Gravity theories with asymptotically flat instantons

We construct explicitly a gravitational instanton which is asymptotically flat and globally euclidean. The instanton is a solution of euclidean gravity coupled to a scalar field in first-order formulation. The presence of a zero in the vierbeins and the consequent non-vanishing torsion allow us to obtain non-trivial topological excitations of the metric field without changing the topologically trivial structure of the underlying (euclidean) spacetime. (orig.)