# Is the Universe growing in size?

mkarger
This is a question about the borders of the Universe (if such a thing actually exists).

I got in a discussion with my astronomy professor and inevitably I brought up the question "if the Universe is expanding, and the Universe is contained, what is it expanding into?"

He said that it's not actually expanding into anything. I said that doesn't make any sense. He gave me an odd look and said "I know".

To me, this would suggest that it's not actually growing larger in size.

narrator
Until about 12 months ago, I had the same difficulty and misunderstanding. As I understand it, there are two theories: The universe is finite, and the universe is infinite. It seems that the latter is the more favored, which I used to have trouble with, but now see its elegance and (within the bounds of my limited knowledge) its validity.

In the infinite model, expansion is discussed in terms of space-time and is difficult, even for the experts, to explain because much of it comes from cosmological/mathematical modeling. But I'll give it a go.

The first thing you have to understand is that the term "big bang" has not helped people's understanding. We think of an explosion as having originated from a coordinate, generally at the center of the explosion. The BB was not like that. With the BB, matter came into existence, everywhere in the same instant. (Suspend philosophical disbelief for the moment, understanding that is a side issue that can be discussed later.)

Next, the typical model of how scientists attempt to explain expansion is by using a 2D analog of our 3D universe. Imagine a balloon as being the universe. Follow a line across that balloon anywhere and you will find no edge. Add some air to that balloon and it expands, creating greater distance between objects on the surface of the balloon.

Objects may be moving on that surface, but the expansion adds distance that has nothing to do with the movement of those objects. Understanding expansion itself is a whole other topic.

If I've explained this wrong, stay tuned for the corrections, but essentially we cannot apply our earthbound understanding of finite things to the universe.

Well, this is one of those things about mathematics that just can't be visualized properly. The problem is that in General Relativity, there is no background: the geometry of space-time is all there is. So if we're talking about an expanding universe described by General Relativity, then that expanding universe is all there is.* There is no background grid against which the expansion is happening. It's just that objects within the expanding universe are getting further apart. Within General Relativity, it doesn't even make sense to talk about what lies "outside" the universe.

*The caveat here is that there are other models that describe our universe as just one expanding bubble of space-time among many. But in these models, our expanding universe still isn't expanding into anything. Basically, the different expanding bubbles become disconnected from one another and act entirely independently.

OCR
He gave me an odd look and said "I know".

Lol... well, at least you have an honest one...

OCR