Could the universe have a boundary? This question is indeed a puzzling one. Astronomers have already discovered evidence that the universe was, at one point in time, compacted into a very small space. This suggests that the universe had a quantum, or finite, beginning. The science that studies this very early state of the universe and how it emerged out of nothing is called quantum cosmology. Quantum cosmology says that the universe arose from a quantum disturbance, known in scientific terms as a quantum fluctuation. For the universe to be infinite, this fluctuation would have had to have infinite energy to cause the universe to expand to infinity. It is not probable that a quantum fluctuation would contain infinite energy; therefore, the universe must be finite.
The idea of a finite universe was first proposed long before the development of quantum cosmology. Its champion was a man named Aristotle. Aristotle was a philosopher in ancient Greece who proposed, among other things, a spherical universe with a background of fixed stars. This fixed background was necessary to provide an absolute frame of reference for moving objects. The idea of a fixed background was furthered by George Berkeley and Ernst Mach. Berkeley and Mach determined that the universe must be finite, or at least contain a finite amount of matter, because objects experience a finite amount of inertia.
Critics of this idea of a finite universe posed the question of what could lay on the other side of the boundary. After all, every edge must have another side. If the answer to what lay on the other side of the boundary was more space, why not simply extend the universe to include that space as well? This riddle was solved by German mathematician George F.B. Riemann in the mid-nineteenth century. He proposed that, rather than a sphere, the shape of the universe is a hyper-sphere. A regular sphere is a three dimensional ball, the surface of which appears as a flat plane. As with the planet Earth, the third dimension is hidden until one looks at the sphere from the outside. A hyper-sphere is the same concept; a four dimensional sphere which appears three-dimensional unless viewed from the outside. This begs the question of what it looks like from the outside. In the case of the universe, however, there is no ‘outside’ from which to view it. Nature simply does not follow the rules.
Even after accepting this, one still may wonder if it would ever be possible to see the end of the universe. After all, space certainly looks infinite. This apparent infiniteness may only be an effect of light traversing the curvature of space. As light wrapped around the universe, it could capture and record multiple images of the cosmos against the background of space creating the illusion of infinity. What appear to be new stars and galaxies could actually be different images of the same stars and galaxies from various periods in the evolution of the cosmos. The Milky Way galaxy, in which Earth resides, would certainly be no exception to this, and multiple images of the Earth would be seen. Astronomers could continue to peer through evermore powerful telescopes, believing that they were discovering something new with every encounter. In reality, however, they would only be looking back in time toward increasingly earlier periods in the history of the universe.
One might continue to wonder, if the universe is finite and has a boundary in the third dimension, would it be possible that one of the many probes launched by humankind into the cosmos should one day encounter the edge and be stopped in its path. Astronomers have also pondered that question. What happens to objects with trajectories perpendicular to the boundary of the universe? Are they destroyed? What happens to light beams that coincide with the cosmic edge? Are they also annihilated? What about the laws that say mass and energy are conserved? Scientists have found a way around this problem by describing the universe as a torus, a geometric shape with interconnected ends. This solution was first proposed by the German physicist Karl Schwarzschild at the end of the nineteenth century. A familiar example of a torus in two dimensions is given by a video game screen, such as that of Atari’s Pac-Man, in which characters seem to leave the screen only to re-appear at the opposite side. Schwarzschild’s model proposes a three dimensional torus for the shape of the universe, basically suggesting that the universe is without a boundary but folds in on itself. This has given rise to the finite universe hypothesis which states that a spatially finite universe does not require an actual edge but uses periodic boundary conditions to create the appearance of an edge.
Recent data collected by NASA’s WMAP probe, which studies temperature fluctuations in the background radiation of the cosmos, reveals that the universe might resemble a soccer ball with interconnected hexagons. This would mean that, overall, the universe is finite and that light originating from one end of the universe will eventually reach its own staring poing as it travels through space. The hexagonal sections of the universe, however, give it the appearance of being infinite.