Since the discovery by Hubble and Slipher in the 1920s of the recession of distant galaxies and the inferred expansion of the universe, cosmology has been a testing ground for gravitational theory. That discovery was thought at the time to be a great confirmation of general relativity for two reasons. First, general relativity, in its original form, predicted a dynamical universe that necessarily either expands or contracts. Of course, Einstein had later modified the theory by introducing the “cosmological constant” into the field equations in order to obtain static cosmological solutions in accord with the current, pre-Hubble observations. To his great joy, following Hubble's discovery, Einstein was allowed to drop the cosmological constant.

Second, was simply the fact that general relativity was capable of dealing with the structure and evolution of the universe as a whole, a capability not shared by Newtonian theory (unless special assumptions are made). However, this capability is more a consequence of the Einstein Equivalence Principle (alternatively of the metric-theory postulates) than a property of general relativity. Because of EEP, spacetime is endowed with a metric g which determines the results of observations made using nongravitational equipment (light rays, telescopes, spectrometers, etc.) and the motion of test bodies (galaxies). Via the field equations provided by each metric theory of gravity, the distribution of matter then determines the metric g, and thereby the entire physical spacetime in which observations are made.