Cosmic microwave background and scalar-tensor theories of gravity

We show that if all observers see an isotropic cosmic microwave back-ground in an expanding geodesic perfect fluid spacetime within a scalar-tensor theory of gravity. then that spacetime must be isotropic and spatially homogeneous. This result generalizes the Ehlers-Geren-Sachs theorem of general relativity, and serves to underpin the important result that any evolving cosmological model in a scalar-tensor theory that is compatible with observations must be almost Friedmann-Lemaitre-Robertson-Walker.