““Let's return to Archytas's staff and to the expanding Universe. A javelin that is thrown straight upward will drop back down a long time before we can sensibly ask the question about the edge of the universe. The javelin cannot escape the gravity of Earth. A spacecraft that runs out of propellant only after it reaches the velocity that permits escape from the gravitational pull of our solar system will continue on its path, but it will remain inside our galaxy. The Milky Way's gravitational field holds onto it. Proceeding like this, we pass, from a practical problem to a fundamental question. Can there be objects that may escape our group of galaxies and that will ultimately leave our universe altogether? The answer must be no. The universe, after all, keeps expanding. Light that reaches us today was emitted shortly after the birth of the universe; its headwaters are moving away from us with a velocity close to that of light. There is no light older than what originated at the time of the Big Bang. Since no information travels at a speed greater than that of light, the observable universe is contained within the surface of a sphere with a radius defined by the distance traveled by light that was emitted at the time of the Big Bang.””
