form, making use of generalized holographic cut-offs first introduced by Nojiri and Odintsov. In addition, the viability of the corresponding bounce cosmological models is investigated, taking into account the actual thermodynamic properties of our universe, by means of a no-singular, generalized entropy function. In the asymptotic case, an expression for the generalized entropy is obtained, which remarkably has the additivity property.

gr-qc↗

arXiv2023

Little Rip, Pseudo Rip and bounce cosmology from generalized equation of state in the Universe with spatial curvature

We consider the Little Rip (LR), Pseudo Rip (PR) and bounce cosmological models in the Friedmann-Robertson-Walker (FRW) metric with nonzero spatial curvature. We describe the evolution of the universe using a generalized equation of state in the presence of a viscous fluid. The conditions of the occurrence of the LR, PR and bounce were obtained from the point of view of the parameters of the generalized equation of state for the cosmic dark fluid, taking into account the spatial curvature. The analytical expressions for the spatial curvature were obtained. Asymptotic cases of the early and late universe are considered. A method of Darboux transformation was proposed in the case of models of an accelerating universe with viscosity.

gr-qc↗

arXiv2019

What can the anthropic principle tell us about the future of the dark energy universe?

An anthropic explanation for evident smallness of the value of the dark energy implies the existence of a time-dependent component of the scalar field, serving, together with a negative-valued cosmological constant, as one of two components to the overall density of the dark energy. The observers (i.e. us) might then only evolve in those regions of the universe where the sum of those two components (a positive and a negative ones) is sufficiently close to zero. However, according to Vilenkin and Garriga, the scalar field component has to slowly but surely diminish in time. In about a trillion years this process will put a cap to the now-observable accelerated expansion of the universe, leading to a subsequent phase of impending collapse. However, the vanishing scalar field might also produce some rather unexpected singularities for a finite non-zero scale factor. We analyse this possibility on a particular example of Sudden Future Singularities (SFS) and come to a startling conclusion: the time required for SFS to arise must be "comparable" to the lifetime of observable universe.

gr-qc↗

arXiv2018

The Landau-Lifshitz equation, the NLS, and the magnetic rogue wave as a by-product of two colliding regular "positons"

In this article we present a new method for construction of exact solutions of the Landau-Lifshitz-Gilbert equation (LLG) for ferromagnetic nanowires. The method is based on the established relationship between the LLG and the nonlinear Schrödinger equation (NLS), and is aimed at resolving an old problem: how to produce multiple-rogue wave solutions of NLS using just the Darboux-type transformations. The solutions of this type - known as P-breathers - have been proven to exist by Dubard and Matveev, but their technique heavily relied on using the solutions of yet another nonlinear equation, Kadomtsev-Petviashvili I equation (KP-I), and its relationship with NLS. We have shown that in fact one doesn't have to use KP-I but can instead reach the same results just with NLS solutions, but only if they are dressed via the binary Darboux transformation. In particular, our approach allows to construct all the Dubard-Matveev P-breathers. Furthermore, the new method can lead to some completely new, previously unknown solutions. One particular solution that we have constructed describes two positon-like waves, colliding with each other and in the process producing a new, short-lived rogue wave. We called this unusual solution (rogue wave begotten after the impact of two solitons) the "impacton".

nlin.SI↗

arXiv2015

The Cauchy problem for the generalized hyperbolic Novikov-Veselov equation

We begin by introducing a new procedure for construction of the exact solutions to Cauchy problem of the real-valued (hyperbolic) Novikov-Veselov equation. The procedure shown therein utilizes the well-known Airy function $\text{Ai}(ξ)$ which in turn serves as a solution to the ordinary differential equation $\frac{d^2 z}{d ξ^2} = ξz$. In the second part of the article we show that the aforementioned procedure can also work for the $n$-th order generalizations of the Novikov-Veselov equation, provided that one replaces the Airy function with the appropriate solution of the ordinary differential equation $\frac{d^{n-1} z}{d ξ^{n-1}} = ξz$.

nlin.SI↗

arXiv2014

The KdV in Cosmology: a useful tool or a distraction?

The letter is a response to the recent article by J. Lidsey. We demonstrate that the Schwarzian derivative technique developed therein is but a consequence of linearizabiliy of the original cosmological equations. Furthermore, we show the required linearized equation to be nothing else but a Schrödinger equation.

gr-qc↗

arXiv2013

Brane cosmology from observational surveys and its comparison with standard FRW cosmology

Several dark energy models on the brane are investigated. They are compared with corresponding theories in the frame of 4d Friedmann-Robertson-Walker cosmology. To constrain the parameters of the models considered, recent observational data, including SNIa apparent magnitude measurements, baryon acoustic oscillation results, Hubble parameter evolution data and matter density perturbations are used. Explicit formulas of the so-called {\it state-finder} parameters in teleparallel theories are obtained that could be useful to test these models and to establish a link between Loop Quantum Cosmology and Brane Cosmology. It is concluded that a joint analysis as the one developed here allows to estimate, in a very convenient way, possible deviation of the real universe cosmology from the standard Friedmann-Robertson-Walker one.

gr-qc↗

arXiv2012

One method for constructing exact solutions of equations of two-dimensional hydrodynamics of an incompressible fluid

We propose a simple algebraic method for constructing exact solutions of equations of two-dimensional hydrodynamics of an incompressible fluid. The problem reduces to consecutively solving three linear partial differential equations for a nonviscous fluid and to solving three linear partial differential equations and one first-order ordinary differential equation for a viscous fluid.

nlin.SI↗

arXiv2011

Friedman vs Abel equations: A connection unraveled

We present an interesting connection between Einstein-Friedmann equations for the models of universe filled with scalar field and the special form of Abel equation of the first kind. This connection works in both ways: first, we show how, knowing the general solution of the Abel equation (corresponding to the given scalar field potential) one can obtain the general solution of the Friedman Equation (and use the former for studying such problems as existence of inflation with exit for particular models). On the other hand, one can invert the procedure and construct the Bäcklund auto-transformations for the Abel equation.

hep-th↗

arXiv2010

Brane-like singularities with no brane

We use a method of linearization to study the emergence of the future cosmological singularity characterized by finite value of the cosmological radius. We uncover such singularities that keep Hubble parameter finite while making all higher derivatives of the scale factor (starting out from the $\ddot a$) diverge as the cosmological singularity is approached. Since such singularities has been obtained before in the brane world model we name them the "brane-like" singularities. These singularities can occur during the expanding phase in usual Friedmann universe filled with both a self-acting, minimally coupled scalar field and a homogeneous tachyon field. We discover a new type of finite-time, future singularity which is different from type I-IV cosmological singularities in that it has the scale factor, pressure and density finite and nonzero. The generalization of $w$-singularity is obtained as well.

astro-ph.CO↗

arXiv2009

The linearization method and new classes of exact solutions in cosmology

We develop a method for constructing exact cosmological solutions of the Einstein equations based on representing them as a second-order linear differential equation. In particular, the method allows using an arbitrary known solution to construct a more general solution parameterized by a set of 3\textit{N} constants, where \textit{N} is an arbitrary natural number. The large number of free parameters may prove useful for constructing a theoretical model that agrees satisfactorily with the results of astronomical observations. Cosmological solutions on the Randall-Sundrum brane have similar properties. We show that three-parameter solutions in the general case already exhibit inflationary regimes. In contrast to previously studied two-parameter solutions, these three-parameter solutions can describe an exit from inflation without a fine tuning of the parameters and also several consecutive inflationary regimes.

astro-ph.CO↗

arXiv2008

Quantum Creation of a Universe in Albrecht-Magueijo-Barrow model

In quantum cosmology the closed universe can spontaneously nucleate out of the state with no classical space and time. The semiclassical tunneling nucleation probability can be estimated as $\emph{P}\sim\exp(-α^2/Λ)$ where $α$=const and $Λ$ is the cosmological constant. In classical cosmology with varying speed of light $c(t)$ (VSL) it is possible to solve the horizon problem, the flatness problem and the $Λ$-problem if $c=sa^n$ with $s$=const and $n<-2$. We show that in VSL quantum cosmology with $n<-2$ the semiclassical tunneling nucleation probability is $\emph{P}\sim\exp(-β^2Λ^k)$ with $β$=const and $k>0$. Thus, the semiclassical tunneling nucleation probability in VSL quantum cosmology is very different from that in quantum cosmology with $c$=const. In particular, it can be strongly suppressed for large values of $Λ$. In addition, we propose the instanton which describes the nucleation of closed universes in VSL models. This solution is akin to the Hawking-Turok instanton in the means of O(4) invariance but, unlike to it, is non-singular. Moreover, using this solution we can obtain the probability of nucleation which is suppressed for large value of $Λ$ too.

hep-th↗

arXiv2007

The Destiny of Universes After the Big Trip

The big trip can be describe with the help of the Wheeler-DeWitt wave equation ${\hat H}ψ(w,a)=0$. The probability to find the universe after big trip in the state with $w=w_0$ will be maximal if $\partialψ(w,a)/\partial w|_{w=w_0}=0$ for any values of the scale factor $a$. It is shown that this will be the case if and only if $w_0=-1/3$. This fact allows one to suggest that vast majority of universes in multiverse must be in this state after their big trips.

astro-ph↗

arXiv2007

Generalized Fubini instantons

We show that $(1+2)$ nonlinear Klein-Gordon equation with negative coupling admits an exact solution which appears to be the linear superposition of the plane wave and the nonsingular rational soliton. We show that the same approach allows to construct the solution of similar properties for the Euclidean $ϕ^4$ model with broken symmetry. Interestingly, this regular solution will be of instanton type only in the $D\le 5$ Euclidean space.

hep-th↗

arXiv2007

The dressing procedure for the cosmological equations and the indefinite future of the universe

In this paper we present a new simple method of construction of infinite number of solutions of Freidmann equations from the already known ones, which allows for a startling conclusion of practical impossibility of correct predictions on the universe's future dynamics which are based solely on astronomical observations on the value of a scale factor. In addition, we present particular examples of newly constructed solutions, such as the ones, describing the smooth dynamical (de)-phantomization, and the models lacking the events horizons (both in classical and brane world cases). The generalization of the method to the simplest anisotropic universes are presented as well.

astro-ph↗

arXiv2005

One more observational consequence of many-worlds quantum theory

Using new cosmological doomsday argument Page predicts that the maximal lifetime of de Sitter universe should be $t_{max}=10^{60}$ yr which is way too small in comparison with strings predictions ($t_f>$googleplex). However, since this prediction is dependant on the total number of human observations, we show that Page arguments results instead in astounding conclusion that this number is the quantum variable and is therefore much greater then Page's estimation. Identifying it with the number of coarse-grained histories in de Sitter universe we get the lifetime of the universe comparable with strings predictions. Moreover, it seems that this result can be considered as another one of the observational evidences of validity of the many-worlds quantum theory. Finally, we show that for the universe filled with phantom energy $t_{max}\sim t_f$ up to very high precision.

hep-th↗

arXiv2005

Darboux Transformation for Dirac Equations with (1+1) potentials

We study the Darboux transformation (DT) for Dirac equations with (1+1) potentials. Exact solutions for the adiabatic external field are constructed. The connection between the exactly soluble Dirac (1+1) potentials and the soliton solutions of the Davey--Stewartson equations is discussed.

hep-th↗