Cosmic microwave background
In 1967, he and Arthur M. Wolfe were the authors of the Sachs–Wolfe effect, which concerns a property of the Cosmic microwave background radiation. The Ehlers–Geren–Sachs theorem, published in 1968 by Jürgen Ehlers, P. Geren and R. Sachs, shows that if, in a given universe, there exists a reference frame at each event such that the cosmic background radiation is isotropic, then under certain conditions that universe is an isotropic and homogeneous FLRW spacetime.