Kerr-Schild--Kundt Metrics are Universal
  We define (non-Einsteinian) universal metrics as the metrics that solve the source-free covariant field equations of generic gravity theories. Here, extending the rather scarce family of universal metrics known in the literature, we show that the Kerr-Schild--Kundt class of metrics are universal. Besides being interesting on their own, these metrics can provide consistent backgrounds for quantum field theory at extremely high energies. 
Metin Gurses, Tahsin Cagri Sisman, Bayram Tekin
gr-qc hep-th