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Here are some images of clusters of galaxies.
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They're exactly what they sound like.
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They are these huge collections of galaxies,
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bound together by their mutual gravity.
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So most of the points that you see on the screen
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are not individual stars,
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but collections of stars, or galaxies.
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Now, by showing you some of these images,
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I hope that you will quickly see that
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galaxy clusters are these beautiful objects,
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but more than that,
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I think galaxy clusters are mysterious,
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they are surprising,
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and they're useful.
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Useful as the universe's most massive laboratories.
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And as laboratories, to describe galaxy clusters
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is to describe the experiments
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that you can do with them.
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And I think there are four major types,
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and the first type that I want to describe
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is probing the very big.
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So, how big?
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Well, here is an image of a particular galaxy cluster.
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It is so massive that the light passing through it
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is being bent, it's being distorted
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by the extreme gravity of this cluster.
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And, in fact, if you look very carefully
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you'll be able to see rings around this cluster.
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Now, to give you a number,
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this particular galaxy cluster
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has a mass of over one million billion suns.
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It's just mind-boggling how
massive these systems can get.
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But more than their mass,
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they have this additional feature.
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They are essentially isolated systems,
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so if we like, we can think of them
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as a scaled-down version of the entire universe.
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And many of the questions that we might have
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about the universe at large scales,
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such as, how does gravity work?
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might be answered by studying these systems.
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So that was very big.
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The second things is very hot.
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Okay, if I take an image of a galaxy cluster,
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and I subtract away all of the starlight,
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what I'm left with is this big, blue blob.
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This is in false color.
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It's actually X-ray light that we're seeing.
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And the question is, if it's not galaxies,
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what is emitting this light?
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The answer is hot gas,
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million-degree gas --
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in fact, it's plasma.
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And the reason why it's so hot
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goes back to the previous slide.
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The extreme gravity of these systems
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is accelerating particles of gas to great speeds,
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and great speeds means great temperatures.
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So this is the main idea,
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but science is a rough draft.
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There are many basic properties about this plasma
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that still confuse us,
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still puzzle us,
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and still push our understanding
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of the physics of the very hot.
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Third thing: probing the very small.
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Now, to explain this, I need to tell you
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a very disturbing fact.
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Most of the universe's matter
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is not made up of atoms.
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You were lied to.
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Most of it is made up of something
very, very mysterious,
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which we call dark matter.
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Dark matter is something that
doesn't like to interact very much,
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except through gravity,
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and of course we would like to learn more about it.
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If you're a particle physicist,
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you want to know what happens
when we smash things together.
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And dark matter is no exception.
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Well, how do we do this?
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To answer that question,
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I'm going to have to ask another one,
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which is, what happens when galaxy clusters collide?
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Here is an image.
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Since galaxy clusters are representative
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slices of the universe, scaled-down versions.
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They are mostly made up of dark matter,
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and that's what you see in this bluish purple.
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The red represents the hot gas,
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and, of course, you can see many galaxies.
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What's happened is a particle accelerator
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at a huge, huge scale.
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And this is very important,
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because what it means is that very, very small
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effects that might be difficult to detect in the lab,
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might be compounded and compounded
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into something that we could
possibly observe in nature.
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So, it's very funny.
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The reason why galaxy clusters
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can teach us about dark matter,
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the reason why galaxy clusters
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can teach us about the physics of the very small,
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is precisely because they are so very big.
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Fourth thing: the physics of the very strange.
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Certainly what I've said so far is crazy.
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Okay, if there's anything stranger
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I think it has to be dark energy.
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If I throw a ball into the air,
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I expect it to go up.
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What I don't expect is that it go up
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at an ever-increasing rate.
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Similarly, cosmologists understand why
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the universe is expanding.
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They don't understand why it's expanding
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at an ever-increasing rate.
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They give the cause of this
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accelerated expansion a name,
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and they call it dark energy.
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And, again, we want to learn more about it.
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So, one particular question that we have is,
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how does dark energy affect the universe
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at the largest scales?
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Depending on how strong it is,
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maybe structure forms faster or slower.
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Well, the problem with the large-scale structure
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of the universe is that it's horribly complicated.
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Here is a computer simulation.
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And we need a way to simplify it.
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Well, I like to think about this using an analogy.
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If I want to understand the sinking of the Titanic,
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the most important thing to do
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is not to model the little positions
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of every single little piece of the boat that broke off.
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The most important thing to do is
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to track the two biggest parts.
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Similarly, I can learn a lot about the universe
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at the largest scales
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by tracking its biggest pieces
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and those biggest pieces are clusters of galaxies.
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So, as I come to a close,
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you might feel slightly cheated.
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I mean, I began by talking about
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how galaxy clusters are useful,
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and I've given some reasons,
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but what is their use really?
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Well, to answer this,
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I want to give you a quote by Henry Ford
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when he was asked about cars.
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He had this to say:
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"If I had asked people what they wanted,
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they would have said faster horses."
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Today, we as a society are faced
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with many, many difficult problems.
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And the solutions to these
problems are not obvious.
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They are not faster horses.
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They will require an enormous amount of
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scientific ingenuity.
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So, yes, we need to focus,
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yes, we need to concentrate,
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but we also need to remember that
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innovation, ingenuity, inspiration --
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these things come
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when we broaden our field of vision
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when we step back
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when we zoom out.
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And I can't think of a better way to do this than
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by studying the universe around us. Thanks.
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(Applause)