Dirac field and gravity in NC $SO(2,3)_\star$ model.

Action for the Dirac spinor field coupled to gravity on noncommutative (NC) Moyal-Weyl space-time is obtained without prior knowledge of the metric tensor. We emphasise gauge origins of gravity (i.e. metric structure) and its interaction with fermions by demonstrating that a classical action invariant under $SO(2,3)$ gauge transformations can be exactly reduced to the Dirac action in curved space-time after breaking the original symmetry down to the local Lorentz $SO(1,3)$ symmetry. The commutative, $SO(2,3)$ invariant action can be straightforwardly deformed via Moyal-Weyl $\star$-product to its NC $SO(2,3)_\star$ invariant version which can be expanded perturbatively in the powers of the deformation parameter using the Seiberg-Witten map. The gravity-matter couplings in the expansion arise as an effect of the gauge symmetry breaking. We calculate in detail the first order NC correction to the classical Dirac action in curved space-time and show that it does not vanish. This significant feature of the presented model enables us to potentially observe the NC effects already at the lowest perturbative order. Moreover, NC effects are apparent even in the flat space-time limit. We analyse NC modification of the Dirac equation, Feynman propagator and dispersion relation for electrons in Minkowski space-time.