We briefly study the dynamics at classical level of the Carrollian limit, with vanishing speed of light and no possible propagation of signals, for a simply effective action in a flat space with a open string tachyon as scalar field. The canonical an
alysis of the theory indicates that the equation of motion is of Dirac type contrary to non-relativistic case where the equation is of Schrodinger type. The ultimate intention is to analize the latter case with electromagnetic fluxes finding that in this case the open string tachyon cannot be interpreted as time.
A twisted covariant formulation of noncommutative self-dual gravity is presented. The formulation for constructing twisted noncommutative Yang-Mills theories is used. It is shown that the noncommutative torsion is solved at any order of the $theta$-e
xpansion in terms of the tetrad and some extra fields of the theory. In the process the first order expansion in $theta$ for the Plebanski action is explicitly obtained.
Unified generalized Chaplygin gas models assuming an interaction between dark energy and dark matter fluids have been previously proposed. Following these ideas, we consider a particular relation between dark densities, which allows the possibility o
f a time varying equation of state for dark energy that crosses the phantom divide at a recent epoch. Moreover, these densities decay during all the evolution of the Universe, avoiding a Big Rip. We find also a scaling solution, i.e. these densities are asymptotically proportional in the future, which contributes to the solution of the coincidence problem.
