I was listening to a YouTube video the other day about the Big Bang and it mentioned that it happened everywhere. I have heard that before, but I don’t understand it.
Pop culture depictions of the Big Bang often show the universe starting as a point in space that explodes outward.
But that’s not really accurate. In the beginning the universe was very, very hot and very, very dense. But it wasn’t a point in space. In fact, if the universe is infinite in extent now, it was also infinite in extent then. And it wasn’t in a particular location because the universe is space itself.
So the Big Bang didn’t happen anywhere. It happened everywhere. The universe came into existence with all parts of it hot and dense, and over time it has become less hot and less dense. For those of us on the inside, this looks like the universe is expanding, but there’s no single point that everything is moving away from.
ETA: Ninja’d by a few seconds. Two ways of saying the same thing may help the OP.
Everything in the universe, including all the empty space in it was once all in a single microscopic spot.
Then it spread out.
A conventional explosion happening in conventional space is different. There is a pre-existing everywhere all around, and then there’s an explosion at some particular single spot. And the effects of the explosion radiate outwards *into *the pre-existing space around that spot. So in that case it makes sense to say "the explosion happened here, not over there.
For the Big Bang everywhere was just in the one spot. And so we can equally say the whole thing happened at the same place then as anyplace is now. It happened equally at what’s now your house, at what’s now my house, at what’s now Jupiter, and at what’s now some distant galaxy.
Because all those places, and all the others in the whole Universe that I didn’t mention were all just one place at the moment it happened.
The first thing to realize is that you’re not dealing with the Newtonian conception of space, you’re dealing with Einsteinian spacetime and things can happen in the latter which can’t can’t happen in the former.
The simplest way to put it is that as you go back in time the (energy) density of the Universe increases and runs away (i.e. becomes arbitrary large) in a finite time. The temperature also became arbitrarily large, though that is a less essential element in understanding what is going.
In Newtonian physics, going to arbitrarily large densities suggests some form of clustering, but in general relativity it doesn’t. Every single point in space, both inside and outside our observable Universe (providing the cosmological principle is in operation beyond the observable Universe) is equally where the big bang happened as the density at every point became equally arbitrarily large.
Distances between spatial locations also become arbitrarily small, but the singularity itself doesn’t have any concept of spatial distance defined on it, so you can’t talk in a precise manner about its extent (i.e. whether it is point-like or infinite, etc.)
It also spaces that the universe has no center. Although every point in it appears to itself to be in the center, with an equal amount of space in every direction one looks, no one point is. This has profound implications because it means that there is no preferred reference frame; all are equivalent. No matter where you go or how far you travel, you would appear to yourself to stay in the middle. Einstein sometimes said he wished he has called his work the theory of equivalence because that is a better way to think about it.
What tends to throw people off with this is that they think of all of the matter in the universe being squished down into a single point, and then that matter explodes. Over here, there’s the entire universe. A million miles away, there’s nothing. Eventually, the matter expands so that some of it reaches that point a million miles away.
That’s easy enough to imagine, but it’s wrong. What actually happened is that space itself expanded.
Let’s say you could pick an arbitrary point in space, which we’ll call A. Put a flag there, and that flag never moves. Not ever. Now pick another point a million miles away from that, and call it B, and put a flag there. What you find is that your two flags are now moving away from each other, even though they aren’t moving. What is happening is that the space in between them is expanding. It’s easy enough to picture matter expanding like a cloud of dust, but it’s a little more difficult to picture the space itself that the dust is in expanding.
So you go backwards in time, and A and B get closer and closer together, until they get squished down into a single dot, along with the rest of the universe. That’s the beginning of the universe. It all started as one little point. Did it start at A or B? Well, you can’t really say it started at either one and not the other, because both of them were squished down into that same little dot. And remember, your little flags at A and B never moved at all, but as you went backwards in time they still got squished together into a single point. The flags didn’t move. Space itself got squished. So the big bang happened at both A and B, and at every other point in the universe. The entire universe, and by that I mean space itself, not just the matter in it, was all squished down into that dot.
The point where you are right now was in that dot. The point in space 4 trillion miles away was in that dot. I’m not talking about whatever star or planet or piece of interstellar dust happens to be 4 trillion miles away. I’m talking about the actual point in space itself. That point in space was in that dot, right next to the point where you are now.
Now let’s look at it from the point of view of your flag A. From its point of view, flag B, as well as the rest of the entire universe, started out right next to it, then expanded away from it. But, if you shift to your flag at point B, from its point of view, the same thing happened. Every point in the universe was right next to it, then flag A and everything else moved away from it. So the big bang happened at A, and it happened at B, and it happened for any arbitrary points you pick in the universe.
In other words, the big bang happened everywhere.
A popular reference is the expanding skin of an inflating balloon. You can make marks with ink or whatever and watch the marks spread out. But even that’s misleading, because the universe isn’t expanding into anything. There’s no “outside.”
Put it this way: Pick any point at all in the Universe. It could be right here, or it could be in a galaxy billions of lightyears away, whatever. But pick that point and stay right there. Now run the clock backwards, at that point. No matter where that point is in the Universe, when the clock goes back to 13.8 billion years before the present time, you’re at the Big Bang.
And our brains think spatially, and are used to using visual processing, so it’s very hard and defeating to use said brains to ‘picture’ what the explanations are describing. Our language and our brain’s desire to paint a picture is why it’s so incredibly difficult to theorize the very things that Einstein did.