Astronomy and Astrophysics – Astrophysics – General Relativity and Quantum Cosmology
Scientific paper
2009-05-11
Astronomy and Astrophysics
Astrophysics
General Relativity and Quantum Cosmology
Electromagnetic as well as gravitational Chern-Simons like terms associated with the four-form included, four references added
Scientific paper
We consider the dynamics of a four-form field $\tilde {w} $, treating it as a distinct physical degree of freedom, independent of the metric. The equations of motion are derived from an action which, besides having the standard Hilbert-Einstein term and the matter part, consists of a new action for $\tilde {w} $. The evolution of $\tilde {w} $ in a flat FRW universe is studied, and it is shown that the parameters of the theory admit solutions wherein it is possible to have an equation of state $p_\phi \approx -\epsilon_\phi $, so that it leads to an accelerating universe. We also put forward electromagnetic as well as gravitational `Chern-Simons' like terms that arise naturally in 4D, entailing a modified Einstein-Maxwell equation and an enlarged system of Einstein equation involving a Cotton tensor. We demonstrate that the scalar-density associated with $\tilde {w} $ can be employed to construct a generalized exterior derivative that converts a p-form density to a (p+1)-form density of identical weight.
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