This paper investigated two scalar field cosmological models in $f(R,T)$ gravity with cosmic transit and varying cosmological constant $Λ(t)$.The cosmological constant tends to have a tiny positive value in the current epoch.The scalar field pressure $p_ϕ$ shows a sign reversal for a normal scalar field. For the phantom field, the scalar potential $V(ϕ)$ is negative and the energy density $ρ_ϕ=E_k+V$ takes negative values when the equation of state parameter $ω_ϕ$ is less than $-1$. While the weak energy condition WEC implies that the total energy density $ρ=\sum_iρ_i\geq 0$, we still can have a negative $ρ$ term as long as the total $ρ$ is positive. In the current work we argue that the WEC, $ρ=\sum_i ρ_i \geq 0$ and $p_i+ρ_i \geq 0$, is not violated but with an instability for the second model at late-times. For a scalar field $ϕ$, The condition $ρ_ϕ+p_ϕ=ρ_ϕ (1+ω_ϕ)=2E_k\geq 0$ allows for $ρ_ϕ<0$ if $ω_ϕ<-1$. The causality and energy conditions have been discussed for both models.
The cosmology in both models was studied using a given function $a(t)$ derived from the desired cosmic behavior, which is the opposite of the traditional view.
The stability of two different bounce scenarios from F(R,G) modified gravity at later times is studied, namely a hyperbolic cosine bounce model and a matter-dominated one. After describing the main characteristics of F(R,G) modified gravity, the two different bounce scenarios stemming from this theory are reconstructed and their stability at late stages is discussed. The stability of the hyperbolic cosine model is proven, while the concrete matter-bounce model here chosen does not seem to accomplish the necessary conditions to be stable at later times.
Chaos and Universality in the Dynamics of Inflationary Cosmologies
我们描述了一个新的统计模式在封闭的膨胀宇宙学的混沌动力学，与哈密顿旋转运动能量和双曲运动能量块的分区，在一个线性邻域的鞍中心存在于相空间的模型。双曲能量的轨道访问附近的鞍中心有一个随机的分布相对于合奏的初始条件，但相关的直方图定义的统计分布规律的形式$p(x) = C x^{-γ}$，几乎整个范围内的双曲能量考虑。我们提出的数值证据表明，$γ$决定的分形盆地边界的初始条件的合奏的维数。这种分布是普适的，因为它不依赖于模型的参数，并且是尺度不变的。我们讨论了这种普适性对暴胀物理学的可能物理后果。暴胀定律的形式为$p(x) = C x^{-γ}$，几乎在整个双曲能量范围内考虑。我们提出的数值证据表明，$γ$决定的分形盆地边界的初始条件的合奏的维数。这种分布是普适的，因为它不依赖于模型的参数，并且是尺度不变的。我们讨论了这种普适性对暴胀物理学的可能物理后果。
Modified expansion rates in the early Universe prior to big bang nucleosynthesis are common in modified gravity theories, and can have a significant impact on the generation of dark matter, matter-antimatter asymmetry, primordial black holes and the primordial gravitational wave (PGW) spectrum. Here we study the PGW spectrum in modified gravity theories, in early Universe cosmology. In particular, we consider scalar-tensor and extradimensional scenarios, investigating the detection prospects in current and future GW observatories. For the scalar-tensor case, PGW could be potentially observed b…
In this paper, we have investigated some accelerating cosmological models at the backdrop of an anisotropic metric in an extended gravity theory. Two viable cosmological models one with a little rip behaviour and the other with a hyperbolic form of Hubble parameter have been constructed. The dynamical aspects of the models along with some physical and geometrical parameters are analysed. Both the models presented here evolve in the phantom-like region and overlap with $Λ$CDM model at late times. We carried out a geometrical diagnosis of the model to show the viability of the models.
We revisit a general minisuperspace (MSS) formalism for scalar-tensor (ST) FLRW type cosmological models in arbitrary frame with perfect fluid source. We discuss how to impose Cauchy data on the corresponding dynamical system in order to reconstruct standard (ΛCDM) cosmological model. So far, the integrability of such models has been extensively studied in the Einstein or Jordan frames mainly. We extend these studies to arbitrary frame taking into account non-minimal coupling between matter and gravity. To this aim we explore a gauge freedom associated with a choice of lapse function. We show …
Positivity bounds - constraints on any low-energy effective field theory imposed by the fundamental axioms of unitarity, causality and locality in the UV - have recently been used to constrain various effective field theories relevant for cosmology. However, to date most of these bounds have assumed that there is a single Lorentz-invariant vacuum in which all fields have zero expectation value and in many cosmologically relevant models this is not the case. We explore ways to overcome this limitation by investigating a simple example model, the covariant Galileon, which possesses a one-paramet…
给出了在理想流体存在下标量场暗能量的精确宇宙学解的一般方法。我们利用Wheeler De Witt（WdW）方程不变变换的存在性。我们证明了WdW方程不变的点变换的存在性等价于场方程的守恒律的存在性，这表明了解析解的存在性。我们扩展了以前的工作，通过提供精确的解决方案的哈勃参数和有效的暗能量状态方程的宇宙学参数包含一个完美的流体和标量场，其自相互作用势是一个双曲函数的幂的组合。我们发现的解决方案时，完美的流体是辐射或冷暗物质，并确定非零空间曲率的影响。利用普朗克2015年的数据，我们确定了暗能量的有效状态方程的演化。最后，我们研究了使用无量纲变量的全局动力学。我们发现，如果目前的宇宙模型是刘维尔可积（承认守恒律），那么有一个唯一的稳定点，描述宇宙的de-Sitter阶段。
Cosmological Constraints on Minimal Cubic Galileon Models in Teleparallel Gravity
School of Physics, Harbin Institute of Technology（哈尔滨工业大学物理学院）；University of Tashkent for Applied Sciences（塔什干应用科学大学）；Department of Physics, New Uzbekistan University（新乌兹别克斯坦大学物理系）；Center for Astronomy and Astrophysics, Center for Field Theory and Particle Physics and Department of Physics, Fudan University（复旦大学天文与天体物理中心、场论与粒子物理中心及物理系）；Institute of Theoretical Physics, National University of Uzbekistan（乌兹别克斯坦国立大学理论物理研究所）；Institute for Advanced Studies, New Uzbekistan University（新乌兹别克斯坦大学高级研究院）；Institute of Space Sciences and Astronomy, University of Malta（马耳他大学空间科学与天文研究所）；Department of Physics, University of Malta（马耳他大学物理系）
The Einstein vacuum equations on an (n+1)-dimensional toroidal manifold $\mathbb{M}^{n+1}=\mathbb{T}^{n}\times\mathbb{R}^{+}$ reduce to a system of n-dimensional nonlinear ODEs in terms of the set of toroidal radii $(a_{i})_{i=1}^{n}$ or the radial moduli fields $(ψ_{i})_{i=1}^{n}=(\log(a_{i}(t))_{i=1}^{n}$ of the n-torus $\mathbb{T}^{n}$. This geometry is also the basis of Kasner-Bianchi-type cosmologies. The equations are trivially satisfied for static solutions $ψ_{i}^{E}=ψ^{E}$ or radii $a_{i}^{E}=a^{E}$, describing an initially static toroidal 'micro-universe' or 'vacuum bubble'. It is Ly…
