[Image: Hubble Heritage Team (AURA/STScI/NASA)]
Einstein's equations can do more than just give us models for specific phenomena such as black holes or gravitational waves; they can also be used to describe the universe as a whole. On the basis of general relativity, physicists, mathematicians and astronomers have developed the so-called big bang models. They describe how our universe evolved from an extremely hot and dense early phase to its present state.
A key property of these models is that the universe has to change over time. Einstein's equation simply do not allow for a stable, eternally unchanging cosmos. According to the big bang models, any universe has to either expand or collapse. Our own universe is expanding, with characteristic consequences for astronomical observation.