Space is the three-dimensional extension in which all things exist and move. Intuitively, it feels that we live in an unchanging space. In this space, the height of a tree or the length of a table is exactly the same for everybody. Einstein's special theory of relativity explains that this intuitive feeling is really an illusion. Neither space nor time is the same for two people moving relative to each other. Only a combination of space and time, called space-time, is unchanged for everyone. Einstein's general theory of relativity states that the force of gravity is a result of a warping of this space-time by heavy objects, such as planets. According to the Big Bang theory of the origin of the universe, the expansion of the universe began from infinitely curved space-time. Scientists still do not know whether this expansion will continue indefinitely, or whether the universe will...