[Start with a screen with options for Text-only and Audio presentations]
Welcome to "The Shape of the Universe", an interactive lesson about shapes, the universe and some other things. Before we get started I feel it would be wise to go over the controls for this lesson. If you know them already, you can skip ahead.
You can go forwards or backwards by clicking or touching the arrows in the corner of the screen. If something in the lesson is happening too fast, you can also restart the current slide using the refresh button. If you prefer to use a keyboard, you can press the left and right arrow keys to go backward and forward, and press "R" to refresh the current slide.
That's all! The next slide will begin the lesson, enjoy!
How did the universe begin?
If you've read books or watched videos on the early universe, you probably know that the universe began as an infinitely dense piece of matter that expanded to a monumental size in an event we call the Big Bang. You've probably seen it illustrated through diagrams like these:
{Diagrams}
If you've looked a little deeper, you've probably also heard that the matter in the early universe wasn't expanding "into" anything, but rather that the universe itself was expanding. That's not to say the universe had an edge or a wall to keep the matter in-- yes I know it looks like there's an edge in these diagrams but, uh, just trust me, there wasn't an edge.
Ahem. So, space expanded, the universe cooled, we learned about this using microwave dishes and everyone lived happily ever.
Um, what.
Do scientists really expect you to digest that kind of explanation and understand what's going on? We might be able to repeat that explanation back to our teachers or write it down on a test, but try to answer this question:
If I'm a particle in this small early universe and I start going left until I hit this "side-thing" over here, what happens? Do I break through the wall? Do I bounce back? If we can't answer that question, it means we don't really know what's going on here.
In fairness to teachers and textbook authors, these ideas are really difficult to explain. We're trying to describe 3 dimensional, and sometimes 4 dimensional concepts using 2 dimensional images and 0 dimensional words. This is a difficult task, and it leads some people to believe that they just aren't smart enough to get how this stuff works.
But it doesn't need to be so hard. The shape of the universe is something that can be understood by anyone who has ever played video games like Asteroids, Pacman or Portal. Even if you haven't played those games, the demonstrations in this lesson will hopefully help you get the hang of things.
Scientists usually talk about the universe as having three possible shapes: Closed, Flat or Open. A Closed universe would have finite mass and limited space. A Flat or Open universe would probably have infinite mass and an infinite amount of space. We can talk another time about how we can measure whether the universe is Closed, Open or Flat, but for now, I'll summarize what we know.
We could be wrong, but we're pretty sure the universe is Flat. However, I'm going to spend most of this lesson talking about a Closed universe. It's easier to wrap our minds around what life would be like in a finite universe. After we've got that under control, we can take a look at how our new knowledge would fit into an infinite universe.
Let's begin by talking about a small universe about 100 meters across. In this universe, we have two items: our space ship, and an asteroid that will always stay put. You can drive the space ship by clicking or touching where you want the ship to go, or by using the WASD keys on your keyboard. These controls will be the same for all the demos you see today.
Drive around this universe to get the hang of things. In particular, try driving off one of the edges to see what happens.
[Triggered by crossing the edge] If you drive off the left edge, you reappear on the right edge. If you drive out the top edge, you reappear through the bottom edge.
The problem with this universe though, is that it appears to have any "edge" or "center" at all. When you're ready, go to the next slide, and we'll fix that problem.
Alright, try driving around this universe and notice what's different. Now our space ship appears to always be in the center, and it's the universe that's moving around us. This is the basic idea behind General Relativity, and what it really means is that the universe itself has no center. To us, the center is wherever we are, and the edge is just the places that are farthest away from us.
Let's review:
- Our tiny universe has a limited amount of space.
- However, it doesn't have an edge or wall to keep us in, we just keep wrapping around to the smae place, over and over.
- The universe also has no center, we just have to look at how far away things are and how fast they are moving relative to us.
Alright, now let's talk about how space expands. Expansion doesn't usually get measured in kilometers per hour, since that wouldn't make sense. Even if we look at how wide the universe is before an expansion (100m) and how big it is after (200m), we still wouldn't say it was expanding at 100m/s. Rather, we'd say that it expanded at 1 meter per second per meter. Basically, if two objects are 50 meters apart, they will be moving away at 50 meters per second. If they are 100 meters apart, they will be moving away from each other at 100 meters per second. [Trigger second round of expansion]. Scientists estimate that our universe is expanding at about 74 kilometers per second per megaparsec. This is much slower that our toy universe, but still fast enough that objects really far away from us are moving away from us faster than the speed of light.
It's also important to note the the universe is not expanding in any direction. No matter where we are, the other things in the universe will appear to be moving away from us.
Up until now our universe has been rather lonely. Let's fill it up with some particles of matter. Now, for the sake of this demonstration, these are going to be huge particles, just so we can more easily see what's happening.
When the particles collide with each other, they heat up and glow. Right now they're fairly well spaced out, so they're cool and dark, and I've outlined them to make it easier to see the dark ones.
Drive around this universe, and notice that particles on the left side of your view are colliding with particles on the right side of your view. There may not be a wall keeping them in, but you can start to see how things could get pretty compressed if you had a lot of these particles in a small universe.
Let's do just that. I'm going to start adding particles to this universe until they start glowing white hot. Feel free to drive around, I've given your space ship a pretty terrific heat shield.
Now that things are super hot, let's expand the universe. Once the particles have more space to move around, they'll run into each other less and cool down.
[Triggered by completion of animation] A quick note before going forward: The universe didn't expand because it was hot and dense, we're just lucky that it did.
Alright, it's time! Let's put all this together and see how the big bang would look if it happened in a tiny rectangular universe.
[Start animation]
The universe starts as a tiny dot that RAPIDLY expands in an event known as inflation. After inflation, expansion slows down, but continues at a steady pace. The next few seconds represent the 300,000 years it would take for the matter to cool and become dark.
Now that we've seen the big bang happen, here's a question: Where in this universe did the big bang happen? If you ask one of the people in the space ship, they'll say "The big bang happened where I was, and then everything started moving away from me." If you ask a particle, it will say "The big bang happened where I was, and then everything started moving away from me." Neither of them are lying: According to everyone, the Big Bang happened everywhere.
Now that we have a handle on how this small, rectangular universe works, let's talk about shapes again. Our 2D rectangular universe has actually been 3D this whole time! Let's look at what that means.
Let's look at the 2D rectangular universe as if it were on a piece of paper. If we fold the paper into a cylinder, the spaceship can leave through what used to be the top edge and come back in through the bottom edge.
But what about the side edges? To make them meet, we'll have to fold the cylinder into a donut shape (often called a torus). This looks like a bit of a mess, because a rectangle cannot be folded into a torus without getting messed up or distorted. This indicates that our rectangular universe isn't mathematically possible.
If we worked with a slightly distorted rectangle, we could make this donut shaped universe work. Although some scientists believe that torus shaped universes are possible, they consider it more likely that a Closed universe would have a more simple shape, like a sphere.
Here's our 2D spaceship on the surface of a sphere.
The controls here are approximately the same as they were in the rectangular universe: W to move forward, S to slow down, A and D to turn left and right. For mouse and touch controls, click or touch above the space ship to go faster, or below it to go slower. Clicking or touching to the left or right of the ship will cause it to turn.
Now, a sphere is easy for us 3D supernatural beings to understand. But for the people on our space ship, they'll see something pretty weird.
If we were in a "circular" 3D universe, it would actually be a 4D hypersphere. To our eyes the universe would appear like some sort of funhouse mirror, with far away objects distorting before getting close again. It wouldn't be exactly like being inside a spherical mirror, but the effect would be similarly twisted and bizarre.
The things we learned about expansion and space still apply. Let's look at a circular universe and see what happens as we fill it up with particles, then let it expand and cool.
Notice that as the universe gets bigger and bigger, the curvature becomes less noticable. It starts to become harder to tell the shape of the universe when we can't see as much of it. If our universe was curved like this, it would have to be so mind-bendingly large that we couldn't see any of these weird curvature effects play out in the sky around us.
Now that we've wrapped our minds around a Closed universe, let's talk about Flat universes. As far as our measurements can tell us, the universe is Flat, not Closed or Open. How we measure this is a story for another time, for now, let's talk about what it means to live in a Flat universe.
This means that the universe goes on forever in all directions without needing to repeat itself, and that it contains an infinite amount of matter. However, our core concepts from the Closed universe haven't changed:
- The universe has no edge and no center.
- Expansion has no direction and is measured in multiples, not distance/time
- The universe started from a point of infinite density
[Part II] Some people who are listening to this may question that last point, asking "What do you mean infinite density? I thought the universe expanded from an infinitely small point?" Let's talk about infinity for a minute.
Infinity is not a number. If we try to do basic math using Infinity, the results may seem to make sense at first, but things get weird pretty quickly.
If you have infinite mass in a finite amount of space, the matter will always be infinitely dense, no matter how large the space becomes.
If you have finite mass in infinite space, the average density of the universe will be 0 (even if all the mass is clumped into one place).
If you have infinite mass in infinite space... well, what's infinity over infinity? The answer is that it could be any density. It could be infinitely dense, or the matter could be spread out so sparsely that the density approaches zero.
So we could start out with infinite matter clumped together really tightly, and then end up with infinite matter clumped together kind of loosely, once space expands a bit.
Infinity is a crazy, mind-bending idea, no matter how you slice it. If this part of the lesson confuses you, I'd advise you to check out this video about a famous riddle called Hotel Infinity. It may help you wrap your mind around what you're about to see.
[Part III] This is an infinite universe at the moment of the big bang, expanding out and cooling. If you head out to the right, you won't wrap around to the beginning, you'll just find... more universe. More matter, more space, a little different than the place you left, but pretty similar.
Although I've worked hard to make these simulations accurate, there are some places where we've been playing a little fast and loose with science. I want to address some of those places before we wrap up:
- First of all, the time scale of these simulations does not match what happened during the real Big Bang:
- Arguably the most interesting part of the Big bang was the first couple seconds, when matter and antimatter annihilated each other and the single unified force broke down into the four forces we know today. Our simulations covered the expansion and cooling of the universe, but left out a lot of this really interesting stuff.
- In our simulations, the universe went from a hot mess of plasma to cool and dark gas in a matter of seconds. However, the early universe was actually hot and bright for hundreds of thousands of years before it expanded enough to cool and become darker. PBS Space Time has an excellent episode describing how this worked.
- Whether our universe is Closed, Flat or Open impacts a lot of things besides whether or not it is finite. Here are some good resources on the topic: [FIND RESOURCES]
This lesson would not have been possible without [Credits].
I enjoyed making this lesson a lot! If you found it helpful and want to see more of these lessons, you can support my work on Patreon.