The expansion of the universe may be the most important fact we have ever discovered about our origins. You would not be reading this article if the universe had not expanded. Human beings would not exist. Cold molecular things such as lifeforms and terrestrial planets could not have come into existence unless the universe, starting from a hot big bang, had expanded and cooled. The formation of all the structures in the universe, from galazies and stars to planets and Scientific American articles, has depended on the expansion. Forty years ago this July, scientists announced the discovery of definite evidence for the expansion of the universe from a hotter, denser, primordial state. They had found the coll afterglow of the big bang: the cosmic microwave background radiation. Since this discovery, the expansion and cooling of the universe has been the unifying theme of cosmollogy, much as Darwinian evolution is the unifying theme of biology. Like Darwinian evolution, cosmic expansion provides the context within which simple structures form and develop over time into complex structures. Without evolution and expansion, modern biology and cosmology make little sense.
The expansion of the universe is like Darwinian evolution in another curious way: most scientists think they understand it, but few agree on what it really means. A century and a half after On the Origin of Species, biologists still debate the mechanisms and implications (through not the reality) of Darwinism, while much of the public still flouders in pre-Darwinian cluenessness. Similarly, 75 years after it's initial discovery, the expansion fo the univers is still widely misunderstood. A prominent cosmologist involved in the interpretation of the cosmic microwave background, James Peebles of Princeston University, wrote in 1993: " The full extent and richness of this picture [the hot big bang model} is not as well understood as I think it ought to be... even among those making some of the most stimulation contributions to the flow of ideas."
Renowned physicists, authors of astronomy textbooks and prominent poularizers of science have made incorrect, misleading or easily misinterpreted statements about the expansion of the univers. Because expansion is the basis of the big bang model, these misunderstandings are fundamental. Expansion is a beguilingly simple idea, but what exactly does it mean to say the universe is expanding, too? to add to the befuddlement, the expansion fo the univers now seems to be accelerating, a process with truly mind-stretching consequences.
When some familiar object expands, such as a sprained ankle or the Roman Empire or a bomb, it gets bigger by expanding into the space around it. Ankles, empires and bombs have centers and edges. OUtside the edges, there is room to expand into. The universe does not seem to have and edge or a center or an outside, so how can it expand?
A good anology is to imaging that you are an ant living on the surface of an inflating balloon. Your world is two dimensional; the only directions you know are left, right, forward, and backward. You have no idea what up and down mean. One day you realize that your walk to milk your aphids is taking longer than it used to: five minutes oneday, six minutes the next day, seven minutes the next. The time it takes to walk to other familiar places is also increasing. You are sure that you are not walking more slowly and that the aphids are milling around randomly in groups, not systematically crawling away from you. This is the important point: the distances to the aphids are increasing even though the aphids are not walking away. They are just standing there, at rest with respect to the rubber of the balloon, yet the distances to them and between them are increasing. Noticing these facts, you conclude that the ground beneath your feet is expanding. That is very strange because you have walked around your world and found no edge or "outside" for it to expand into.
The expansion of our universe is much like the inflation of a balloon. The distance to remote galaxies are increasing. Astronmers casually say that distant galaxies are "receding" or "moving away" from us, but the galaxies are not traveling through space from us. They are not fragments of a big bang bomb. Instead the space between the galaxies and us is expanding. Individual galaxies move around at random within clusters, but the clusters of galaxies are essentially at rest. The term "at rest" can be defined rigorously. The microwave background radiation fills the universe and defines a universal referance frame, analogous to the rubber of the balloon, with respect to which motion can be measured.
This balloon analogy should not be stretched too far. From our point of view outside the balloon, the expansion of the curved two-dimensional rubber is not possible only because it is embedded in three-dimensional space. Within the third dimension, the balloon has a center, and it's surface expands into the surrounding air as it inflates. One might conclude that the expansion of our three-dimensional space requires the presence of a fourth dimension. But in Einstien's general theory of relativity, the foundation of modern cosmology, space is dynamic. It can expand, shrink, and curve without being embedded in a higher-dimensional space.
In this sense, the universe is self-contained. It needs neither a center to expand away from nor empty space on the outside (wherever that is) to expand into. When it expands, it does not claim previouls unoccupied space from its surroundings. Some newer theories such as string theory do postulate extra dimensions, but as our three-dimensional univers expands, it does not needs these extra dimensions to spread into.
In our universe, as on the surface of the balloon, everything receds from everything else. Thus, the big bang was not an explosion in space; it was more like an explosion of space. IT did not go off at a particular location and spread out from there into some imagined preexisting void. It occured everywhere at once.
If one imagines running th e clock backward in time, any given region of the universe shrinks and all galaxies in it get closer and closer until they smach together in a cosmic traffic jam -- the big bang. This traffic -jam analogy might imply local congestion that you could avoid if you listened to the traffic report on the radio. But the big bang was an unavoidable traffic jam. It was like having the surface of the Earth and all it's highways shrink while cares remained the same size. Eventually the cars will be bumper to bumper on every road. No radio broadcast is going to help you around that kind of traffic jam. The congestion is everywhere.
Similarly, the big biang happened everywhere--in the room in which you are reading this article, in a spot just to the left of Alpha Centauri, everywhere. It was not a bomb goin off at a particular spot that we can identify as the center of the explosion. Likewise, in the balloon analogy, there is no special place on the surface of the balloon that is the center of the expansion. The unbiquity of the big bank holds no matter how big the universe is or even whether it is finite or infinite in size. Cosmologists sometimes state that thte universe used to be the size of a grapefruit, but what they mean is that the part of the universe we can now see--our observable universe--used to be the size of a grapefruit.
Observers living in the Adromeda galaxy and beyond have their own observable universe that are different from but overlap with ours. Andromedans can see galaxies we cannot, overlap with ours. Andromedans can see galaxies we cannot, simply by virtue of being slightly closer to them, and vice versa. Their observable universe also used to be the size of a grapefruit. Thus, we can conceive of the early universe as a pile of overlapping grapefruits that stretches infinitely in all directions. Correspondingly, the idea that the big bang was "small" is misleading. The totality of space could be infinite. Shrink an infinite space by an arbitrary amount, and it is still infinite.
