The time evolution of a wormhole in a Friedmann universe approaching the Big Rip is studied. The wormhole is modeled by a thin spherical shell accreting the superquintessence fluid – two different models are presented. Contrary to recent claims that the wormhole overtakes the expansion of the universe and engulfs it before the Big Rip is reached, it is found that the wormhole becomes asymptotic...

From a Twin Universe perspective, it is proposed that stellar nuclear fusion can account for the negative-energy pressure (Dark Energy) that drives our present-day accelerated cosmic expansion. In the mirror twinned universe all processes are duplicated but with reverse negative polarity. Both the Positive and Negative Universes exist on the opposing surfaces of a topological two-dimensional me...

We extend the Wald cosmic no–hair theorem to a general class of scalar–tensor nonminimally coupled theories of gravity where ordinary matter is also present in the form of a perfect fluid. We give a set of conditions for obtaining a de Sitter expansion independently of any initial conditions, generalizing the treatment of such a problem as given by Wald. Finally we apply the results to some spe...

We present a new solution in the heterotic M–theory in which the metric depends on (cosmic) time. The solution preserves N = 1 supersymmetry in 4 dimensions in the leading order of the κ2/3 expansion. It is the first example of the time–dependent supersymmetric solution in M–theory on S/ZZ2. It describes expanding 4–dimensional space–time with shrinking orientifold interval and static Calabi–Ya...

The observed properties of stars and especially the neutrino signal of the supernova 1987A provide an upper limit to the axion mass, while the age and expansion rate of the universe provide a lower limit. There remains a “window of opportunity” 10 eV ∼ < ma ∼ < 10 eV, with large uncertainties on either side, where axions could still exist and where they would provide a significant fraction or a...

Standard cosmology connects the scale generated at late times by the cosmological constant with that generated in early times by the energy dynamics. Assuming dark energy de-coherence occurs during early times when the scale parameter expansion rate is no longer supra-luminal specifies a class of cosmological models in which the cosmic microwave background fluctuation amplitude at last scatteri...

It is well known the our Universe is expanding. In principle, we can estimate the expansion speed either directly, by observing the current state of the Universe, or indirectly, by analyzing the cosmic background radiation. Surprisingly, these two estimates lead to somewhat di erent expansion speeds. This discrepancy is a important challenge for cosmologists. Another challenge comes from recent...

If there is a shortest length in nature, for example at the Planck scale of 10m, then the cosmic expansion should continually create new comoving modes. A priori, each of the new modes comes with its own vacuum energy, which could contribute to the cosmological constant. I discuss possible mathematical models for a shortest length and an explicit model for a corresponding mode-generating mechan...

We extend the Wald cosmic no–hair theorem to a general class of scalar–tensor nonminimally coupled theories of gravity where ordinary matter is also present in the form of a perfect fluid. We give a set of conditions for obtaining a de Sitter expansion independently of any initial conditions, generalizing the treatment of such a problem as given by Wald. Finally we apply the results to some spe...

Among various phenomenological Λ models, a time-dependent model Λ̇ ∼ H is selected here to investigate the Λ-CDM cosmology. Using this model the expressions for the time-dependent equation of state parameter ω and other physical parameters are derived. It is shown that in H model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has al...