In an affine prolongation of general relativity, the minimal coupling of Dirac fields to gravity naturally provides an axial current interaction. We demonstrate that the cancellation of the translational curvature, i.e. torsion, in the chiral anomaly induces a dynamical axion coupled with gravitational strength. Because of a geometrical identity, our torsion-induced pseudoscalar couples to the Einstein equations with an effective energy-momentum tensor which automatically satisfies the quintessence condition $w<-1/3$ for the equation of state parameter. In a toy model of an axion-dominated Universe, this leads to an anharmonic oscillatory evolution for which the deceleration parameter is within the range of current observations.

• Received 1 August 2005

DOI:https://doi.org/10.1103/PhysRevD.73.043521