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  • 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.

MEGHAN: Today, we're going to talk about dark matter.

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暗物質--60個符號 (Dark Matter - Sixty Symbols)

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    林宜悉 發佈於 2021 年 01 月 14 日
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