Cosmic accelerated expansion of the Universe with phantom fluid

The phantom behavior of the Universe is discussed in an extended version of Gauss–Bonnet gravity. Following the method proposed by the author (Int. J. Mod. Phys. D, 27, 1850078. 2018. doi: 10.1142/S0218271818500785 ), we obtain a viable cosmological model for the phantom phase of the Universe. We find a condition for m in the model ∼Gm, which shows phantom expansion of the Universe. On the other hand, using a phantom source term ∼T2n in the model we observe that the term ∼Gn, with n > 3/4, gives a phantomic space–time expansion. This form (Gn + T2n) obtained for the phantom phase of the Universe exhibits a similar form to Einstein’s gravity theory (R + Lm). However, we addressed the cosmic coincidence problem for the model.

Tsallis Holographic Dark Energy in f(G,T) Gravity

In this paper, we study the reconstruction paradigm for Tsallis holographic dark energy model using generalized Tsallis entropy conjecture with Hubble horizon in the framework of f ( G , T ) gravity (G and T represent the Gauss-Bonnet invariant and trace of the energy-momentum tensor). We take the flat Friedmann-Robertson-Walker universe model with dust fluid configuration. The cosmological evolution of reconstructed models is examined through cosmic diagnostic parameters and phase planes. The equation of the state parameter indicates phantom phase while the deceleration parameter demonstrates accelerated cosmic epoch for both conserved as well as non-conserved energy-momentum tensor. The squared speed of the sound parameter shows instability of the conserved model while stable non-conserved model for the entire cosmic evolutionary paradigm. The trajectories of the ω G T - ω G T ′ plane correspond to freezing as well as thawing regimes for the conserved and non-conserved scenario, respectively. The r - s plane gives phantom and quintessence dark energy epochs for conserved while Chaplygin gas model regime for the non-conserved case. We conclude that, upon the appropriate choice of the free parameters involved, the derived models demonstrate a self-consistent phantom universe behavior.

Changes in the role of energy conditions in f(R) cosmology by the acceleration of universe

Recently, the role of energy conditions in [Formula: see text]-cosmology has been investigated. We generalize these results to BIonic systems in accelerating systems and show that the energy conditions can be changed in this system. We show that [Formula: see text]-gravity produces a wormhole between two universes and forms a BIonic system. On the other hand, the acceleration creates two regions in the Rindler space-time which BIon in each region acts reverse to other in another region. This means that by the expansion of universes of the BIon in one region, universes of another BIon in other regions contract. Also, in this model, by increasing the order of curvatures in [Formula: see text]-gravity, the energy and the entropy of system in one region increases and in other region decreases. Amount of this growth or decrease in the energy depends on the acceleration of universes. Finally, by calculating the dark energy equation of state, we observe that one universe enters to phantom phase and goes toward the big rip singularity and another goes out of phantom state.

Viewing the Cosmological Consequences of Modified Holographic Dark Energy in Various Interaction Scenarios

In the work reported here, we have considered the interaction between modified holographic Ricci dark energy, a particular choice of Nojiri–Odintsov cut-offs, and pressureless dark matter for three different choices of scale factor, namely logamediate, intermediate, and emergent. Two different interaction terms have been considered. In one case, the interaction has been taken proportional to the dark energy density; in the other case, it has been taken proportional to the matter density. In the case of Q=3Hδρx, we have observed that there is a crossing of phantom boundary and the equation of state parameter has been observed to behave like quintessence. The influence of various parameters in the scale factors has also been observed. Also, an exit from phantom phase has been observed for the choice of scale factor in the emergent form. Considering the interaction term Q to be proportional to the matter density for logamediate scale factor, wx has been found to behave like quintessence, i.e. >−1. For the emergent scale factor in this interaction scenario, the model has been found to lead to a wx parameter lying in the region ≤−1. Therefore, in general, it has been found that Q=3Hδρx is more suitable than Q=3Hδρm in creating a model having a wx parameter crossing the phantom boundary.

Accreting Scalar-Field Models of Dark Energy Onto Morris-Thorne Wormhole

The present paper reports a study on accreting tachyon, Dirac-Born-Infeld essence and h-essence scalar field models of dark energy onto Morris-Thorne wormhole. Using three different parameterisation schemes and taking $H\, = \,{H_0}\, + \,{{{H_1}} \over t}$, we have derived the mass of the wormhole for all of the three parameterisation schemes that are able to get hold of both quintessence and phantom behaviour. With suitable choice of parameters, we observed that accreting scalar field dark energy models are increasing the mass of the wormhole in the phantom phase and the mass is decreasing in the quintessence phase. Finally, we have considered accretion with power law form of scale factor and without any parameterisation scheme for the equation of state parameter and observed the fact that phantom-type dark energy supports the existence of wormholes.

Removing the big rip singularity from anisotropic universe in super string theory

Recently, various observational data predict the possibility that dark energy could be in the form of phantom field. The positive phantom-energy density grows without limit with the expansion of the Universe and leads to a big-rip singularity at a finite future time. The main question arises: what is the origin of the big rip singularity in a four-dimensional Universe? To answer this question, in this paper, we propose a new model in super string theory that allows taking into account the Dirac and vector string tachyon in addition to the scalar one, which stretches between branes and antibranes. In this model, scalar and Dirac string tachyons cancel each other’s effects and the only effect induced by the vector tachyon can be observed in density and pressures of the universe. We observe that different scale factors, pressures, and dark energy equation of state parameters are produced in different directions because of inhomogeneous tachyon dynamics and consequently one anisotropic universe is formed. Also, these observations are given in terms of effective tachyon potential and the separation between branes and antibranes. Thus, we have shown that the expansion of the anisotropic Universe is controlled by the vector string tachyon and evolves from the non-phantom phase to the phantom one and consequently, the phantom-dominated era of the universe accelerates and ends up in a big-rip singularity.

Cosmological Evolution of Pilgrim Dark Energy inf(G)Gravity

We analyze the behavior of pilgrim dark energy with Hubble horizon inf(G)gravity. We reconstruct thef(G)models through correspondence phenomenon by assuming two values of pilgrim dark energy parameter(u=2, -2). We evaluate the equation of state parameter which shows evolution of the universe in the quintessence, vacuum, and phantom phase for both cases ofuand give favor the pilgrim dark energy phenomenon. Also, squared speed of sound exhibits the stability off(G)model for both cases ofu. ThewPDE-wPDE′also provides freezing and thawing regions in this scenario. In this framework, ther-splane also corresponds to different dark energy scenarios.

(m, n)-TYPE HOLOGRAPHIC DARK ENERGY MODELS

We construct (m, n)-type holographic dark energy models at a phenomenological level, which can be viewed as a generalization of agegraphic models with the conformal-like age as the holographic characteristic size. For some values of (m, n) the holographic dark energy can automatically evolve across ω = -1 into a phantom phase even without introducing an interaction between the dark energy and background matter. Our construction is also applicable to the holographic dark energy with generalized future event horizon as the characteristic size. Finally, we address the issue on the stability of our model and show that they are generally stable under the scalar perturbation.

MASSIVE GRAVITONS DARK MATTER SCENARIO REVISITED

We reexamine the massive graviton dark matter scenario (MGCDM) which was recently considered as an alternative to dark energy models. When introducing the native and effective equations of state (EoS), it is shown that there is no phantom phase in the evolution toward the far past. Also we show that the past accelerating phase arises from the interaction between massive graviton and cold dark matter.