MG11 
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Traversable wormholes supported by cosmic accelerated expanding equations of state

We shall explore the possibility that traversable wormholes be supported by specific equations of state responsible for the present accelerated expansion of the Universe, namely phantom energy, the generalized Chaplygin gas, and the van der Waals quintessence equation of state. Firstly, phantom energy possesses an equation of state of the form $\omega\equiv p/\rho<-1$, consequently violating the null energy condition. As this is the fundamental ingredient to sustain traversable wormholes, this cosmic fluid presents us with a natural scenario for the existence of these exotic geometries. In this context, we investigate the physical properties and characteristics of these phantom energy traversable wormholes. Secondly, the generalized Chaplygin gas (GCG) is a candidate for the unification of dark energy and dark matter, and is parametrized by an exotic equation of state given by $p_{ch}=-A/\rho_{ch}^{\alpha}$, where $A$ is a positive constant and $0<\alpha \leq 1$. In this context, we shall explore exact solutions of spherically symmetric traversable wormholes supported by the GCG. Thirdly, the van der Waals quintessence equation of state is an interesting scenario for describing the late universe, and seems to provide a solution to the puzzle of dark energy, without the presence of exotic fluids or modifications of the Friedmann equations. Solutions of traversable wormholes where the exotic matter is constrained to the throat neighborhood, as well as an exact solution of an asymptotically flat wormhole, are found.

Stable dark energy stars: An alternative to black holes?

The Schwarzschild solution has played a fundamental conceptual role in general relativity, and beyond, for instance, regarding event horizons, spacetime singularities and aspects of quantum field theory in curved spacetimes. However, one still encounters the existence of misconceptions, ambiguities and much scepticism in the literature, which has served to inspire new and fascinating ideas. A new final state of gravitational collapse, an alternative to the concept of a black hole, is that of a gravastar. In this work, a generalization of the latter is explored, by considering a matching of an interior solution governed by the dark energy equation of state, $\omega\equiv p/ \rho<-1/3$, to an exterior Schwarzschild vacuum solution at a junction interface, situated near to where the event horizon is expected to form. The motivation for implementing this generalization arises from the fact that recent observations have confirmed an accelerated cosmic expansion, for which dark energy is a possible candidate. Several relativistic dark energy stellar configurations are analyzed by imposing specific choices for the mass function. The dynamical stability of the transition layer of these dark energy stars to linearized spherically symmetric radial perturbations about static equilibrium solutions is further explored. Large stability regions are found that exist sufficiently close to where the event horizon is expected to form, so that it would be difficult to distinguish the exterior geometry of these dark energy stars from astrophysical black holes.