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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)