No abstract

It is shown that the addition of a non-linear term to the Lagrangian of the electromagnetic field yields a fluid with an asymptotically super-negative equation of state, causing an accelerated expansion of the universe. Some general properties of nonlinear electromagnetism in cosmology are also discussed.

It will be shown that a given realization of nonlinear electrodynamics, used as source of Einstein's equations, generates a cosmological model with interesting features, namely a phase of current cosmic acceleration, and the absence of an initial singularity, thus pointing to a way to solve two important problems in cosmology.

Analytical wormhole solutions in Brans-Dicke theory in the presence of matter are presented. It is shown that the wormhole throat must not be necessarily threaded with exotic matter.Comment: Minor corrections, to be published in Phys. Rev.

12 We use Clebsch potentials and an action principle to derive a complete closed system of gauge-invariant equations for sound 13 superposed on a general background flow. Our system reduces to the Unruh [Phys. Rev. Lett. 46 (1981) 1351] and Pierce 14 [J. Acoust. Soc. Am. 87 (1990) 2292] wave equations when the flow is irrotational, or slowly varying. We illustrate our for-15 malism by applying it to waves propagating in a uniformly rotating fluid where the sound modes hybridize with inertial waves. 16