Strings, Branes And An Accelerating Universe

Velocity-dependent interactions in a fundamental-string dominated universe lead quite naturally, with reasonable assumptions on initial conditions, to an accelerating expanding universe without assuming the existence of a cosmological constant. Using a mean-field approximation, exact solutions are obtained for various string/brane cosmological toy models. A multi-body model is also investigated. It is also shown that the repulsive velocity-dependent force arises in more general contexts and can lead to cosmic structure formation. Finally, we discuss a toy model including both ordinary and extremal matter, in which the latter accounts for dark matter while simultaneously acting as effective dark energy. Eternal acceleration is once more seen to arise in this case.