  The curvature-squared model of gravity, in the affine form proposed by Weyl
and Yang, is deduced from a topological action in 4D. More specifically, we
start from the Pontrjagin (or Euler) invariant. Using the BRST antifield
formalism with a double duality gauge fixing, we obtain a consistent
quantization in spaces of double dual curvature as classical instanton type
background.
  However, exact vacuum solutions with double duality properties exhibit a
`vacuum degeneracy'. By modifying the duality via a scale breaking term, we
demonstrate that only Einstein's equations with an induced cosmological
constant emerge for the topology of the macroscopic background. This may have
repercussions on the problem of `dark energy' as well as `dark matter' modeled
by a torsion induced quintaxion.
