## What is the universe expanding into? (Intermediate)

*I am very confused about things my science book says about the expanding universe. Every book I have seen has defined the universe as “everything”. If the universe is expanding what is it expanding into? It would have to expand into even more universe. I understand that the red spectra indicates that things are moving away from us but that is drifting not expanding, right? If you could help me to understand this, it would be appreciated. Thank you for your time.*

*anything*; instead, what is happening is that every region of the universe, every distance between every pair of galaxies, is being “stretched”, but the overall size of the universe was infinitely big to begin with and continues to remain infinitely big as time goes on, so the universe’s size doesn’t change, and therefore it doesn’t expand into anything. If, on the other hand, the universe has a finite size, then it may be legitimate to claim that there is something “outside of the universe” that the universe is expanding into. However, because we are, by definition, stuck within the space that makes up our universe and have no way to observe anything outside of it, this ceases to be a question that can be answered scientifically. So the answer in that case is that we really don’t know what, if anything, the universe is expanding into.

*through*space at all, they get farther away from each other as time goes on because the space in between them has been stretched.

*there*, and which everything else in the universe exists within. But according to Einstein’s theory of general relativity, space isn’t really as simple as our common sense tells us. If we want to understand the actual way that the universe functions, we need to find some way to incorporate Einstein’s ideas into our mental picture and imagine space as a more complicated entity which is capable of doing things like “bending” and “stretching”.

*Relativity for the Million*.) We have no idea how big the dough is at this point – all we know is that it is

*very*big, and we, sitting on some raisin somewhere inside it, are so far away from the “edge” that the edge can’t possibly have any effect on us or on what we see.

*relative to the dough*they don’t move at all – the same particles of dough that start off near a particular raisin will always be next to that raisin. That is what I meant when I said that the galaxies aren’t really moving through space as the universe expands – here, the raisins aren’t moving through the dough, but the distance between the raisins is still getting larger.

*much*different from the old picture in which the galaxies are all moving through space away from some point at the center. A lot of concepts and definitions that seem simple to us in the old picture are much more complicated now. For example:

**What is the distance between two galaxies?**In the old picture, this is an easy question to answer theoretically (though not necessarily in practice!). Just get yourself a giant tape measure and clip it to a faraway galaxy, then come back to our galaxy and hold on tight. As the galaxy moves away, it will pull on the tape measure, and you will easily be able to read off the distance as the tape measure unwinds… one billion light-years, one and half billion light-years, two billion light-years, etc.

*also*will be stretched, causing it to have a longer wavelength and therefore causing its color to appear more towards the red end of the spectrum. This is what leads us to see redshifted light when we look at faraway galaxies, and it is measurements of this redshift that allow us to estimate the distances to these galaxies.)

**Where is the center of the universe?**In the old picture, it is easy to say where the center of the universe is – it’s the point in space that all the galaxies are moving away from. In the new picture, though, this isn’t so clear. Remember, the galaxies aren’t actually moving away from each other – they’re sitting still! Let’s go back to the dough analogy – sure, you can imagine that even if the dough is really really big, it has some point within it which is the geometric center. But this definition is not very useful. Since the dough represents the space that we live in, we have no way to see “outside” of the dough to get a sense of the entire shape and figure out where the center is. So if you are stuck inside the dough, and have no way to see anything except the dough, and if you are so far from the “edge” of the dough that you can’t see it and it can’t have any effect on you, then what difference do you notice between the point where you’re at and the point that is actually at the geometric center of the entire blob of dough? The answer is that there is no difference, absolutely none. The concept of the “center of the universe” loses all meaning, so we don’t even think about it.

*infinitely*big – that is, you could walk forever in a straight line and

*never*reach a place where the dough ends. In that case, there really would be no center of the universe – the only way you can define the center is to mark out the edges and find the point that’s equally in between all of them. So if the universe is infinitely big and has no edges, then it also has no center, not even on a theoretical level.

**What does the universe expand into?**Finally, we can return to the original question. In our old picture of the universe, the answer would be simple, although very unsatisfying. The collection of galaxies that make up the universe is moving through space; therefore, the universe is expanding into even more space than it already encompassed. In our new picture, though, the galaxies are just raisins spread throughout the dough – their presence is largely irrelevant to the question of the universe’s expansion. What we really care about is the dough, and whether or not it has a boundary.

*does*have a boundary, we know that the boundary is so far away from us that we can’t currently see it and it doesn’t have any effect on us.

*anything*to expand into. Thinking about infinity is always complicated, but a good analogy can be made with simple math. Imagine you have a list of numbers: 1,2,3,etc., all the way up to infinity. Then you multiply every number in this list by 2, so that you now have 2,4,6,etc., all the way up to infinity. The distance between adjacent number in your list has “stretched” (it is now 2 instead of 1), but can you really say that the total extent of all your numbers has “expanded”? You started off with numbers that went up to infinity, and you finished with numbers that went up to infinity. So the total size is the same! If these numbers represent the distances between galaxies in an infinite universe, then it is a good analogy for why the universe does not necessarily expand even though it stretches.