Gravity as a topological gauge theory

We describe a topological gauge theory with underlying Cartan geometry G/H and principal H-bundle. The topological action is constructed as a linear combination of the Euler and Pontrjagin numbers coming from entries of the curvature of a Cartan connection. As an example we show that for G = SO (4, 1) (respectively SO (3, 2) ) for ₀ > 0, (respectively ₀ < 0) and H=SO (3, 1), a very particular linear combination yields the Holst + Euler and Pontrjagin of the curvature R of the spin connection + Nieh-Yan + bare cosmological constant ₀ terms. Additionally, in this construction, the coupling constants of these different terms are inherently linked together. The other example treated is the one of the M\"obius group G= SO (4, 2) /\ I\ and H = CO (4, 2) R^3, 1. By considering a constraint on the "pair of frames" (, ) we retrieve an action comprising all terms compatible with diffeomorphism invariance and local Lorentz invariance described in the previous example + kinetic term for a scalar field + an interaction of that scalar field with the Torsion T, the scalar field being related to dilation. Finally, in both examples, we study the equations of motion associated to the total (Gauge + Matter) action. It is especially shown that one of the solutions in the M\"obius case consists of Einstein's equations modified by the Holst and Nieh-Yan terms with an additional source term for curvature depending on specific variations of spin density of matter.