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Dark Energy: What Is It?

Neil Smith speaking at Milton Keynes Geek Night in January, 2017
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About this talk

"63% of the universe is dark energy": what does that mean? In just five minutes, Neil Smith explains general relativity, the shape of space-time, and how dark energy fits in.


1915 or so, he started thinking about what happens if you turn on a torch inside a spaceship, as you do. From that, he got to general relativity and working out that gravity isn't actually a force, it's the shape of spacetime. And so there's a question, why is gravity so different from every other force in the universe? We don't quite know, but we'll move on to that. So he said, "I've got this great theory of relativity. Can I use that to write down an equation that describes the entire universe?" And in 1922, someone called Friedmann said, "Yes, you can." That's it. So, we've got this big, scary equation. We're going to spend a bit of time with this, so go over what this means. This bit here is talking about the size of the universe. Here's a bunch of boring constants, we don't care about this. This row is the density of mass energy in the universe. This is how much stuff there is in here, and this is a bit about the shape of space. Okay now, it turns out that space is completely flat in the same way that the surface of a cylinder is completely flat, so that last bit disappears. The density depends on how much stuff there is and how big the universe is, and so you actually want to split out radiation and matter, and how it works. So, this bit is all the matter in the universe. That includes all the dark matter, which is 85% of the stuff we see. I'm not talking about dark matter. I'm talking about dark energy. So, although we don't know what dark matter is, that's not what I'm talking about. So, we've got this. First order, nonlinear differential equation, describes how the universe changes. So you can quite happily solve that in your head, can't you? Yes, you can. You absolutely can with the right analogy. So, this is the universe, okay? And the model, it's quite small, yeah. So, we're going to represent the size of the universe by how high this ball is, okay? So this is a big universe and this is a little universe. And relativity says that the thing that's acting on the universe is gravity, so in this model we're going to represent gravity by gravity. So, what we want to do is, given this equation, starting from some initial conditions, as in, I'm holding a ball, if I let the universe evolve naturally, what happens to the size of the universe? It... - [Man] It shrinks. - Yeah. If I drop the ball, it goes down. There you go, you're cosmologists. You're doing that. So, this says the universe will tend to collapse over time. Now in 1922, Einstein, along with all right-thinking people, knew the universe was static and unchanging. So, he didn't like this answer, so he invented the cosmological constant to hold things in place. So, basically, he invented the hand, is a bit of a hack. That's where we were. 1929, this guy turns up, Edwin Hubble. He invents a couple of things, or discovers a couple of things. One is that the universe is actually quite a bit bigger than just our galaxy, and he also actually was able to prove general relativity. So the good news is, the universe is not static. The bad news is, it's expanding. So, the question is, as the universe is expanding, how can you get this ball moving upwards? How can I make this ball go up? - [inaudible 00:04:07]. - Yeah. So I could throw the ball up, okay? So that means that, at some point in the past, there must have been something quite big, like a bang, that made the universe move up. But as it does, what happens to the size of the universe over time? So, watch the ball. What's it doing? - Going down. - Going up and coming down. So, what does that mean for the size of the universe? - It's bigger and smaller. - Bigger, and then smaller. So, we end up with a big crunch. There's another solution to these equations. What happens if I throw the ball up really hard? Really, really hard? Sort of NASA-levels of hard? - It will just go up. - It's just going to go up and stay up. It's going to go on forever. And that's another way the universe could end. It could be with stuff just drifting away, in the heat death of the universe. And of course, there's this dividing line between the two, where the universe kind of hovers between expanding and contracting. And that's where we were with our understanding, until 1998 when a large team of astronomers started looking at supernovas. Because it's really unclear which side of this line we're on, we're not too sure which way. We seem to be right on the critical value. Now, why that is we don't quite know. There, the size of the universe is doing this. It's accelerating. The expansion's accelerating. It's like I throw the ball up in the air and it goes up, and it slows down, and after a while, it starts speeding up. And the ball speeds up more and more the higher it gets. This is strange. Now, even for cosmologists, they think this is strange. So, what's happening? So, it's like there's this extra force pushing on, and we don't know what it is, this thing, so it's got to be "dark" something. And the mathematics means it looks more like energy than matter, and we can't use the name matter because we've already got dark matter. So, we call it dark energy. It's this stuff that's pushing up, and it seems like the bigger the universe is, the more it's pushing out. It's like the density of this pushing out is the same. So, we've got this thing that's acting on the cosmological scale that's a bit constant pushing out. So, we're back to the cosmological constant, so Einstein's ugly hack was actually correct all the time. And if you work out the numbers for how this works, radiation is having no particular effect at the minute. Matter, including dark matter, is about a third of the universe, and dark energy is about two-thirds of it. So, we've got some idea of what dark energy is, we know how it's behaving, we've got an idea of the size of its effects. What is it? What's the mechanism that's underpinning this? There's actually a fair bit of consensus in the cosmological community about this. Ask a bunch of cosmologists, "What is dark energy?" You will get the same answer from all of them. Thank you.