We investigate the restrictions on the equation-of-state parameter of phantom cosmology, due to the minimum quantum gravitational requirements. We find that for all the examined $w_Λ(z)$-parametrizations and for arbitrary phantom potentials and spatial curvature, the phantom equation-of-state parameter is not restricted at all. This is in radical contrast with the quintessence paradigm, and makes phantom cosmology more robust and capable of constituting the underlying mechanism for dark energy.
Some phantom cosmological models without big rip singularity have been constructed in a simple extended theory of gravity. In the geometrical part of the action, a minimally coupled linear function of the Ricci Scalar and the trace of the energy momentum tensor have been considered in place of the Ricci scalar. Four Little Rip and Pseudo Rip models have been investigated where the equation of state parameter evolves asymptotically and sufficiently rapidly to $-1$. The effect of the coupling constant of the extended gravity theory on the dynamics has been discussed. Possible wormhole solutions …
We show that the recent results of \ [Int. J. Mod. Phys. D 25 (2016) 1650051] on the application of Lie/Noether symmetries in scalar field cosmology are well-known in the literature while the problem could have been solved easily under a coordinate transformation. That follows from the property, that the admitted group of invariant transformations of dynamical system is independent on the coordinate system.
弦气体宇宙学的主要优点之一是它解决了宇宙学奇点。由于宇宙在暴胀时代结束时可以近似为局部渐近的德西特时空，奇点定理意味着这些宇宙学实际上违反了高能条件[而不仅仅是强能量条件]。我们强调，这是一个非常可靠的结果，它并不依赖于假设空间截面在早期宇宙中保持精确平坦。然而，这意味着弦宇宙学必须有可能跨越最近讨论得很多的"幻影鸿沟"[从w-1开始，其中w是状态方程参数]。< -1 to w >这自然提出了一个问题，即是否可以再次跨越幻影鸿沟，以解释最近的观测结果，即目前w <-1。我们认为，非微扰弦效应排除了这种可能性，即使NEC违反问题只是“有效”。
Motivated by the exciting features and a recent proposed general form of the function of non-metricity scalar Q, we investigate the cosmological implications in $f(Q)$ gravity, through the resulting effective dark energy sector, extracting analytical expressions for the dark energy density, equation-of-state parameter and the deceleration parameters. We show that even in the absence of a cosmological constant, the universe exhibits the usual thermal history, with the sequence of matter and dark energy eras, and the dark-energy equation-of-state parameter always lie in the phantom regime. Addit…
We study the quantum avoidance of the big rip singularity in the Eddington-inspired-Born-Infeld (EiBI) phantom model. Instead of considering a simple phantom dark energy component, which is described by a perfect fluid, we consider a more fundamental degree of freedom corresponding to a phantom scalar field with its corresponding potential, which would lead the classical universe to a big rip singularity. We apply a quantum geometrodynamical approach by performing an appropriate Hamiltonian study including an analysis of the constraints of the system. We then derive the Wheeler-DeWitt (WDW) eq…
Effective equation of state for dark energy: mimicking quintessence and phantom energy through a variable Lambda
虽然在实验宇宙学的各个方面都有越来越多的证据表明宇宙中存在着不消失的暗能量成分，但我们离理解它的最终性质还很远。一个基本的宇宙学常数，λ，是最自然的候选者，但许多动力学机制，以产生一个有效的λ已被设计出来，假设存在一个特殊的标量场（所谓的精华，及其推广）。这些模型本质上是特设的，但它们导致了具有非平凡状态方程（EOS）的时间演化暗能量的吸引人的可能性。大多数，如果不是全部，未来的精密宇宙学实验研究（例如SNAP和普朗克项目）非常仔细地解决了EOS参数化“a la quintessence”的确定。在这里，我们表明，通过将宇宙学数据拟合到这种状态方程，也可以解释为一个基本的，但随时间演化的宇宙学项的暗示：λ = λ（t）。我们通过研究与λ的重整化群（RG）模型相关的有效状态方程来验证这种可能性。我们发现有效EOS可以对应于正常本质和幻影暗能量，具体取决于RG模型单个参数的值。我们的结论是，背后的一个非平凡的状态方程的一个所谓的精髓或幻影标量场实际上可以有一个运行的宇宙学项的基本量子场论。
Using a hyperbolic complex plane, we study the realization of the underlying hyperbolic symmetry as an internal symmetry that enables the unification of scalar fields of cosmological and particle physics interest. Such an unification is achieved along the universal prescriptions used in physics, avoiding the use of concepts as Euclideanization, non-canonical Lagrangians and hidden structures, that have appeared in other approaches. The scalar potentials constructed within the present scheme are bounded from below, and the realization of the spontaneous symmetry breaking of the aforementioned n…
We consider the possibility to solve the issues of the phantom field cosmology by means of the PT-symmetric quantum theory. The Born-Oppenheimer approximation is applied to the Wheeler-DeWitt equation to study the inhomogeneous fluctuations over the homogeneous minisuperspace. The evolution of the longitudinal inhomogeneous modes in WKB-time is described using a time-dependent pseudo-Hermitian effective Hamiltonian.
We present modified cosmological scenarios that arise from the application of the "gravity-thermodynamics" conjecture, using the Barrow entropy instead of the usual Bekenstein-Hawking one. The former is a modification of the black hole entropy due to quantum-gravitational effects that deform the black-hole horizon by giving it an intricate, fractal structure. We extract modified cosmological equations which contain new extra terms that constitute an effective dark-energy sector, and which coincide with the usual Friedmann equations in the case where the new Barrow exponent acquires its Bekenst…
We construct modified cosmological scenarios through the application of the first law of thermodynamics in the universe horizon, but using the generalized, nonextensive Tsallis entropy instead of the usual Bekenstein-Hawking one. We result to modified cosmological equations that possess the usual ones as a particular limit, but which in the general case contain extra terms that appear for the first time, that constitute an effective dark energy sector quantified by the nonextensive parameter $δ$. When the matter sector is dust, we extract analytical expressions for the dark energy density and …
We construct a holographic dark energy scenario based on Kaniadakis entropy, which is a generalization of Boltzmann-Gibbs entropy that arises from relativistic statistical theory and is characterized by a single parameter $K$ which quantifies the deviations from standard expressions, and we use the future event horizon as the Infrared cutoff. We extract the differential equation that determines the evolution of the effective dark energy density parameter, and we provide analytical expressions for the corresponding equation-of-state and deceleration parameters. We show that the universe exhibit…
We analyze from a classical and quantum point of view the behavior of the universe close to a little rip, which can be interpreted as a big rip sent towards the infinite future. Like a big rip singularity, a little rip implies the destruction of all bounded structure in the Universe and is thus an event where quantum effects could be important. We present here a new phantom scalar field model for the little rip. The quantum analysis is performed in quantum geometrodynamics, with the Wheeler-DeWitt equation as its central equation. We find that the little rip can be avoided in the sense of the …
