general relativity also set the stage for cosmology (theories of the structure of our universe at large distances and over long times). einstein thought that the universe may curve a little bit in both space and time, so that the universe always had existed and always will exist, and so that if an object moved through the universe without bumping into anything, it would return to its starting place, from the other direction, after a very long time. he even changed his equations to include a "cosmological constant," in order to allow a mathematical model of an unchanging universe. the general theory of relativity also allows the universe to spread out (grow larger and less dense) forever, and most scientists think that astronomy has proved that this is what happens. when einstein realized that good models of the universe were possible even without the cosmological constant, he called his use of the cosmological constant his "biggest blunder," and that constant is often left out of the theory. however, many scientists now believe that the cosmological constant is needed to fit in all that we now know about the universe.