One of Hawking's major achievements (which he shares with Jim Hartle) was to come up with the theory that the universe has no boundaries in 1983.
In 1983, the effort to understand the nature and shape of the universe, Hawking and Hartle combined the concepts of quantum mechanics (the study of the behavior of microscopic particles) with general relativity (Einstein's theories about gravity and how mass curves space) to show that the universe is a contained entity and yet has no boundaries.
To conceptualize this, he tells people to think of the universe like the surface of the Earth. As a sphere, you can go in any direction on the surface of the Earth and never reach a corner, an edge or any boundary where the Earth can be said to "end." However, one major difference is that the surface of the Earth is two-dimensional (even though the Earth itself is three-dimensional, the surface is only two-dimensional), while the universe is four-dimensional.
Hawking explains that spacetime (see the sidebar on this page) is like the lines of latitude on the globe. Starting at the North Pole (the beginning of the universe) and going south, the circumferences get bigger until beyond the equator, when they would get smaller. This means that the universe is finite in spacetime and will re-collapse eventually -- however, not for at least 20 billion years [source: Hawking]. Does this mean that time itself would go backwards? Hawking grappled with this question, but decided no, because there is no reason to believe that the universe's trend from ordered energy into disordered energy will reverse [source: Hawking].