The idea that the universe is expanding raises an interesting question: where is the center of the universe? We are used to thinking that an expansion will occur about a particular point—for example, the center. Where is this point? The answer is that it's everywhere. From our perspective on Earth, almost all galaxies seem to be moving away from us; although this might lead you to think that Earth is the center of the universe, observers on any other planet (or anywhere else) would make the same observations.

How do we quantify the expansion of the universe? We do so via the scale factor, which we symbolize with a. This is the ratio of the distance between two points in space at one time with respect to another time. For instance, suppose that, in the early universe, you and your friend are separated by 1 meter. You are untethered to each other or anything else, so that you are free to move in space. As the universe expands, the distance between you and your friend increases; in fact, this is what we mean when we say that the universe is expanding: it is space itself that is expanding. Say that you are engaged in an engrossing conversation for some time. You then measure the distance between you and your friend, which is now, say, 1000 m. This means that the scale factor is now 1000/1 = 1000. Since the scale factor is a ratio, we are free to define it as 1 whenever we like; then, the distance between two points in the universe will be identical to whatever the scale factor is at some later time, assuming this distance was 1 initially. It is important to bear in mind that there is no force pulling apart objects in an expanding universe. Rather, it is the background space that is stretching. Thus, objects which are bound do not get pulled apart. For instance, galaxies, solar systems, and humans do not expand in an expanding universe, since each of those objects consists of atoms and molecules that are bound by forces and/or gravity.