Form invariant transformations between n-- and m-- dimensional flat Friedmann--Robertson--Walker cosmologies

We illustrate how the group of symmetry transformations, which preserve the form of the n--dimensional flat Friedmann--Robertson--Walker cosmologies satisfying Einstein equations, acts in any dimension. This group relates the energy density and the isotropic pressure of the cosmic fluid to the expansion rate. The freedom associated with the dimension of the space time yields assisted inflation even when the energy density of the fluid is a dimensional invariant and enriches the set of duality transformations leading to phantom cosmologies.Comment: Accepted for publication in Iternational Journal of Modern Physics

Radiation in (2+1)-dimensions

In this paper we discuss the radiation equation of state $p=\rho/2$ in (2+1)-dimensions. In (3+1)-dimensions the equation of state $p=\rho/3$ may be used to describe either actual electromagnetic radiation (photons) as well as a gas of massless particles in a thermodynamic equilibrium (for example neutrinos). In this work it is shown that in the framework of (2+1)-dimensional Maxwell electrodynamics the radiation law $p=\rho/2$ takes place only for plane waves, i.e. for $E = B$. Instead of the linear Maxwell electrodynamics, to derive the (2+1)-radiation law for more general cases with $E \neq B$, one has to use a conformally invariant electrodynamics, which is a 2+1-nonlinear electrodynamics with a trace free energy-momentum tensor, and to perform a volumetric spatial average of the corresponding Maxwell stress-energy tensor with its electric and magnetic components at a given instant of time $t$.Comment: 7 pages. Accepted for publication in Phys. Lett.

(N+1)-dimensional Lorentzian evolving wormholes supported by polytropic matter

In this paper we study $(N+1)$-dimensional evolving wormholes supported by energy satisfying a polytropic equation of state. The considered evolving wormhole models are described by a constant redshift function and generalizes the standard flat Friedmann-Robertson-Walker spacetime. The polytropic equation of state allows us to consider in $(3+1)$-dimensions generalizations of the phantom energy and the generalized Chaplygin gas sources.Comment: 6 pages, 2 figures, accepted for publication in European Physical Journal