There is always a good deal of confusion that students have when it comes to understanding what expansion actually means. As we will discuss later, our modern cosmological model is known as the Big Bang model and that always conveys the wrong impression to students concerning the driver of the expanding Universe. We can best demonstrate what is occurring by considering the following thought experiment (which could be a real one if you found some cooperative ants).

The Ant Universe

Next, get a balloon and inflate it about half way. Sprinkle some ants onto the surface of the balloon and continue to inflate it. Notice that while you are inflating the balloon the ants are walking randomly around its surface. Some will be walking towards each other and some away. There will be no well-defined correlation between the distance between any two ants and their relative motion. This again is not what we observe and indicates there is some controlling agent that is causing the relative motion between galaxies to depend on distance.

Finally, get some glue (super glue works best) and glue the ants to the balloon (this is actually quite hard to do unless you have a really good pair of tweezers). As you inflate the balloon you will notice that the ants are no longer moving. The ants are stationary but the separation between each ant is increasing as the surface of the balloon increases as a result of it being inflated. If you pick any one ant on the balloon, all the other ants appear to be moving away from that ant. It matters not which ant you pick. Hence, every ant believes they are in the center of the ant distribution when in fact you know that there is no center because the ants are distributed on a surface. So what happens when you let the air out of the balloon? The ants all approach each other until they are all together (more or less) in one big ant glue-ball.

While this analog may seem silly, in fact, it's quite appropriate to the real Universe. Gravity is the "glue" that fixes the galaxies to the surface of the Universe. In the case of the Universe, this surface is a four dimensional spacetime surface which is embedded in some five dimensional volume. As this is rather difficult to draw on a two dimensional piece of paper, we use the analogy of the two dimensional balloon surface embedded in a three-dimensional volume. The key is to adopt the ant perspective. The ant is a two dimensional creature that really doesn't know it's on a surface. As we just saw above, the surface of the balloon has no center and the collapse of the balloon means the surface area has decreased (in principle, to zero).

In the case of our Universe, as the surface expands, the separation between all the galaxies increases and the observational manifestation of this is galaxy redshifts. This uniform expansion of the Universe makes a clear and important prediction. If galaxies are getting farther apart from each other due to the expansion of the surface, then in the past the galaxies were much closer to one another. Indeed, there must have even been a time when all the galaxies (all the matter) in the Universe were together in the same space at the same time. This means that the early Universe was a very small dense place and in a physical state well-removed from how it is observed to be today.