  We propose a new solution to the cosmological constant problem building on a
nonperturbative quantum theory of gravity with torsional instantons. These
pseudoparticles, which were recently found to exist in a first order
formulation of Giddings-Strominger axionic gravity, carry nontrivial Nieh-Yan
topological charge. The nonperturbative ground state as generated due to
tunneling effects is shown to be stable under quantum fluctuations. Within this
framework, the associated vacuum angle, namely the Barbero-Immirzi topological
parameter, gets fixed to a numerical value determined by the Hubble constant.
