Let's just go ahead and assume that you're a little confused. Don't worry, I won't judge. We've all been there. Trust me. Answers cosmological don't come easy—it's not like our brains were hardwired for pondering the expansion of a universe billions of light-years across, so there's bound to be some poor connections made between the mathematics that we use to describe our reality and the words that I'm using to describe the mathematics. So let's take a little break from our breakneck race through cosmology and cosmological history to settle some old scores, once and for all.

Like the good teacher I pretend to be, I've tried to anticipate some of your questions:

WHAT DOES IT MEAN FOR THE UNIVERSE TO “EXPAND”?

Although the analogies come in handy in some limited cases, I want you to stop thinking of balloons inflating, bread rising in the oven, or anything else. Just simply absorb this bare-naked observational statement: on average, at large scales, all galaxies in our universe are moving away from each other. Take a snapshot of all the galaxies in the universe and record the distances between them. Wait a while. How long? It depends—how long is your ruler? Take another snapshot and repeat your distance measurements. The second set will be bigger numbers than the first set. And there it is: the expansion of the universe.

WHERE IS THE CENTER?

It's right here, where you're standing. Surprised? I thought we junked that whole Earth-centered business. The new wrinkle is that “You're the center of the universe” is now a meaningless statement. From any perspective, anywhere, on any planet in any galaxy, they will view themselves to be the center. Since every galaxy moves away from every other galaxy, the “center” is arbitrary. From our perspective, it looks like everybody is fleeing from the Milky Way. From Andromeda, same story. Pick the farthest, dimmest galaxy you can spot. If there are aliens in the galaxy, they'll come to the same conclusions: the universe is expanding from them. “Center” doesn't have any meaning in an expanding universe.

WHERE DID THE BIG BANG HAPPEN?

You really won't let this go, will you? The universe has no center. The big bang didn't happen somewhere over there. It happened everywhere. It happened in the room you're sitting in, in the distant galaxies, and in all the voids between. There's no place in the sky you can vaguely gesture to and say, “It happened roughly over there.” The big bang wasn't an explosion in space, it was an explosion of space. It happened everywhere simultaneously. It was a distinct moment in time that sits in the finite past of everything in the universe, not a point in space that everyone can, er, point to.

WHERE IS THE EDGE OF THE UNIVERSE?

Just as it has no center, it has no edge. I know, I know, it's very hard to imagine something with structure but without any outside. Look, if the universe had an edge, like a wall, separating universe from not-universe, then the not-universe would be a thing, and since by definition the universe is all the things, we would have to include it in the definition, and we'd be right back to where we started. If the universe is infinite, then we don't even have to think about it anymore. If the universe is finite, it gets a little harder to contemplate, since we're not used to the concept of objects having limited spatial extent but no boundary.

CAN I GET A METAPHOR, PLEASE?

Fine. Imagine the surface of the Earth. Just the surface, nothing else. Every location on the surface can be perfectly described by two numbers: latitude and longitude. Where is the center of the surface of the Earth? Notice I did not say “the center of the Earth.” I'll ask again to really make it clear: where is the center of the two-dimensional surface of the Earth? There isn't one. Where's the edge of the surface of the Earth? There isn't one (in two dimensions). Does the surface of the Earth have finite size? Yeah, 196.9 million square miles. Does it have a boundary? No.

ARE YOU SAYING THAT THE UNIVERSE IS CURVED LIKE THE EARTH?

Yes, but no. The universe may be curved at very large scales, but it doesn't matter. The point is that finite structures don't need to have a boundary in order to be a thing. Sure, we can easily visualize the surface of the Earth by imagining it embedded in a higher three-dimensional setting, but it's a mistake to think that everything must be embedded like that. The three-dimensional universe (ahem, four-dimensional, but let's focus on just space right now) could be embedded in a four-dimensional spatial structure, but it doesn't have to be for all the math to work out. It's just one of these weird things where the math is totally chill about it, but it's hard to express in English, let alone paint a picture in our brains. We're just not wired to conjure up images of three-dimensional, expanding, finite things. Sorry.

I THOUGHT YOU SAID THE UNIVERSE WAS FLAT.

One, that's not a question, and two, you're skipping ahead in the book.

SO WHAT IS IT EXPANDING INTO?

OK, I can't be too hard on you for not cracking this nut. The answer isn't “nothing” or even “something else.” The universe has no outside, and no edge. This is the real kicker: this question doesn't even make sense when it comes to the universe. There simply isn't an answer, but not because we don't know, but because the question can't be formulated. “What color is a cat's meow?” has all the correct English words and grammatical structure, but it's not a question we can answer. There is simply no such thing as an outside to the universe.

HOW BIG IS THE UNIVERSE?

Current estimates (which are very good, thank you very much) pin the whole observable universe to be a sphere about ninety-three billion light-years across. That's a big universe.

HOLD ON A SEC, WHAT DO YOU MEAN BY “UNIVERSE”?

I'll admit it, I've been playing a little fast and loose with my definitions. In one chapter I say that the universe is impossibly large—due to inflation—or maybe even infinitely big, but here I just gave it a size. When people (like me) refer to the size of the universe, we usually mean the observable universe, the limit of what we can see based on the age of the universe. It's the distance to the “horizon,” which, like a horizon on Earth, is…the limit of what you can see. The cosmic microwave background sits juuuust inside this horizon, and so for all practical purposes, it is the farthest thing we can see. But there is much more universe that we can't see—more galaxies, stars, solar systems, farm fields. That's the whole entire universe, but since it's completely inaccessible to us, can never affect us, and frankly doesn't care much for us, it doesn't matter. At all, so just ignore it.

UH, OK. WELL, HOW OLD IS THE UNIVERSE?

13,799,000,000 years, plus or minus 21 million years “since” the big bang. Yes, we know it that precisely,1 and yes, we're very proud of ourselves.

DOESN'T THAT MEAN THE UNIVERSE EXPANDS FASTER THAN LIGHT?

Well, yeah. The radius of the universe is bigger than 13.8 billion light-years. So superluminal expansion is a thing, especially during inflation (that was kind of the point). And even now! In an expanding universe, the farther you go from the Milky Way, the faster the rate of recession. Double the distance, double the speed. Eventually you'll come upon a distance that gives a speed faster than light. What's the big deal? Oh, you think that's a problem with special relativity, which set a universal speed limit? Haven't we, like, tested that or something?

Come here, child; let me share a secret with you. Special relativity is a local law of physics. The quantity we call “speed” is really only something you can measure nearby: you'll never measure the speed of a rocket blasting in front of your face to be greater than light. But a galaxy on the distant edge of the universe? It can have any “speed” it wants—if its redshift implies that it's going faster than light, it just means that the expansion of the universe is carrying it away so fast that you'll never, ever catch up to it. Even if you really want to.

HOW CAN WE ALL AGREE ON THE AGE? ISN'T TIME RELATIVE?

Yes, time is relative and not all clocks tick at the same tock…in special relativity. But the game of cosmology is played with general relativity, which is, you guessed it, more general. While usually time and space are all mixed together in a single cotton-poly blend of a fabric, in a homogeneous, isotropic universe (like the one we live in), time splits off from the spatial dimensions and ticks away comfortably at its own preferred rate. Due to the way the cosmic microwave background was emitted (across the entire universe at about the same time in about the same way), you can use that to find the heart of the cosmological clock. No matter your motion through the universe, once you measure the CMB, you can compute a frame of reference that is, at rest, relative to the universe. Once you know that, you can measure the universally common time (called the conformal time if you're feeling fancy) since the big bang.

IS MORE SPACE BEING CREATED, OR DOES THE EXISTING SPACE STRETCH?

Does it matter?

HOW CAN GRAVITY MAKE THINGS EXPAND?

It's not that gravity is, per se, making everything expand. It's that the behavior of the universe writ large is determined by its initial conditions and its contents. In the framework of general relativity, the universe found itself in an expanding state, and so that's what it's going to do unless somebody tells it otherwise.

IF THE EARLY UNIVERSE WAS SO DENSE, WHY DIDN'T IT COLLAPSE INTO A BLACK HOLE?

Remember that black holes, those infinitely dense bastards of the cosmos, are things in space. It's hard for all of space (i.e., the universe) to be a black hole: it's a different setup. The catastrophic, unstoppable gravitational collapse that leads to the formation of a black hole is driven by differences in density from place to place. But the extremely early universe was equally extremely uniform: despite its overall global density, there wasn't much difference within the cosmos at the time. It's only later that contrast arises.

IF BOTH SOLUTIONS ARE POSSIBLE, WHY IS THE UNIVERSE EXPANDING RATHER THAN CONTRACTING?

Now that's a good question.