Radiation in Yang-Mills formulation of gravity and a generalized pp-wave metric

Variational methods applied to a quadratic Yang-Mills-type Lagrangian yield two sets of relations interpreted as the field equations and the energy-momentum tensor for the gravitational field. A covariant condition is imposed on the energy-momentum tensor in order for it to represent the radiation field. A generalized pp-wave metric is found to simultaneously satisfy both the field equations and the radiation condition. The result is compared with that of Lichnerowicz.