A simple characterization of doubly twisted spacetimes

In this note, we characterize 1+n doubly twisted spacetimes in terms of “doubly torqued” vector fields. They extend Bang–Yen Chen’s characterization of twisted and generalized Robertson–Walker spacetimes with torqued and concircular vector fields. The result is a simple classification of 1 + n doubly-twisted, doubly-warped, twisted and generalized Robertson–Walker spacetimes.