1-1 What Is The Universe?

The universe, informally, means everything there is. Mountains and oceans and the moon and hot nuclear balls of hydrogen. And a whole lot more. But if the universe is everything, does that mean it includes the things that we don't know that we don't know exist? Or even the things that we believe exist but haven't yet seen or observed?

For example, is the future a part of the universe? In physics, we usually distinguish between these two notions of universe as, one, the observable universe, which is everything whose existence we've thus far been able to confirm or observe, or could in principle observe if we pointed our telescopes at it, and two, the universe with a capital U, or the whole universe, which is everything that exists, has existed, or will exist, anytime, anywhere, regardless of whether or not we're aware of it yet, or ever will be.

Of course, the main reason for this distinction is so that we can talk scientifically about parts of the universe we haven't yet been able to observe, and the two biggest reasons we might not have been able to observe a piece of the universe yet are either it's in the future, or it's so far away that light from it hasn't had time to reach us.

Yet. To explain, as far as we know, the universe had a beginning, and is now around 13.8 billion years old. Light takes time to travel through space, so anywhere too far away from us just isn't visible yet in the same way that thunder from a distant lightning strike isn't audible. Until you wait long enough. If you were blind, the thunder would be the first sign you have of the storm's existence, once you hear it, you can infer that the storm must have existed all along.

So within the observable universe, what do we know exists? Pretty much just three things, spacetime, particles, and the physical laws that describe how spacetime and those particles interact to give rise to gravity and matter and forces and everything else. It's open for debate whether or not mathematics is a part of the universe or if it exists outside the universe in some sort of mathematical heavens.

But if you think that mathematics exists, and if the universe is everything that exists, then well, I'll leave that up to you. There's one other thing, though. You may have heard of parallel universes, or multiverses, an idea proposed by researchers or sci-fi fanatics that perhaps what we think of as the universe is actually just a small piece of a larger thing which is really everything there is.

But that's another verse of the story. This episode of MinutePhysics is supported by Gazelle.com, the fast and simple way to sell your used smartphones and Apple products. Simply tell Gazelle the condition of your gadget, find out what it's worth, and Gazelle pays you quickly by check or PayPal. That means you'll get cash to upgrade to the latest version of your favorite gadgets.

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The universe. How big is it? Does it have a center? Does it have an edge? Is it getting bigger? And if so, why? Well, we know that there are two different meanings for universe. First, the observable universe is everything that we've been able to see or observe thus far. And second, the universe, or the whole universe, means everything that exists or has existed or will exist.

More specifically, the observable universe is the region of space visible to us from Earth. And since the universe is only about 13.8 billion years old, and light takes time to travel through space, then regardless of what direction we look, we see light that's been traveling at most 13.8 billion years. So it's logical to think that the observable universe must then be 2 times 13.77 equals 27.5 billion light years across. But it's not. That's because over time, space has been expanding, so the distant objects that gave off that light 13.8 billion years ago have since moved even farther away from us.

Today, those distant objects are a bit more than 46 billion light-years away. Multiply times two and you get 93 billion light-years, the diameter of the observable universe. To give you a sense of scale, the size of the Earth within the observable universe is roughly equivalent to the size of a virus within the solar system. Although that doesn't help much because we can't really appreciate the incomprehensible smallness of a virus, nor the bewildering bigness of our solar system either.

So let's just say that the observable universe is stupendously big. But the whole universe, as far as we can tell, is a lot bigger. Space is most likely infinite, or at least it doesn't have an edge, though the difference between those is another story unto itself. Now, what about the center of the universe? Well, the observable universe has a center.

Us. We're at the center of the observable universe because the observable universe is just the region of space visible from Earth. And kind of like how the view from a very tall tower is a circle centered on the tower, the piece of space we can see from here is naturally centered here. In fact, if you want to be more precise, each one of us is the center of our own observable universe.

But that doesn't mean we're at the center of the whole universe, just like the tower isn't the center of the world. It's the center of the piece of the world that it can see, up to the horizon. But just because you can't see beyond the horizon doesn't mean there's nothing there. And so it is with the observable universe.

Looking up at the sky, we see light that's at most 13.8 billion years old and coming from stuff that's now 46 billion light years away. Anything farther is beyond the horizon. But each second, we see new, even older light coming from slightly farther away, three light seconds farther to be precise. And so our view of the cosmos is literally getting bigger all the time.

All we have to do is wait and watch as the universe ages and light from more distant places has the time to get to us. So here we are, sitting at the center of our observable piece of the whole universe. How big is the universe? Well, the observable universe is currently 93 billion light-years across. The whole universe is probably infinite.

Does the universe have an edge? The observable universe does. It's 46 billion light-years away in any direction. And the whole universe has a temporal edge, or what we call a beginning, but almost certainly not a spatial one. Does the universe have a center? Again, the observable universe does. You. The universe as a whole? Almost certainly not. And is the universe getting bigger?

Yes! Space is expanding, which makes both the observable universe and the whole universe bigger. Plus over time, we see older and older light coming from farther and farther away, so our observable universe gets bigger that way, too. And that, in a nutshell, is our view from the tower. You are the center of the universe. And so am I.

And so is everyone else. And so is no one.

Thank you. a free audiobook at audible.com slash minute physics