New Almost Universal Metrics

Plane waves and pp-waves are well-known universal metrics that solve all metric-based gravitational field equations. Similarly, the Kerr-Schild-Kundt class of metrics is almost universal: all metric-based gravitational field equations reduce to a linear scalar partial differential equation that always admits a solution. Here, we add a new member to this class of metrics and show that nonzero constant curvature pp-wave metrics are also almost universal. They reduce the generic gravity field equations to those of cosmological Einstein-Maxwell theory with null dust. The background of the pp-waves has the topology ${\mathbb{R}}^{1,1}\times S^2$ and provides the missing partner to the Nariai metric with ${\mathrm{d}\mathrm{S}}^2\times S^2$ and the Bertotti-Robinson metric with ${\mathrm{A}\mathrm{d}\mathrm{S}}^2\times S^2$ topologies. These quantum-protected metrics are of clear interest. We exemplify our results by using the quadratic and cubic gravity theories.