Alexander E. Pavlov
In the present paper we study equations of state of Casimir vacuum of massive scalar field and massive bispinor field in compact Friedmann Universe. With use of the Abel-Plana formula the renormalization of divergent series for calculation of the quantum means of operators is implemented.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
继续探索
The quantum Casimir condensate of a conformal massive scalar field in a compact Friedmann universe is considered in the static approximation. The Abel-Plana formula is used for renormalization of divergent series in the condensate calculation. A differential relation between the static Casimir energy density and static Casimir condensate is derived.
In a previous work [S. Rahbardehghan et al. in Phys. Lett. B 750, 627 (2015)], we considered a simple brane-world model; a single $4$-dimensional brane embedded in a $5$-dimensional de Sitter (dS) space-time. Then, by including a conformally coupled scalar field in the bulk, we studied the induced Casimir energy-momentum tensor. Technically, the Krein-Gupta-Bleuler (KGB) quantization scheme as a covariant and renormalizable quantum field theory in dS space was used to perform the calculations. In the present paper, we generalize this study to a less idealized, but physically motivated, scenari…
We calculate the total internal energy, total energy density and pressure, and the free energy for the neutrino and electromagnetic fields in Einstein and closed Friedmann cosmological models. The Casimir contributions to all these quantities are separated. The asymptotic expressions for both the total internal energy and free energy, and for the Casimir contributions to them are found in the limiting cases of low and high temperatures. It is shown that the neutrino field does not possess a classical limit at high temperature. As for the electromagnetic field, we demonstrate that the total int…
We present exact solutions to the Friedmann equation in standard cosmology in Weierstrass and Jacobi functions. The right hand side of the Friedmann equation describing various contributions of matter sources is considered in generic form. It is proved that the problem of integration of the Friedmann equation for simple EoS of medium is reduced to solving Abel integrals for algebraic functions.
We evaluate the Hadamard function, the vacuum expectation values (VEVs) of the field squared and the energy-momentum tensor for a massive scalar field with general curvature coupling parameter in the geometry of two parallel plates on a spatially flat Friedmann-Robertson-Walker background with a general scale factor. On the plates, the field operator obeys the Robin boundary conditions with the coefficients depending on the scale factor. In all the spatial regions, the VEVs are decomposed into the boundary-free and boundary-induced contributions. Unlike to the problem with the Minkowski bulk, …
Horizon entropy consistent with FLRW equations for general modified theories of gravity and for all EoS of the matter field
本文推导连接任意引力理论FLRW方程与表观视界热力学的广义视界熵，并提出对所有物质场EoS均成立的修正热力学定律$TdS=-dE+\rho dV$。
We evaluate the two-point functions of the electromagnetic field in (D+1) -dimensional spatially flat Friedmann-Robertson-Walker universes with a power-law scale factor, assuming that the field is prepared in the Bunch-Davies vacuum state. The range of powers are specified in which the two-point functions are infrared convergent and the Bunch-Davies vacuum for the electromagnetic field is a physically realizable state. The two-point functions are applied for the investigation of the vacuum expectation values of the field squared and the energy-momentum tensor, induced by a single and two paral…
We investigate the Wightman function, the vacuum expectation values of the field squared and the energy-momentum tensor for a massless scalar field with general curvature coupling parameter in spatially flat Friedmann-Robertson-Walker universes with an arbitrary number of toroidally compactified dimensions. The topological parts in the expectation values are explicitly extracted and in this way the renormalization is reduced to that for the model with trivial topology. In the limit when the comoving lengths of the compact dimensions are very short compared to the Hubble length, the topological…
We describe the accelerated expansion of the late-time universe using a generalized equation of state (EoS) when account is taken of bulk viscosity. We assume a homogeneous and isotropic Friedmann-Robertson-Walker spacetime. Solutions of the gravitational equations for dark energy are obtained in implicit form. Characteristic properties of the Universe evolution in the presence of the viscosity effects are discussed. Finally, the dynamics of the accelerated expansion of the viscous universe is discussed, on the basis of a modified logarithmic-corrected EoS.
We study the Friedmann inflation in general covariant Horava-Lifshitz (HL) gravity without the projectability conditions and with detailed and non-detailed balance conditions. Accordingly, we derive modifications in the Friedmann equations due to a single-scalar field potential describing minimal-supersymmetrically extended inflation. By implementing two time-independent equations of state (EoS) characterizing the cosmic background geometry filled up with dark energy, the dependence of the tensorial and scalar density fluctuations and their ratios on the inflation field are determined. The lat…
The accelerating Friedmann flat universe, filled with an ideal fluid with a linear (oscillating) inhomogeneous equation of state (EoS) depending on time, is reviewed. The equations of motions are solved. It is shown that in some cases there appears a quasi-periodic universe, which repeats the cycles of phantom-type space acceleration.
We propose a novel equation of state (EoS) which explains the evolutionary history of a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe. The uniqueness of this EoS lies in the fact that it incorporates the Lambert $W$ function in a special fashion. It is explicitly demonstrated that with observationally relevant values of the unknown parameters $\vartheta_\mathrm{1}$ and $\vartheta_\mathrm{2}$, all the evolutionary phases of the Universe can be reproduced. Moreover, it also shows that the initial singularity is unavoidable and asserts that the late-time acceleration of the Universe wo…
In this paper, we try to consider the stability conditions of a modified gravity coupled by Weyl tensor. In this way, we indicate the suitable conditions for a successful bounce while the equation of state (EoS) parameter crosses the phantom divider for our new corrected modified gravity. In the spatially flat Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) universe, according to the ordinary version of the holographic dark energy (HDE) model, describing accelerated expansion of the universe, will be considered. Our model's stability conditions and its general properties of attractors for scalar …
In recent years there has been a growing interest in the field of Casimir wormhole. In classical general relativity (GR), it is known that the null energy condition (NEC) has to be violated to have a wormhole to be stable. The Casimir effect is an experimentally verified effect that is caused due to the vacuum field fluctuations in quantum field theory. Since the Casimir effect provides the negative energy density, thus this act as an ideal candidate for the exotic matter needed for the stability of the wormhole. In this paper, we study the Casimir effect on the wormhole geometry in modified s…
We discuss the exact solutions of brane universes and the results indicate the Friedmann equations on the branes are modified with a new density term. Then, we assume the new term as the density of dark energy. Using Wetterich's parametrization equation of state (EOS) of dark energy, we obtain the new term varies with the red-shift z. Finally, the evolutions of the mass density parameter $Ω_2$, dark energy density parameter $Ω_x$ and deceleration parameter q_2 are studied.
Cosmological Analysis of $f(R, Σ, T)$ Gravity with EoS Parameterization
本文研究了 $f(R, Σ, T)$ 引力理论中通过方程状态参数化对宇宙学的分析，利用观测数据验证了该模型与 $Λ$CDM 情景的一致性，并展示了其在解释晚期宇宙加速方面的可行性。
We generalize the results of hep-th/0008140 to the case of the (n+1)-dimensional closed FRW universe satisfying a general equation of state of the form p=wρ. We find that the entropy of the universe can no longer be expressed in a form similar to the Cardy formula, when w\neq 1/n. As a result, in general the entropy formula does not coincide with the Friedmann equation when the conjectured bound on the Casimir energy is saturated. Furthermore, the conjectured bound on the Casimir energy generally does not lead to the Hubble and the Bekenstein entropy bounds.
Scalar field cosmologies with a generalized harmonic potential and a matter fluid with a barotropic Equation of State (EoS) with barotropic index $γ$ for the Locally Rotationally Symmetric (LRS) Bianchi I and flat Friedmann-Lemaître-Robertson-Walker (FLRW) metrics are investigated. Methods from the theory of averaging of nonlinear dynamical systems are used to prove that time-dependent systems and their corresponding time-averaged versions have the same late-time dynamics. Therefore, the simplest time-averaged system determines the future asymptotic behavior. Depending on the values of $γ$, th…
Holographic dark energy in modified Barrow cosmology
本文在修正Barrow熵框架下重新审视全息暗能量（HDE）模型，探讨其对Friedmann方程及能量密度的影响，并分析Hubble视界和未来事件视界作为IR截断的宇宙学后果，发现Barrow指数δ显著影响HDE行为，使状态方程参数可跨越幽灵线并改变宇宙相变时间。
In this paper, the model of interaction is studied between $f(T,\mathcal{T})$ gravity and modified Chaplygin gas in FRW-flat metric. We obtain the Friedmann equations in the framework of teleparallel gravity by vierbein field. We consider that Universe dominates by components of cold matter, dark energy and modified Chaplygin gas. In what follows we separately write the corresponding continuity equations for components of Universe. Also, dark energy EoS and effective EoS are obtained with respect to redshift, thereafter the corresponding cosmological parameters are plotted in terms of redshift…
Little Rip and Pseudo Rip cosmological models with coupled dark energy based on a new generalized entropy
本研究基于新广义熵，在含体粘滞的耦合暗能量-暗物质FRW宇宙中，推导Little Rip和Pseudo Rip出现的条件，并修正EoS参数与热力学量，揭示早期及晚期宇宙性质。
We study the cosmological evolutions of the equation of state (EoS) for the universe in the homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker (FLRW) space-time. In particular, we reconstruct the cyclic universes by using the Weierstrass and Jacobian elliptic functions. It is explicitly illustrated that in several models the universe always stays in the non-phantom (quintessence) phase, whereas there also exist models in which the crossing of the phantom divide can be realized in the reconstructed cyclic universes.
Holographic description of the dissipative unified dark fluid model with axion field
本文扩展轴子F(R)引力模型，应用全息原理结合含体积粘度的状态方程，利用广义红外截断全息暗能量，在平坦FRW宇宙中实现了轴子物质存在下早期与晚期宇宙的统一描述。
We apply a quadratic teleparallel torsion scalar of the $f(T)=T+αT^{2}$ field equations to the spatially flat Friedmann-Robertson-Walker (FRW) model. We assume two perfect fluid components, the matter component has a fixed equation of state (EoS) parameter $ω$, while the torsion component has a dynamical EoS. We obtain an effective scale factor allowing a graceful exit inflation model with no need to slow roll technique. We perform a standard cosmological study to examine the cosmic evolution. In addition, the effective EoS shows consistent results confirming a smooth phase transition from inf…
Scalar field cosmologies with a generalized harmonic potential and a matter fluid with a barotropic Equation of State (EoS) with barotropic index $γ$ for Locally Rotationally Symmetric (LRS) Bianchi III metric and open Friedmann-Lemaître-Robertson-Walker (FLRW) metric are investigated. Methods from the theory of averaging of nonlinear dynamical systems are used to prove that time-dependent systems and their corresponding time-averaged versions have the same late-time dynamics. Therefore, simple time-averaged systems determine the future asymptotic behavior. Depending on values of barotropic in…
Probing Dark Energy Properties with Barrow Holographic Model in f(Q, C) Gravity
本研究在f(Q,C)引力框架下探究巴罗全息暗能量（BHDE）模型，分析平直FRW度规下的关键宇宙学参数，发现其EoS参数符合宇宙加速膨胀特征且渐近趋近ΛCDM模型结果，为解释暗能量提供了动态框架。
Exploring Dark Energy via Non-Minimal Coupling in $f(Q,L_m)$ Gravity with Gong-Zhang Parameterization
本文在非最小耦合$f(Q,L_m)$引力中结合Gong-Zhang参数化推导修正Friedmann方程，利用CC和Pantheon+SH0ES数据约束模型，揭示了暗能量从精质到幽灵型的演化，为$\Lambda$CDM提供了可行替代。
We investigate quantum vacuum effects for a massive scalar field, induced by two planar boundaries in background of a linearly expanding spatially flat Friedmann-Robertson-Walker spacetime for an arbitrary number of spatial dimensions. For the Robin boundary conditions and for general curvature coupling parameter, a complete set of mode functions is presented and the related Hadamard function is evaluated. The results are specified for the most important special cases of the adiabatic and conformal vacuum states. The vacuum expectation values of the field squared and of the energy-momentum ten…
Bouncing Theory in the Modified $f(R,G,T)$ Gravity
本研究提出修正$f(R,G,T)$引力模型，通过穿越幻影分界线并反转尺度因子实现反弹宇宙学，解决初始奇点问题，并验证了五个模型样本的有效性。
We study the dynamics of expansion of the homogeneous isotropic Universe and the evolution of its components in the model with nonminimally coupled dynamical dark energy. Dark energy, like the other components of the Universe, is described by the perfect fluid approximation with the equation of state (EoS) $p_ {de}=wρ_{de}$, where the EoS parameter $w$ depends on time and is parameterized via the squared adiabatic sound speed $c_{ a}^2$ which is assumed to be constant. On basis of the general covariant conservation equations for the interacting dark energy and dark matter and Einstein equation…
Scalar field cosmologies with a generalized harmonic potential and matter with energy density $ρ_m$, pressure $p_m$, and barotropic equation of state (EoS) $p_m=(γ-1)ρ_m, \; γ\in[0,2]$ in Kantowski-Sachs (KS) and closed Friedmann--Lemaître--Robertson--Walker (FLRW) metrics are investigated. We use methods from non--linear dynamical systems theory and averaging theory considering a time--dependent perturbation function $D$. We define a regular dynamical system over a compact phase space, obtaining global results. That is, for KS metric the global late--time attractors of full and time--averaged…
We investigate a brane model based on Randall-Sundrum scenarios with a generic dark energy component. The latter drives the accelerated expansion at late times of the Universe. In this scheme, extra terms are added into Einstein Field equations that are propagated to the Friedmann equations. To constrain the dark energy equation of state (EoS) and the brane tension we use observational data with different energy levels (Supernovae type Ia, $H(z)$, baryon acoustic oscillations, and cosmic microwave background radiation distance, and a joint analysis) in a background cosmology. Beside EoS being …
We demonstrate two periodic or quasi-periodic generalizations of the Chaplygin gas (CG) type models to explain the origins of dark energy as well as dark matter by using the Weierstrass $\wp(t)$, $σ(t)$ and $ζ(t)$ functions with two periods being infinite. If the universe can evolve periodically, a non-singular universe can be realized. Furthermore, we examine the cosmological evolution and nature of the equation of state (EoS) of dark energy in the Friedmann-Lemaître-Robertson-Walker cosmology. It is explicitly illustrated that there exist three type models in which the universe always stays …
We summarize the present state of research on the darkon fuid as a model for the dark sector of the Universe. Nonrelativistic massless particles are introduced as a realization of the Galilei group in an enlarged phase space. The additional degrees of freedom allow for a nonstandard, minimal coupling to gravity respecting Einstein's equivalence principle. Extended to a self-gravitating fluid the Poisson equation for the gravitational potential contains a dynamically generated effective gravitational mass density of either sign. The equations of motion (EOMs) contain no free parameters and are …
In this work, in the framework of dynamical system analysis, we focus on the study of the accelerated expansion of the Universe of $f(Q)$ gravity theory where $Q$ be the non-metricity that describes the gravitational interaction. We consider the linear form of $f(Q)$ gravity i.e. $f(Q)=-α_{1}Q-α_{2}$ where $α_{1}$ and $α_{2}$ are constants. We consider an interaction between dark matter (DM) and dark energy (DE) in $f(Q)$ gravity. To reduce the modified Friedmann equations to an autonomous system of first-order ordinary differential equations, we introduce some dimensionless new variables. The…
Based on recent perturbative and non-perturbative lattice calculations with almost quark flavors and the thermal contributions from photons, neutrinos, leptons, electroweak particles, and scalar Higgs bosons, various thermodynamic quantities, at vanishing net-baryon densities, such as pressure, energy density, bulk viscosity, relaxation time, and temperature have been calculated up to the TeV-scale, i.e. covering hadron, QGP and electroweak (EW) phases in the early Universe. This remarkable progress motivated the present study to determine the possible influence of the bulk viscosity in the ea…
Within the context of scalar-tensor gravity, we explore the generalized second law (GSL) of gravitational thermodynamics. We extend the action of ordinary scalar-tensor gravity theory to the case in which there is a non-minimal coupling between the scalar field and the matter field (as chameleon field). Then, we derive the field equations governing the gravity and the scalar field. For a FRW universe filled only with ordinary matter, we obtain the modified Friedmann equations as well as the evolution equation of the scalar field. Furthermore, we assume the boundary of the universe to be enclos…
Modified $f(R,G,T)$ Gravity in the Quintom model, and Inflation
本文研究Quintom模型中耦合两类标量场的f(R,G,T)修正引力，在平坦FLRW宇宙中实现反弹条件，证明仅含暴胀场时EoS参数不能穿越phantom分界线，并表明该模型可涵盖Weyl共形几何与暴胀时期。
We consider a class of metric f(R) modified gravity theories, analyze them in the context of a Friedmann-Robertson-Walker cosmology and confront the results with some of the known constraints imposed by observations. In particular, we focus in correctly reproducing the matter and effective cosmological constant eras, the age of the Universe, and supernovae data. Our analysis differs in many respects from previous studies. First, we avoid any transformation to a scalar-tensor theory in order to be exempted of any potential pathologies (e.g. multivalued scalar potentials) and also to evade any u…
Dark Energy Phenomenology in a $f(R,Σ,T)$ Gravity Framework: $O_m(z)$ Parameterization Approach
本文通过$O_m(z)$诊断的对数参数化重构Hubble参数，探讨$f(R,Σ,T)$引力的宇宙学影响，揭示暗能量方程状态参数动态变化，保持 quintessence 行为，避免phantom行为，且与观测数据一致。
Holographic bounce cosmological models induced by viscous dark fluid from a generalized non-singular entropy function
本文在空间平坦FRW宇宙中研究含暗粘性流体的反弹宇宙学模型，基于Odintsov和Paul提出的非奇异广义熵函数描述物质反弹，构建了三种标度因子形式的模型并推导了红外截断解析表达式，验证了其热力学可行性。
We consider the Chiral cosmological model consisting of two scalar fields minimally coupled to gravity. In the context of a Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) space-time, and for massless fields in the presence of a cosmological constant, we present the general solution of the field equations. The minisuperspace configuration that possesses maximal symmetry leads to scenarios which - depending on the admissible value of the parameters - correspond to a quintessence, quintom or phantom case. The canonical quantization of the model retrieves this distinction as different families of…
In this paper, we study the $f(T,\mathcal{T})$ gravity model in the presence of the bulk viscosity by the flat-FRW metric. The field equation is obtained by teleparallel gravity with tetrad field. The universe components are considered as matter and dark energy which the dark energy component associates from the viscous $f(T,\mathcal{T})$ gravity. After calculating the Friedmann equations, we obtain the energy density, the pressure and the EoS of dark energy in terms of the redshift parameter. Afterward, we plot the corresponding cosmological parameters versus the redshift parameter and examin…
Models of f(Q) gravity with electromagnetic field
本文研究了具有电磁场的f(Q)引力模型，通过重构技术生成不同物质源的f(Q)拉格朗日量，并探讨了宇宙加速、本质暗能量和宇宙学常数等场景。
EDSFD parametrization in $ f(R,T) $ gravity with linear curvature terms
本文在 $f(R,T)$ 引力下利用 EDSFD 参数化研究平坦 FLRW 模型，结合 Hubble、Pantheon 和 BAO 数据约束参数，并与标准 $Λ$CDM 模型对比分析减速参数等物理量的演化。
Holographic cosmology with logarithmic equation of state based on a new generalized entropy
本文基于新广义熵提出的一种修正对数方程状态，研究了暗粘性流体在平坦FRW宇宙中的应用，探讨了暗物质与粘性暗能量耦合下的宇宙演化及全息形式的表达。
The R_h=ct Universe is a Friedmann-Robertson-Walker (FRW) cosmology which, like LCDM, assumes the presence of dark energy in addition to (baryonic and non-luminous) matter and radiation. Unlike LCDM, however, it is also constrained by the equation of state (EOS) p=-rho/3, in terms of the total pressure p and energy density rho. One-on-one comparative tests between R_h=ct and LCDM have been carried out using over 14 different cosmological measurements and observations. In every case, the data have favoured R_h=ct over the standard model, with model selection tools yielding a likelihood ~90-95% …
Reconstructing FHDE with Scalar and Gauge Fields
本文在平坦 FRW 宇宙中用六类标量场与规范场重构 FHDE，计算 1<α≤2 时的 EoS 及动能、势能特征，证明其可支持晚期加速、避免幽灵分界，并渐近趋近 ΛCDM。
Constrained transit cosmological models in $f(R,L_{m},T)$-gravity
本文研究了$f(R,L_{m},T)$-引力中的约束过渡宇宙学模型，通过施加状态方程约束推导出能量守恒方程，并利用观测数据对宇宙学参数进行限制，揭示了宇宙的减速到加速膨胀转变历史。
In this paper, we study the viscous $f(T, B)$ gravity model as a source of dark energy, and the Extended Bose-Einstein Condensate (EBEC) as a source of dark matter, in a flat-FRW metric. In the presence of bulk viscosity, we obtain Friedmann equations and write two continuity equations of dark energy and dark matter by interacting them. Using the generalized Gross-Pitaeveskii equation, we earn Equation of State (EoS) of dark matter by EBEC regime as $p_m = αρ_{m} + βρ_{m}^2$ in which the both of terms are respectively introduced as normal dark matter and dark matter halo. The innovation of the…
We derive an exact $f(T)$ gravity in the absence of ordinary matter in Friedmann-Robertson-Walker (FRW) universe, where $T$ is the teleparallel torsion scalar. We show that vanishing of the energy-momentum tensor $\mathcal{T}^{μν}$ of matter does not imply vanishing of the teleparallel torsion scalar, in contrast to general relativity, where the Ricci scalar vanishes. The theory provides an exponential (inflationary) scale factor independent of the choice of the sectional curvature. In addition, the obtained $f(T)$ acts just like cosmological constant in the flat space model. Nevertheless, it …
Cosmological Evolution, Analytical Parametrization, and Dynamical Stability of a Bouncing Universe in Non-Minimal Kinetic Coupling Gravity
本文提出一种非最小动能耦合引力模型，通过直接获得标度因子并拟合指数函数，研究宇宙 bounce 阶段的演化与稳定性，探索早期宇宙的动态行为及暴胀时期的稳定性。
扫码添加微信好友，提出您的宝贵建议 👇
💡 备注请填写：网站反馈
发表机构由 AI 从论文文本抽取，如发现错误，欢迎反馈修正建议。