If The Universe Is Expanding, What's It Expanding Into?S

Last week the readers of io9 asked their most nagging questions about physics to a real-live physicist. The first question: What is the Universe expanding into?

You responded to my call for questions in droves. There were over 350 comments to the post, and at least a hundred unique questions. For a few, I couldn't wait to answer. With others, I waited in vain for closure, including the reader asked me out, and another who apparently has a theory for interstellar travel that he's been working on for 20 years, but after being burned by physicists before was unwilling to provide any details.

Lots of you wanted to know what would happen if you were driving at speed of light and turn on your headlights, and I'd be remiss if I didn't shamelessly mention that my new book answers many of the questions. Chapter 1, in fact, is entitled, "What happens if I'm traveling at the speed of light and I try to look at myself in the mirror?" but this being the Internet, you want your content for free.

Over the next several days, I'll be answering the best and the weirdest, and what the hell, if you're the asker and you send me an email, we'll send you a free book!

Today's question comes from PVIII (and others, but PVIII asked first):

Ok, so I've always had this question, and I know it's very, very dumb:
So the universe is redshifting (or is blueshifting, I always forget), but either way it's either expanding or contracting from the big bang. That being said, what is it expanding INTO or FROM if it's the freakin' universe! If it's not endless, where is the end?!? How is there an end to the universe!
My mind just exploded.

I get this question a lot, and with good reason. You've seen a Science Channel special in which a dapper (presumably British) cosmologist talks knowingly about how the universe is expanding as though it's the most natural thing in the world. But to someone not trained in general relativity, it can be hard to grok what the expanding universe really means. This sort of question comes up so often, in fact, that it (or a variant of it) is the title of Chapter 5 in my book, and was the first question, chronologically, that my co-author, Jeff Blomquist, and I knew we had to answer.

So first, a few things about what the expansion of the universe isn't.

1. You are not currently expanding. Neither is the earth. Nor the solar system. Nor the Milky Way Galaxy. The expanding universe is due to gravity, which means that in regions of high density, the dominant gravitational effects are entirely local. As a case in point, not all galaxies are moving away from the Milky Way. Our nearest neighbor, Andromeda, is actually hurtling toward us at around 80 miles per second, and will collide with us in a few billion years.

2. Don't believe the metaphors. In every science special worth its salt, you'll see some image of a balloon being inflated. "This is just like the universe!" the dapper British cosmologist will say. But you, being clever, will note that there really is something outside the balloon universe, and that the 2-dimensional surface of the balloon is inflating into a 3-dimensional studio. But our universe only has 3-dimensions. *

3. The universe doesn't have a center or an edge. We're not actually sure whether the universe is infinitely large, or merely very, very big, but in even supposing it is, that just means that if you traveled far enough in one direction, you'd eventually return to where you started. Think Pac-Man, but without the ghosts or fruit. Practically speaking, though, the speed of the expansion (and the size of the universe). As for the center, that's where the balloon analogy is actually pretty helpful. Sure, it seems like all of the galaxies are moving away from us, but from their own perspective, every galaxy appears to be at the center of the universe. I assure you that it's just an illusion.

So what is the universe really expanding into? Nothing. There is no cosmic storage locker just waiting to be filled up with stuff. But in order to understand why, let's see what general relativity has to say about space-time.

In GR (as the pros say), the most important property of space (and time) is the distance (and time interval) between two points. In fact, the distance measure defines space entirely. The evolution of the distance scale is governed by the amount of matter and energy in the universe, and as time passes, the scale increases and the distance between galaxies increases. But, and here's the weird part, it does so without the galaxies actually moving.

I know your intuition breaks down at this point. Mine does, too, but that doesn't prevent us from exploring some of the weird implications of all of this.

For one thing, you've probably been told that galaxies are flying away from us. They're not. This is one of those realities that are usually swept under the rug because science channel producers don't have the same faith in you that I do.

"But wait!" one of the more scientifically astute among you will say, "We measure the Doppler shifts of distant galaxies." This is the so-called "redshift" that PVIII referred to in his question, and here on earth, a Doppler shift, like with an ambulance siren, is a sure sign of motion. But that's not what's happening on cosmological scales. What's really going on is that the scale of space increases from the time that the light was emitted at the distant galaxy to the time that it reaches your eye. As space expands, so do the wavelengths of the photons, and as wavelengths increase, they appear redder.

This picture of the universe even helps you to understand another popular question: "Is the Universe expanding faster than light?" It's absolutely true that the most distant galaxies are increasing their distance from us at faster than the speed of light, but so what? The galaxies aren't moving faster than light (they're sitting still), and more importantly, the fast expansion doesn't actually help you do anything. For example, you couldn't use it to send a care package – or any information – to a distant galaxy faster than a light signal, which means that the speed of light is still the universal speed limit.

I've described the rock-solid (or at least relativity-solid) consensus view of how cosmological expansion works, but I'll finish with something we don't currently understand. Everything I've described above works perfectly fine if you have already have space to stretch. But what happened at the very beginning to create space out of literally nothing? Physics doesn't have an answer for that yet, I'm afraid, and we'll presumably have to wait (at least) until a theory of quantum gravity comes along before we have one.

* I know that at least one person is going to write in the comments section about string theory and how the universe might really be 10 dimensional. You win a gold star. Still, a) We don't need string theory to understand the expanding universe. Ordinary general relativity works just fine, and b) There is a heck of a lot more evidence (i.e. some) that GR is right than string theory is.

Top image: The Whirlpool Galaxy, from the Hubble Space Telescope.

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (Wiley: 2010). He is an associate professor of Physics at Drexel University. Most recently, he wrote "An Open Letter to the Writers and Directors of Hot Tub Time Machine" in io9.