Rip Cosmology Research Articles

Behaviours of rip cosmological models in f(Q,C) gravity

In this study, the Universe's rip cosmology theories have been provided for the f(Q,C) gravity theory, where Q and C stand for the non-metricity scalar and boundary term. We assume f(Q,C)=αQn+βC and analyze the nature of the physical parameters for the Little Rip (LR), Big Rip (BR) and Pseudo Rip (PR) models. In the LR and PR models, the EoS parameter exhibits phantom characteristics but remains closely aligned with the ΛCDM line. The non-metricity term Q has direct effect on the rip models. After investigating the energy conditions, we recognise that our model violates the strong energy constraint. Avoiding singularity situations has been noted in all of these accelerated models. The characteristics of the jerk and snap parameters have been investigated. Our model provides an effective description of the Universe's evolutionary history and fits well with contemporary cosmic data.

Inhomogeneous equation of state and its consequences in a coupled fluid scenario and realization of Little Rip, Pseudo Rip and Bounce cosmology

Motivated by the work of P. H. Frampton et al. [Phys. Lett. B 708 (2012) 204; I. Brevik et al., Phys. Rev. D 84 (2011) 103508, arXiv:1107.4642 [hep-th]; I. Brevik et al., Phys. Rev. D 86 (2012) 063007, arXiv:1208.4770 [gr-qc]] this study reports on the reconstruction schemes for the inhomogeneous equation of state (EoS) first introduced by S. Nojiri et al., [Phys. Rev. D 72 (2005) 023003, arXiv:hep-th/0505215 [hep-th]] and the cosmology in the scenario of coupled fluid. We have considered the viscosity through the Eckart approach in an interacting system. The EoS parameter for the Little Rip, the [Formula: see text] in terms of cosmic time for Pseudo Rip, is reconstructed for the EoS for Bounce cosmology inhomogeneous viscous fluid. The EoS parameter is rebuilt for the coupled dark fluid in the nonviscous system for the Little Rip, Pseudo Rip and Bounce cosmology. The reconstructed EoS parameter for inhomogeneous viscous fluid was phantom for the Little Rip cosmology. The reconstructed EoS parameters of holographic Ricci dark energy (HRDE) of the coupled fluid in nonviscous systems are phantom for both the Little Rip and Pseudo Rip cosmology. The model HRDE of the coupled fluid in the nonviscous scenario for Pseudo-Rip cosmology indicates the possibility of singularity. The same model for the bounce cosmology violates the null energy condition and has the EoS parameter almost symmetric around the bouncing point.

Dynamic study of Weyl tensor corrected f(R) gravity

We investigate the cosmological and thermodynamic aspects of Weyl tensor corrected [Formula: see text] gravity. For this purpose, we assume some well-known cosmological bouncing scenarios such as symmetric bounce cosmology, oscillatory cosmology, matter bounce cosmology, little rip cosmology, superbounce cosmology and develop some cosmological parameters. For instance, the equation of state parameter [Formula: see text] describes the quintessence phase for symmetric bounce cosmology, vacuum phase for oscillatory, little rip and matter bounce cosmology while it gives both quintessence and vacuum phases for matter bounce cosmology. It is also observed that the squared speed of sound [Formula: see text] gives positive behavior for all models resulting in that the models assumed are stable. We evaluate generalized second law of thermodynamics which remains valid for all cosmological models except symmetric bounce cosmology. Moreover, we also investigate the thermal equilibrium condition [Formula: see text] and found its validity for all models except symmetric bounce cosmological model.

Little rip in classical and quantum f(R) cosmology

The little rip is a cosmological abrupt event predicted by some phantom dark energy models that could describe the future evolution of our Universe. This event can be interpreted as a big rip singularity delayed indefinitely, although in those models bounded structures will be destroyed in a finite cosmic time in the future. In this work, we analyse the little rip cosmology from a classical and quantum point of view within the scheme of alternative metric $f(R)$ theories of gravity. The quantum analysis is performed in the framework of $f(R)$ quantum geometrodynamics by means of the modified Wheeler-DeWitt equation. In this context, we show that the DeWitt criterion can be satisfied. Similar to what happens in general relativity, this result points towards the avoidance of the little rip in $f(R)$ quantum cosmology.

Rip cosmologies, wormhole solutions and big trip in the f(T,𝒯 ) theory of gravity

Rip cosmological models have been investigated in the framework of [Formula: see text] theory of gravity, where [Formula: see text] denotes the torsion and [Formula: see text] is the trace of the energy–momentum tensor. These phantom cosmological models revealed that at initial epoch a EoS parameter [Formula: see text] tends asymptotically at late phase to [Formula: see text] [Formula: see text]. On the other hand, Wormhole Solutions and Big Trip have been subject to an investigation. The wormhole throat radius [Formula: see text] and the conditions to be satisfied to produce the Big Trip phenomenon have been discussed.

Black holes in the turbulence phase of viscous rip cosmology

We study the phantom fluid in the late universe, thus assuming the equation of state parameter [Formula: see text] to be less than [Formula: see text]. The fluid is assumed to consist of two components, one laminar component [Formula: see text] and one turbulent component [Formula: see text], the latter set proportional to [Formula: see text] as well as to the Hubble parameter, [Formula: see text] with [Formula: see text] a positive constant associated with the turbulence. The effective energy density is taken to be [Formula: see text], and the corresponding effective pressure is [Formula: see text], with [Formula: see text] constant. These basic assumptions lead to a Big Rip universe; the physical quantities diverging during a finite rip time [Formula: see text]. We then consider the mass accretion of a black hole in such a universe. The most natural assumption of setting the rate [Formula: see text] proportional to [Formula: see text] times the sum [Formula: see text] leads to a negative mass accretion, where [Formula: see text] goes to zero linearly in [Formula: see text] near the singularity. The Hubble parameter diverges as [Formula: see text], whereas [Formula: see text] and [Formula: see text] diverge as [Formula: see text]. We also discuss other options and include, for the sake of comparison, some essential properties of mass accretion in the early (inflationary) universe.

Linking little rip cosmologies with regular early universes

Cosmological fluids with a Generalized Equation of State (GEoS) are here considered, whose corresponding EoS parameter $\omega$ describes a fluid with phantom behavior, namely $\omega<-1$, but leading to universes free of singularities at any past or future, finite time. Thus avoiding, in particular, the Big Bang and the Big Rip singularities, the last one considered to be typical in phantom fluid models. More specifically, such GEoS fluid cosmologies lead to regular Little Rip universes. A remarkable new property of these solutions is proven here, namely that they avoid the initial singularity at early times; therefore, they are able to describe emergent universes. Solutions of this kind had been studied previously, but only either as late time or as early time solutions; never as solutions covering both epochs simultaneously. Appropriate conditions are proposed here that relate the Little Rip cosmologies with the initial regular universe, for the future and past regimes, respectively. This is done by taking as starting point the conditions under which a given scale factor corresponds to a Little Rip universe.

Teleparallelism by inhomogeneous dark fluid

In this paper, we investigate \(f(T)\) cosmology and find an exact solution for \(f\) which gives a Little Rip cosmology. Also, considering accelerating cosmology with dark matter, the time-dependent solution is found. For these cases, by using solutions obtained from \(f(T)\) gravity we find expressions for \(\omega\) and \(\varLambda\) defined as time functions via equivalent description in terms of inhomogeneous fluid. This puts the question: which theoretical model describes the observational cosmology?

Locally rotationally symmetric (LRS) Bianchi type II string dust viscous fluid cosmological models in Lyra geometry

In the present study, a locally rotationally symmetric Bianchi type II string dust cosmological model with varying bulk viscosity in the framework of Lyra geometry is investigated. To get the deterministic solution, we have assumed that shear (σ) is proportional to the expansion (θ) and ζθ = constant, where ζ is the coefficient of bulk viscosity. The ansatz ζθ = constant, was found to coincide with the occurrence of Little Rip (LR) cosmology using Friedmann–Robertson–Walker metric (Breviket al. Phys. Rev. D, 84, 103508-1-6 (2011)). The model describes the pre-inflationary stage of the universe. The presence of viscosity leads to inflationary-like solutions. Anisotropy is maintained because of the presence of strings. The presence of bulk viscosity prevents the matter density from vanishing. In a special case, the model represents the isotropy of the universe. The universe passes through a decelerating stage to an accelerating phase, which is in agreement with Ries et al.’s (Astrophys. J. 607, 665 (2004). doi:10.1086/383612 ) type Ia supernovae astronomical observations. The displacement vector (β) of Lyra geometry is a decreasing function of time analogous to the cosmological constant (Λ) of general relativity. We have attempted to explore the possibility of constructing a string dust viscous fluid cosmological model in Bianchi type II space–time with displacement vector (β) in Lyra geometry using the geometric condition that shear is proportional to expansion.

Thermodynamics in Little Rip cosmology in the framework of a type of f(R, T) gravity

The cosmological reconstruction of the Little Rip model in f (R, T) gravity is investigated, where R is the curvature scalar and T the trace of the energy momentum tensor. The model perfectly reproduces the present stage of the universe, characterized by the $ \Lambda$ CDM model, without singularity at future finite-time (without the Big Rip). The input parameters are determined according to Supernovae cosmology data and perfectly fit with the WMAP around the Little Rip. Moreover, the thermodynamics is considered in this Little Rip cosmology and it is illustrated that the second law of thermodynamics is always satisfied around the Little Rip universe for the temperature inside the horizon being the same as that of the apparent horizon. Moreover, we show the existence of a stable fixed point in the Little Rip universe which confirms that this is actually a late-time attractor in the phantom-dominated universe. The linear perturbation analysis is performed around the critical points, showing that the Little Rip model obtained is stable.

LRS Bianchi type I stiff fluid inflationary universe with variable bulk viscosity

Stiff fluid inflationary cosmological model with variable bulk viscosity in the frame work of locally rotationally symmetric (LRS) Bianchi type I space–time, is investigated. To get the deterministic solution, we have also assumed that σ is proportional to θ and ζθ = constant = α where σ is shear, θ the expansion, and ζ the coefficient of bulk viscosity. The ansatz ζθ = constant, was found to connect with the occurrence of Little Rip cosmology using Friedmann–Robertson–Walker metric given by Brevik et al. (Phys. Rev. D, 84, 103508 (2011)). We find that the model has constant expansion but represents accelerating phase. The model in general represents an anisotropic universe but in special case, it leads to an isotropic one. The spatial volume increases exponentially with time representing the inflationary scenario. The presence of bulk viscosity tends to increase the inflationary phase. Also, the model leads to the de Sitter model in a special case. The presence of bulk viscosity prevents the matter density to vanish at late time. The energy conditions as given by Kolassis et al. (Classical Quantum Gravity, 5, 1129 (1988)). Chatterjee and Banerjee (Gen. Relativ. Gravitation, 36, 303 (2004)) are also discussed. We find that the energy conditions are satisfied for stiff fluid inflationary universe with variable bulk viscosity.

Rip Cosmology via Inhomogeneous Fluid

The conditions for the appearance of the Little Rip, Pseudo Rip and Quasi Rip universes in the terms of the parameters in the equation of state of some dark fluid are investigated. Several examples of the Rip cosmologies are investigated.

Turbulence accelerating cosmology from an inhomogeneous dark fluid

Specific dark energy models with a linear inhomogeneous time-dependent equation of state, within the framework of 4d Friedman-Robertson-Walker (FRW) cosmology, are investigated. It is demonstrated that such 4d inhomogeneous fluid models may lead to a turbulence FRW cosmology. Both one-component and two-component models from 4d inhomogeneous dark fluid models are considered. In the one-component model the universe may develop from a viscous era with, for instance, a constant bulk viscosity, into a turbulent era. In the two-component model the fluid can be decomposed into two components, one non-turbulent (ideal) and another turbulent part, obeying two different equations of state. Conditions for the appearance of the turbulent dark energy universe in terms of the parameters in the equation of state (EoS) without introducing the turbulence concept explicitly are are obtained. An equivalent description in terms of an inhomogeneous fluid for the viscous Little Rip (LR) cosmology is also developed.

The Little Rip cosmology with the time-dependent equation of state

The discovery of accelerated expansion of the Universe resulted in the development of new theoretical models in cosmology [1, 2]. In this work, we discuss the cosmological models, where the parameter w in the equation of state is smaller than 1 − ( ) 1 w < − , however, 1 w →− asymptotically in such a way that there arises no singularity at finite time t . This nonsingular cosmology is referred to as Little Rip [3]. The LR models describe an intermediate evolution of the Universe between the model with the de Sitter asymptotic expansion and those with a Big Rip (BR) singularity. In so doing, two cases are possible. In the first case, the dark-energy density is increased with time without occurrence of the BR singularity. In the second one, the dark-energy density tends to a cosmological constant value with time (de Sitter asymptotic space). This case corresponds to the Pseudo Rip (PR) model discussed in [4]. An interesting scenario is connected with the LR cosmology where the Hubble parameter tends to infinity in the infinite future [5–10]. The LR cosmology can be described in terms of an ordinary liquid with a complicated equation of state [11, 12]. The nonsingular LR cosmology results in the decay of the objects bound by gravity force (galaxies, solar system, etc.) in the future similar to the BR cosmology. This work is concerned with the study of the effect of time-dependent parameters w and Λ on the LR occurrence in different cosmological models of the effect.

Rip brane cosmology from 4d inhomogeneous dark fluid universe

Specific dark energy models with linear inhomogeneous time-dependent equation of state, within the framework of 4d Friedman-Robertson-Walker (FRW) cosmology, are investigated. It is demonstrated that the choice of such 4d inhomogeneous fluid models may lead to a brane FRW cosmology without any explicit account of higher dimensions at all. Effectively, we thus obtain a brane dark energy universe without introducing the brane concept explicitly. Several examples of brane Rip cosmology arising from 4d inhomogeneous dark fluid models are given.

LITTLE RIP COSMOLOGICAL MODELS WITH TIME-DEPENDENT EQUATION OF STATE

Specific dark energy models, leading to the Little Rip (LR) cosmology in the far future, are investigated. Conditions for the occurrence of LR in terms of the parameters present in the proposed equation of state for the dark energy cosmic fluid are studied. Estimates about the time needed before the occurrence of the small singularity in the standard LR model in which the universe approaches the de Sitter spacetime asymptotically, are given.

Turbulence and little rip cosmology

A variety of conditions are considered under which the cosmic dark fluid may be able to develop a future Big Rip or Little Rip singularity. Both one-component and two-component models are considered. In the latter case we present a way in which the fluid can be decomposed into two components, one nonturbulent (ideal) and one turbulent part, obeying two different equations of state. For the nonturbulent part, the thermodynamical parameter, commonly called $w$, is assumed to be less than $\ensuremath{-}1$ throughout. For the turbulent part, it turns out that it is sufficient that ${w}_{\mathrm{turb}}$ lies in the quintessence region in order to lead to a singularity. Both Big Rip and Little Rip behavior for dark energy are found. In the one-component case, we examine how the universe may develop from a viscous era with constant bulk viscosity into a turbulent era, the turbulence in effect protecting the universe from encountering the singularity at all. The equivalent description of the same cosmology in terms of inhomogeneous (imperfect) fluid is also presented.

Reconstruction off(T)gravity: Rip cosmology, finite-time future singularities, and thermodynamics

We demonstrate that there appear finite-time future singularities in $f(T)$ gravity with $T$ being the torsion scalar. We reconstruct a model of $f(T)$ gravity with realizing the finite-time future singularities. In addition, it is explicitly shown that a power-law-type correction term ${T}^{\ensuremath{\beta}}$ ($\ensuremath{\beta}&gt;1$) such as a ${T}^{2}$ term can remove the finite-time future singularities in $f(T)$ gravity. Moreover, we study $f(T)$ models with realizing inflation in the early universe, the $\ensuremath{\Lambda}\mathrm{CDM}$ model, little rip cosmology and pseudo-rip cosmology. It is demonstrated that the disintegration of bound structures for little rip and pseudo-rip cosmologies occurs in the same way as in gravity with corresponding dark energy fluid. We also discuss that the time-dependent matter instability in the star collapse can occur in $f(T)$ gravity. Furthermore, we explore thermodynamics in $f(T)$ gravity and illustrate that the second law of thermodynamics can be satisfied around the finite-time future singularities for the universe with the temperature inside the horizon being the same as that of the apparent horizon.

Phantom inflation in little rip

We study the phantom inflation in little rip cosmology, in which the current acceleration is driven by the field with the parameter of state w<−1, but since w tends to −1 asymptotically, the rip singularity occurs only at infinite time. In this scenario, before the rip singularity is arrived, the universe is in an inflationary regime. We numerically calculate the spectrum of primordial perturbation generated during this period and find that the results may be consistent with observations. This implies that if the reheating happens again, the current acceleration might be just a start of phantom inflation responsible for the upcoming observational universe.

Phantom cosmology without Big Rip singularity

We construct phantom energy models with the equation of state parameter w which is less than −1, w<−1, but finite-time future singularity does not occur. Such models can be divided into two classes: (i) energy density increases with time (“phantom energy” without “Big Rip” singularity) and (ii) energy density tends to constant value with time (“cosmological constant” with asymptotically de Sitter evolution). The disintegration of bound structure is confirmed in Little Rip cosmology. Surprisingly, we find that such disintegration (on example of Sun–Earth system) may occur even in asymptotically de Sitter phantom universe consistent with observational data. We also demonstrate that non-singular phantom models admit wormhole solutions as well as possibility of Big Trip via wormholes.