People have been asking me: “If the Big Bang began at a some point in space, then what–”
Hold it! The Big Bang did not begin at any point in space. The Big Bang is everywhere, it has no center. The standard analogy is that space is like the surface of an expanding balloon, and as you can see a balloon has no center on its surface.
Image from “Misconceptions About the Big Bang“, which incidentally is a really good article.
People find this confusing though, since balloons do have centers (not on their surface). Scientists also say that the universe as we know it used to be the size of a grapefruit. Doesn’t a grapefruit have a center? Well you got me there. So where’s the center of the universe?
The answer is that you are the center of the universe. I’m not referring to “you” in the abstract, I mean the literal person that is currently reading this. You are the center of the universe.
I will drop the balloon analogy and just draw some diagrams. Let’s say that each colored square is a galaxy. Your galaxy is the grapefruit-colored square, the one with a circle around it. The circle represents the universe as you know it. Successive diagrams show how the universe looks as time moves forward.
At first, the universe is small, and galaxies are closely packed. Then galaxies start to move away from you, and the universe gets bigger. At all points in time, you are at the center of the universe.
I know what you’re about to say. You’re about to say, “But you’re assuming I’m human! I’m actually a space alien in a galaxy far away, and I’m reading this from radio signals transmitted from planet Earth a long time ago. I don’t live on the grapefruit-colored galaxy in the center of the universe, I live on the avocado-colored galaxy at the edge of it!”
To that I say, you’re right. I’m going to have to change my story. Here it is.
You are the center of the universe. I’m not referring to “you” in the abstract, I mean the literal space alien that is currently reading this. You are the center of the universe.
Let’s say that each colored square is a galaxy. Your galaxy is the avocado-colored square, the one with a circle around it. The circle represents the universe as you know it. Successive diagrams show how the universe looks as time moves forward.
The next question you might ask is, how come I drew so many squares outside the universe? Ah, the circle doesn’t represent the entire universe, it just represents the universe as you know it.* The universe as you know it is limited by the speed of light and finite age of the universe. It could be that the galaxies we see are the only ones that exist. But probably the galaxies go on and on beyond our vision. Possibly they go on forever. We don’t really know.
*I’m referring to the universe as you currently know it. The universe as you will know it in the future will be bigger, not just because the universe is expanding, but also because light will have had more time to travel.
Given that the universe as we know it is limited by our own vision, it is not surprising that we are at the center. All that really says is that we can see equally far in all directions.
Based on what we can see, all far away galaxies are receding away from us. But the motion is such that everyone, even space aliens, will see the same. The space aliens on the avocado-colored galaxy will also find that they are the center of the universe as they know it, and will also find that all far away galaxies are receding away from them. They will conclude that at one time, their universe was as small as an avocado, and they were at the center.
The Big Bang has no special center point. If you define the center as the point from which every galaxy is receding, then every point in the known universe is a center of the Big Bang.