Narrow Results

The Observed Perlmutter-Riess Acceleration (OPRA) implies that the expansion of the Universe is currently increasing and is motivation for the so-called phantom energy models. We consider the dynamics of phantom scalar field models. An important physical time constraint, which can be used to rule out many cosmological models, is obtained from the condition that all forms of energy density, including the field causing OPRA (e.g., the phantom field), must be non-negligible for an extended period, which is conservatively estimated to be of the order of a few Gyr. We find that this physical time constraint cannot be satisfied in conventional phantom cosmological models.

We construct a model of phantom energy using the graded Lie algebra SU(2/1). The negative kinetic energy of the phantom field emerges naturally from the graded Lie algebra, resulting in an equation of state with w < -1. The model also contains ordinary scalar fields and anticommuting (Grassmann) vector fields which can be taken as two-component dark matter. A potential term is generated for both the phantom fields and the ordinary scalar fields via a postulated condensate of the Grassmann vector fields. Since the phantom energy and dark matter arise from the same Lagrangian, the phantom energy and dark matter of this model are coupled via the Grassmann vector fields. In the model presented here phantom energy and dark matter come from a gauge principle rather than being introduced in an ad hoc manner.

This paper deals with the dynamical behavior of phantom field near five-dimensional charged black hole. We formulate equations of motion for steady-state spherically symmetric flow of phantom fluids. It is found that phantom energy accretes onto black holes for u < 0. Further, the location of critical point of accretion are evaluated that leads to mass to charge ratio for 5D charged black hole. This ratio implies that accretion cannot transform a black hole into a naked singularity. We would like to mention here that this work is an irreducible extension of 4D charged black hole.

We discuss whether the future extrapolation of the present cosmological state may lead to a singularity even in case of "conventional" (negative) pressure of the dark energy field, namely w=p/ρ≥-1. The discussion is based on an often neglected aspect of scalar–tensor models of gravity: the fact that different test particles may follow the geodesics of different metric frames, and the need for a frame-independent regularization of curvature singularities.

Dimming mechanisms based on the conversion of photons into axions have been recently suggested which could justify the observation of equation of state values w = P/ρ less than -1 that do not correspond to the presence of phantom energy in the universe. It is argued that, though such a mechanism may in fact increase the actual value of w, it does not actually solve the phantom problem because the produced axions precisely are of the kind which has been considered to make up phantom energy and therefore the suggested mechanism can never make the actual value of w to exactly reach the value -1 from below, such as it also happens with other suggested mechanisms intended to get rid of phantoms from cosmology. It is conjectured that the actual current value of w must likely be less than -1 but so close to it that present observational accuracies could not allow discriminating between a cosmological constant and the phantom energy as the form currently taken by dark energy in the universe.

We discuss the generalized second law of thermodynamics (GSLT) in three different systems by taking quantum corrections (logarithmic and power law) to cosmological horizon entropy as well as black hole (BH) entropy. First, we consider phantom energy accretion onto the Schwarzschild BH in the closed Friedmann–Robertson–Walker universe and investigate the validity of the GSLT on the apparent and event horizons. In another scenario, we evaluate the critical mass of the Schwarzschild BH with upper and lower bounds under accretion process due to phantom-like modified generalized chaplygin gas. It is found that the GSLT is respected within these bounds and BH cannot accrete outside them. Finally, we explore this law for a closed universe filled with interacting n-components of fluid (in thermal equilibrium case) and with noninteracting dark matter and dark energy components (in thermal nonequilibrium case) on the apparent and event horizons and find conditions for its validity.

We investigate spherically symmetric spacetime filled with global monopole and perfect fluid in $f(R)$-gravity. We consider field equations of $f(R)$-gravity in order to understand the global monopole and the perfect fluid curve to the spacetime. It has taken advantages of conformal symmetry properties of the spacetime to solve these equations. The obtained solutions are improved in case of phantom energy. It is shown that obtained $f(R)$ function is consistent with well-known models of the modified gravity. Also, it is examined whether the obtained solutions support a traversable wormhole geometry. Obtained results of the solutions have been concluded.

It has been argued that wormholes are as good a prediction of Einstein’s theory as black holes but the theoretical construction requires a reverse strategy, specifying the desired geometric properties of the wormhole and leaving open the determination of the stress–energy tensor. We begin by confirming an earlier result by the author showing that a complete wormhole solution can be obtained by adopting the equation of state $p=\omega \rho$ and assuming that the wormhole admits a one-parameter group of conformal motions. The main purpose of this paper is to use the assumption of conformal symmetry to show that the wormhole is stable to linearized radial perturbations whenever $-1.5<\omega <-1$.

In this article, we have discussed Morris and Thorne (MT) wormhole solutions in a modified theory of gravity that admits conformal motion. Here, we explore the wormhole solutions in $f(R,T)$ gravity, which is a function of the Ricci scalar ($R$) and the trace of the stress–energy tensor ($T$). To study wormhole geometries, we make assumption of spherical symmetric static spacetime and the existence of conformal Killing symmetry to get more acceptable astrophysical outcomes. To do this, we choose the expression of $f(R,T)$ as $f(R,T)=R+2\gamma T$. Here, we employ the phantom energy EoS relating to radial pressure and density given by $p_r=\omega \rho$ with $\omega <-1$ to constrain our model. Following a discussion of wormhole geometry and behavior of shape function, the study moves on to the computation of proper radial distance, active mass function, the nature of total gravitational energy and a discussion on the violation of energy conditions. We have shown that the wormhole solutions exist for positive as well as negative values of the coupling constant $\gamma$. From our analysis, we see that no wormhole solution exists for $\gamma =-4\pi ,-\pi (3+\omega )$. All the physical parameters have been drawn by employing the values of $\gamma$ as $\gamma =-0.3,-0.2,-0.1,0,0.1$ and $0.2$, where $\gamma =0$ corresponds to general relativity (GR) case. It is found that for our proposed model, a realistic wormhole solution satisfying all the properties can be obtained.

Traversable wormhole solutions in General Relativity require exotic matter sources that violate the null energy condition (NEC), and such behavior may be avoided in modified gravity. In this study, we analyze the energy conditions for static, spherically symmetric traversable Morris–Thorne wormholes in a recently proposed viable $f(R)$ gravity model. We numerically analyze solutions considering both constant and variable redshift functions, and present wormhole spacetimes respecting the NEC, supported by a phantom energy-like equation of state for the source. Moreover, we analyze the stability of the spacetimes using the generalized Tolman–Oppenheimer–Volkov equation. We demonstrate the effects of certain parameters in the $f(R)$ model in determining energy condition violations, and establish that stable wormholes can be formulated only at the expense of violating the NEC.

We consider it worthy if we could construct a realistic model universe that would enable us to identify a clue about the source of dark energy. So, we develop a Scale Covariant Theory model universe considering a 5D spherically symmetric space-time. It is predicted that the constructed model itself behaves as a phantom energy model/ source that tends to a de Sitter phase avoiding the finite-time future singularity (big rip). The model universe is isotropic and is free from an initial singularity. The gravitational constant $G$ decreases with a variation of $-7.2\times 10^{-11}$${\mathrm{\text{yr}}}^{-1}$ and the Hubble parameter is estimated to be $H=68$. We also provide a thorough analysis of the cosmological findings with graphical representations.

In this paper, we studied the interactions of dark energy with dark matter, black hole, and wormhole. It was shown that, in phantom case, the interaction terms make arise the new behaviors, such as avoidance from big rip, the decrease of black hole mass, and the increase of the wormhole throat size.