In the 1920’s, Edwin Hubble found that galaxies appear to fly away from us in all directions. The farther a galaxy, on average, the faster it recedes: this is Hubble’s Law. We now know that receding galaxies are caused by an expanding universe, a universe which began in a Big Bang 13.7 billion years ago and has been expanding ever since. But it seems odd, does it not, that galaxies recede in all directions from our point of view? Does this mean we’re at the center of the universe? And if we’re not, then where is the center of the universe, the place where the Big Bang happened?
In our everyday experience, all explosions, whether a firecracker or a supernova, have a center from which they expand outward into space. We can see the effect of the explosion in the form of hot gas and light moving outward from a central point. But the Big Bang was not an explosion IN space. It was an explosion OF space! According to the physics as we now understand it, there was no space, matter, energy, or time before the Big Bang. Because the Big Bang was everything, it happened everywhere. Observation seems to bear this out. On a very large scales, there is no motion or distribution of galaxies that suggest a center in the universe, or any privileged point where matter flows to or from.
To explain the expansion of the universe, astronomers often turn to the famous “expanding balloon analogy”. It was first used by the great astronomer Arthur Eddington in 1933, and later used by Fred Hoyle in the 1960’s. In this analogy the two dimensional surface of a balloon is like our three dimensional space. The dots on the balloon represent galaxies. As the balloon inflates the dots get farther apart, and the further any two dots lie apart on the surface of the balloon, the faster they seem to move away from each other. From the point of view of each dot, all other galaxies are moving away, but none has a privileged position at the center, because there is no center, just as the Earth’s surface has no center.
The balloon analogy can be a bit confusing, and it’s not precise. So when thinking about how it relates to our expanding universe, you must keep in mind:
- The 2-dimensional surface of the balloon is an analog to our 3-dimensional space.
- The 3-dimensional space in which the balloon is expands is not analogous to any higher dimensional space. Points off the surface of the balloon are not in the “universe” in this simple analogy.
- The center of the balloon corresponds to nothing in our universe.
- The universe may be limited in size and growing like the surface of the expanding balloon. But it could also be infinite. No one knows.
But what about the picture you saw in the last article (see above), which showed our solar system at the center of giant sphere, the surface of which is the edge of the observable universe, some 46.5 billion light years away.
How could such a puny planet as ours sit at the center of the universe? It seems absurd!
But this picture does not show us at the center of THE universe. It shows us at the center of the piece of the universe we can observe. Every other observer in the universe sees their own sphere of their own observable universe. We might see a distant galaxy some 12 billion light years away on the edge of our sphere, its ancient light just now reaching our telescope. An observer in this galaxy today sees ancient light from our own Milky Way on the edge of their observable universe. Everyone seems like they’re at the center. But no one has a privileged point of view. On a large scale, there are no bad seats… everyone in the universe has the same view.