字幕列表 影片播放 列印英文字幕 MEGHAN: Today, we're going to talk about dark matter. PROF. COPELAND: Where is it? Haha, it's dark. You can't see it. MEGHAN: We know there's a lot of it out there, but it's quite mysterious. PROF. MERRIFIELD: Astronomers kind of rely on light in order to figure out what's going on in the universe, and things in the universe which inconveniently don't give out any light, causes some problems because it's very hard for us to actually infer very much about it apart from the fact that the material is there. MEGHAN: If I can introduce my prop here... PROF. COPELAND: About 95% of the total energy density in the universe is made of stuff that we don't know. MEGHAN: Well, it's dark obviously. It's appropriate. It's chocolate and it smells really good. If you imagine that this pie represents the entire matter and energy budget in the universe, and so the biggest slice of this pie is something mysterious - something really mysterious, called Dark Energy. Nearly three-quarters of our pie so that... Let's say, about that much. So all of this bit here is this mysterious stuff called dark energy, but we don't nearly have time to talk about that today So I'm going to leave this whole piece of the pie for the moment, okay? We'll just stick it over here So what's left is the stuff that has mass. This is all the matter in the universe. Now I'm going to divide this again. So this little slice of the pie here, which is supposed to represent about 4% of the total, this is all of the normal matter in the universe. This is all of the periodic table, so it's all chemistry, all of biology... It's everything that we can see, it's all of the stuff that we're made of, that the earth is made of, it's all of the stars and galaxies and the gas and dust out there in the universe. Most of it is actually hydrogen, so an even tinier sliver is the stuff that makes, you know, our universe interesting. And what's left, about... just about 23% or so of the universe is this dark matter - this mysterious stuff. We call it dark because it neither emits nor absorbs electromagnetic radiation so it doesn't shine and neither does it cast a shadow. There's almost certainly dark matter streaming through this room right now, but we have no way of knowing that it's there And we call it matter because it has mass. And that's very important because although it doesn't interact through the rest of the normal forces it does interact with itself, and with normal matter and light, through gravity because it has mass. And that's the only way that we can figure out that it's out there in such large quantities. *nom* Haha, a little less of it now. PROF. MERRIFIELD: And so we can tell it's there because we can see its gravitational influence on things we can actually see that the this dark matter was actually, you know, gravitating and pulling other stuff towards it. MEGHAN: And the first observation of that kind was made by an astronomer named Fritz Zwicky back in the 30s. PROF. COPELAND: ...who was looking at the rotation of galaxies. You know, they go around, and he was looking at the speed of the rotation as you move away from the center of the galaxy, so he'd pick some object that was emitting light and he'd look at how rapidly it was going around. MEGHAN: Think about the solar system for a moment. In our solar system, most of the mass is right in the middle - it's made up of the sun. So the planets close to the sun, they feel a strong force of gravity. So Mercury for example is zipping around the sun while Neptune, further away, not feeling such a strong force of gravity, is just sort of pootling along very slowly. You'd expect something of the same to be happening in galaxies, because if you look at a galaxy - if you look at a spiral galaxy, it looks like it's got this big concentration of stars in the middle and this disk that extends out even further. PROF. COPELAND: So you expect the speed to sort of rise up to a maximum and then drop off again. That's what Newton would have told you, given what you could see. And what he noticed was that this... this speed went up to a maximum, and then stayed there. MEGHAN: They were going just as fast on the outside as they were in the inside. And what this meant was, again: What you see is not what you get. It's not the whole story, there must be some other component. Part of this galaxy, providing enough mass to keep these stars moving. As an astronomer, I don't know what the dark matter is. But what I can tell you is how much of it is out there, and what kind of structures it forms. And so, again it goes back to this key idea that, whatever these particles are, they interact gravitationally. BRADY: A bit of a misnomer calling it Dark Matter though, it seems like it's almost transparent. PROF. COPELAND: Yeah, I hadn't thought about that until I said the word transparent today. It's dark only in that light doesn't seem to interact with it. And so we're inferring that there's something there that we can't see. The way you actually do perceive it is, light will go past it and as it goes past it, or through it, it will get bent. MEGHAN: In a sense, imagine yourself looking through a window. On a normal day, you just see right through the window you don't even notice it's there. On a rainy day, there might be raindrops on the window, and that kind of distorts your view, and that's exactly what dark matter is doing. It's distorting our view of the distant universe. Using data from the Hubble space telescope, the distortions in this case are so small you can't actually see them, but by adding up the shapes that we observe of tens of thousands of tiny little galaxies we can measure this and reconstruct. And what my colleague Catherine Heymans has done to make this beautiful map is to use those distortions to figure out how much dark matter is in this particular part of the universe, and where it is, and we've color-coded it pink here. So you can see these big pink blobs of dark matter making up what is actually a super cluster of galaxies And what's interesting is, if we look closely - and we've overlaid the actual pictures of galaxies here themselves, you see the galaxies are embedded in these blobs of dark matter. Dark Matter is of course invisible, so for the purposes of this picture, we've chosen just to color it pink so you can see it. BRADY: Why can't we find this stuff? If it's everywhere; if it's in this room; if it makes up such a big piece of the pie, why can't we find it? MEGHAN: Well people are looking for it, and this is again a very interesting connection where people like myself who study the universe on very very large scales interact with people who are studying it on the small scales: The particle physicists. PROF. MERRIFIELD: As with most things in astronomy, as soon as you come up with some observation there's a whole bunch of theoretical astrophysicists who say "I have an explanation for that!" And so there are a whole bunch of possible explanations out there for dark matter. The particle physicists for example would very much like it to be some form of exotic particle, so one of these sort of supersymmetric particles that comes out of their theories. MEGHAN: And so there are actual experiments that are trying to basically catch the dark matter particle in action, as it flies by. They're really, really, really difficult. Because as I said, the best candidate that we... that the particle theory people have for dark matter, is something called a weakly interacting massive particle. So it's not a normal type of atom. A good candidate for the dark matter particle would be the lightest supersymmetric particle, the Neutralino. So this is something that's been thought of in theory but hasn't been observed. And so these dark matter experiments, they usually take place deep underground because they have to be shielded from all sorts of radiation, from neutrons, that sort of thing. BRADY: What would happen if I drove my car into a big concentrated clump of dark matter? PROF: COPELAND: Into a big concentrated clump of dark matter... That would be... you'd... That's a good question, that I've not thought of the answer to. PROF: MERRIFIELD: There are a whole bunch of experiments around the world, trying to detect this. For example if it is these weakly interacting massive particles, then they're everywhere. They're very small, each one, but there's millions and millions of them, and they're, you know, they're passing through this room at this moment. And they're very hard to detect because they don't really interact very much with normal matter, but once in a while they do, and so there's a whole bunch of experiments sort of scattered around the world, trying to detect these things and when one of them... IF and when one of them gets a detection, then we'll have an answer tomorrow as to as to what the dark matter is - or at least we'll have detected it. BRADY: Does it frustrate you, as someone who dreams of dark matter being discovered, to know that it's here in this room? PROF. COPELAND: Yeah, it's amazing 'cause it is everywhere, and then... it's just so elusive. It won't interact it all - barely, it just passes straight through you. And indeed, in order to try and find it, you have to go to areas where you increase the chance of it interacting with something, and so that typically is to go deep underground so that you can hide it from all the other type of signals, from particles that might mimic dark matter. MEGHAN: Because we think that dark matter makes up such a huge piece of the pie in our universe it means that we can actually run very very high... very very large computer simulations to predict what a dark matter universe would be. You have to figure out what the force of gravity is between that particle and every other particle and how that's going to make it move. And then you do it again and again and again So yeah, It does take a large amount of computing power. So this is... this is a simulation of our universe. So what you're seeing is a slice - a chunk of the universe at very early times, when the distribution of matter was very very smooth. And let's fast-forward through the history of universe and see what happens. We have what's called a hierarchical universe being built up. Small bits of dark matter merged together to form larger halos. That means the force of gravity is greater there and that acts as a well. And this... Dark matter tends to drain down along these filamentary structures and collect in these increasingly larger and larger halos. You can see we've got this beautiful, detailed picture of a universe that's invisible to us. BRADY: What will happen to the man or woman who discovers dark matter? PROF. COPELAND: They'll be going to Stockholm, I think, probably. MEGHAN: We're coming across the road from the astronomy building, to the Cripps Computing Centre. So what we're going to see is a supercomputer. And basically some of my colleagues across the road use this computer to simulate whole universes full of Dark Matter. I'll have to put earplugs in for health and safety reasons, because we're told that this is going to be very very loud [very very loud ventilation system noises] So it's really loud in here and it's really cold, because they have to keep pumping cold air through. And I've just learned that if the air conditioning fails, it would get so hot so quickly, with all these machines running that things would start to basically break down within an hour. I should say that this facility isn't just used for astronomy. It's also used across the University by other disciplines to do simulations on protein folding in cells and all sorts of other applications that require a large amount of computing power.