# How does the concept of a universe with no center work?

I understand basically that the universe is homogenous (looks the same from every point) and I was told in my astro class that it's supposed to have no center, but how that works boggles my mind. I can't get over the feeling that from some point in the universe a civilization could look in a certain direction and see a sign indicative of the direction towards the edge or the center, like younger galaxies or a lack of galaxies moving a certain direction.

EDIT: Thanks to Sir Cumference for highlighting some things that are unclear about my question. Sure, to a certain extent you might be able to think of the universe as being without a center, but if you think of a ball growing larger from nothing, and you pick a random point in the growing sphere, you can always say that the point is further from some points on the sphere than others, or in other words, that there is a direction towards a "center".

Any other details you can give that are related at all would be extremely welcome.

• I remember reading Brief History of Time a long time ago, where It says that every point on a imaginary infinite line/space is a center. I guess that makes sense looking out from Earth or any planet. – ObiWanKenobi Mar 5 '16 at 17:38

When we talk about the universe, we are really talking about one of two things:

• The observable universe, which is everything we can possibly see.
• The Universe, which is everything that has ever existed, currently exists, and will exist.

The observable universe has its own center, usually the Earth. It is a spherical region of everything that we can see, essentially anything whose light has reached us. We usually refer to this when we say things like "there are $10^{86}$ atoms in the universe."

In reality, everyone has their own observable universe, and it can change depending on where you are. An exoplanet far away has its own observable universe, and can receive light from different places. Essentially, you are the center of your own observable universe.

I assume you're talking about the latter, though. The Universe (notice the capital "U") is all of space and time and its contents. Anything that has existed, will exist, and currently exists is part of it.

The Universe is thought to be infinitely large, so it can't have a center. The center of something is the point equidistance from the edges, but if something spans infinitely long, it would just keep going. It wouldn't have an edge, and thus it wouldn't have a center. You couldn't find the point equidistant from the edges if it just spans infinitely.

You might ask, "then where did the Big Bang start? Surely it must've been the center of the Universe, right?" Well, you can say the Big Bang happened everywhere. Before the Big Bang, matter filled most of the Universe's emptiness. It was essentially dense, and so got extremely hot to the point where no hadrons could form.

It is thought that this temperature caused space itself to expand. Essentially, more space was created in between all the matter, until everything was able to cool down. This is what we call the Big Bang. It didn't happen at a certain point, but rather it happened everywhere.

EDIT: I understand your confusion. Let me just clear some common misconceptions:

The Universe is not like a ball. Rather, you can think of it like a flat grid, and its "expansion" just means that the distances between objects on the grid are getting larger. In essence, more space is being created between the objects. That's what we mean by expansion — that objects are moving away from each other, since more space is being created between them.

Here's an easy analogy: imagine you are walking your dog. Suddenly, the ground begins expanding between you. You and your dog will separated and continue receding away from each other.

That's essentially happening everywhere: space is expanding between everything, so we are drifting away from other galaxies. The Universe is infinite, and we can constantly drift apart from other objects because space is being created in between us. Here's a GIF I made that might help you get it:

You can see how the galaxies drift apart as the space between them increases. And this happens everywhere in the Universe. So let's keep that in mind and clear up the Big Bang.

The Big Bang did not happen at a single point, nor did the Universe begin at a single point. The Universe is, and has always been, infinite. The Big Bang was just when the Universe's expansion really began — that is, when objects started drifting away from each other. The Universe was still infinite, but there was less space between the matter.

This density caused the Universe to get extremely hot and expand. So here, space itself was distorted and began to expand. More and more space was created between the matter, and still is now (although now it is mainly due to dark energy instead of heat).

• The issue is more that I hear a lot of stuff like this when I ask similar questions, and i just don't get it. First of all, the idea that it is infinitely large seems just flat out wrong. It started out at a small, finite size, and expanded at a finite speed. Actually, I have a ton to say about my confusion. I'll add it into my question details. – Pulchritude Mar 4 '16 at 3:31
• @Pulchritude Ah, I'm editing my question now. – Sir Cumference Mar 4 '16 at 6:44
• @Pulchritude Hopefully I've cleared things up. – Sir Cumference Mar 4 '16 at 7:44

I suspect that what the class told you was that the universe might be infinite. We currently have no way of knowing if that was true or not. The issue is that there is no way of knowing when inflation stopped outside of our Hubble sphere. I believe that there is some evidence in the CMBR that it has a minimum size which is several times the Hubble radius, as there would be resonances that aren't apparent in the spectrum. However there is no upper limit. Note, however, that this theory does include the possibility that the universe wasn't always infinite but that it was a finite size early.