Let’s start out with something that we can say for certain: we live in an expanding universe. Every single day, the universe gets a little bit bigger than it was the day before. But right away, when we say something like “we live in an expanding universe” certain questions start to pop up, and they’re far and away the most common kinds of questions that I get asked. If the universe is expanding, then what is it expanding into? And what is it expanding from? Where’s the edge of the universe, and where is it’s center?
I’m going to be honest with you. I dread this kind of question, especially when I give public talks. Not because I know what the answer is, but because I know how totally unsatisfying that answer feels. It’s like sneaking into your kid’s candy bag the night after Halloween and eating half their chocolates (don’t judge me, I know you’ve done it too): it feels good in the moment, but you instantly have regrets.
Here's the answer: the big bang has no center, and it has no edge. That’s it. That’s the answer. No more, no less.
And that doesn’t really feel good because it doesn’t make a lot of sense.
It’s easy to imagine an expanding universe, and there are plenty of analogies out there to help guide our thinking. We can imagine drawing little galaxies on the surface of a balloon, and inflating that balloon to see the galaxies getting farther apart. We can imagine baking a load of bread with raisins in it (why it’s always raisins, and not chocolate chips, is an enduring mystery) and seeing how as the bread rises the raisins get farther apart.
Yay, expanding universe! But the balloon has a center, and it has an edge. And the bread has a center, and a crust. So…where’s the center of the universe, and where is its edge?
This is why we can’t rely on analogies too much when dealing with many concepts in fundamental physics and cosmology. In both the subatomic and cosmological realms, we’re grappling with structures and concepts and physics that is far and above what our puny human brains can imagine. That’s why we have mathematics. Mathematics is a tool that we can use to understanding the universe around us, even when we can’t envision what’s going on. That’s right kids: imagination is important, but mathematics can be even more powerful.
Let’s start with the center. Where did the big bang start? Right here. And right there. And right over there. And in the next room over. The big bang happened everywhere, all at once. The big bang was an event that the entire universe participated in. The big bang was NOT an explosion that happened somewhere in space, it was an explosion OF space – it was when the expansion of the universe first got started. It was not a place we can point to, it was a TIME that we can point to.
The big bang was a singular event that happened in the past of every single object in the universe. If you take any particle, any bit of energy, and trace its path backwards through time, you’ll end up at the big bang, where that particle, that bit of energy, gets all mushed together with every other particle and bit of energy in a singularity.
Think about it. Let’s say the big bang happened over there, say, in the Andromeda galaxy. Well then, where were we, in the Milky Way, when that happened? If we traveled back in time 13 and change billion years, would we be watching the big bang unfold? But…aren’t we a part of the universe? How can we be a part of the universe but somehow outside of the event the created it?
The big bang had to happen everywhere, because everywhere is by definition part of the universe. You can’t have a universe, especially an expanding one, without a big bang. So no matter where you are, your point in space had, at one time, to participate in the big bang.
Now what about the other side of the coin? If the universe is expanding, what is it expanding into? Where’s the crust in our expanding loaf of bread, and what’s the oven that we’re sitting in?
Go ahead, have another piece of chocolate, because this is going to get weird. There is no edge. The universe has no edge. I don’t even want to say something like “the universe isn’t expanding into anything” because that still conjures up the wrong mental image. It’s too tempting to imagine a wall, or boundary, with galaxies and stuff on one side and nothingness on the other, with the universe expanding to fill up that nothingness with somethingness.
But that’s wrong. Even the vacuum of space is something. There are still point, locations, and existence. There’s no “outside” to the universe because “outside” implies existence, even an empty one. But the universe is, by definition, all there is. There is nothing to physical reality except the universe. Walls separate one region from another, but the universe is all the regions simultaneously.
If there were an edge, you could imagine working hard enough to get outside that edge. But that’s not possible. There is no outside. There is no…side. There just is, us, the universe. All there is.
Like I said, it feels a little thrilling at first, but then the stomach aches kick in.
