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  • As a particle physicist, I study the elementary particles

  • and how they interact on the most fundamental level.

  • For most of my research career, I've been using accelerators,

  • such as the electron accelerator at Stanford University, just up the road,

  • to study things on the smallest scale.

  • But more recently, I've been turning my attention

  • to the universe on the largest scale.

  • Because, as I'll explain to you,

  • the questions on the smallest and the largest scale are actually very connected.

  • So I'm going to tell you about our twenty-first-century view of the universe,

  • what it's made of and what the big questions in the physical sciences are --

  • at least some of the big questions.

  • So, recently, we have realized

  • that the ordinary matter in the universe --

  • and by ordinary matter, I mean you, me,

  • the planets, the stars, the galaxies --

  • the ordinary matter makes up only a few percent

  • of the content of the universe.

  • Almost a quarter, or approximately a quarter

  • of the matter in the universe, is stuff that's invisible.

  • By invisible, I mean it doesn't absorb in the electromagnetic spectrum.

  • It doesn't emit in the electromagnetic spectrum. It doesn't reflect.

  • It doesn't interact with the electromagnetic spectrum,

  • which is what we use to detect things.

  • It doesn't interact at all. So how do we know it's there?

  • We know it's there by its gravitational effects.

  • In fact, this dark matter dominates

  • the gravitational effects in the universe on a large scale,

  • and I'll be telling you about the evidence for that.

  • What about the rest of the pie?

  • The rest of the pie is a very mysterious substance called dark energy.

  • More about that later, OK.

  • So for now, let's turn to the evidence for dark matter.

  • In these galaxies, especially in a spiral galaxy like this,

  • most of the mass of the stars is concentrated in the middle of the galaxy.

  • This huge mass of all these stars keeps stars in circular orbits in the galaxy.

  • So we have these stars going around in circles like this.

  • As you can imagine, even if you know physics, this should be intuitive, OK --

  • that stars that are closer to the mass in the middle will be rotating at a higher speed

  • than those that are further out here, OK.

  • So what you would expect is that if you measured the orbital speed of the stars,

  • that they should be slower on the edges than on the inside.

  • In other words, if we measured speed as a function of distance --

  • this is the only time I'm going to show a graph, OK --

  • we would expect that it goes down as the distance increases

  • from the center of the galaxy.

  • When those measurements are made,

  • instead what we find is that the speed is basically constant,

  • as a function of distance.

  • If it's constant, that means that the stars out here

  • are feeling the gravitational effects of matter that we do not see.

  • In fact, this galaxy and every other galaxy

  • appears to be embedded in a cloud of this invisible dark matter.

  • And this cloud of matter is much more spherical than the galaxy themselves,

  • and it extends over a much wider range than the galaxy.

  • So we see the galaxy and fixate on that, but it's actually a cloud of dark matter

  • that's dominating the structure and the dynamics of this galaxy.

  • Galaxies themselves are not strewn randomly in space;

  • they tend to cluster.

  • And this is an example of a very, actually, famous cluster, the Coma cluster.

  • And there are thousands of galaxies in this cluster.

  • They're the white, fuzzy, elliptical things here.

  • So these galaxy clusters -- we take a snapshot now,

  • we take a snapshot in a decade, it'll look identical.

  • But these galaxies are actually moving at extremely high speeds.

  • They're moving around in this gravitational potential well of this cluster, OK.

  • So all of these galaxies are moving.

  • We can measure the speeds of these galaxies, their orbital velocities,

  • and figure out how much mass is in this cluster.

  • And again, what we find is that there is much more mass there

  • than can be accounted for by the galaxies that we see.

  • Or if we look in other parts of the electromagnetic spectrum,

  • we see that there's a lot of gas in this cluster, as well.

  • But that cannot account for the mass either.

  • In fact, there appears to be about ten times as much mass here

  • in the form of this invisible or dark matter

  • as there is in the ordinary matter, OK.

  • It would be nice if we could see this dark matter a little bit more directly.

  • I'm just putting this big, blue blob on there, OK,

  • to try to remind you that it's there.

  • Can we see it more visually? Yes, we can.

  • And so let me lead you through how we can do this.

  • So here's an observer:

  • it could be an eye; it could be a telescope.

  • And suppose there's a galaxy out here in the universe.

  • How do we see that galaxy?

  • A ray of light leaves the galaxy and travels through the universe

  • for perhaps billions of years

  • before it enters the telescope or your eye.

  • Now, how do we deduce where the galaxy is?

  • Well, we deduce it by the direction that the ray is traveling

  • as it enters our eye, right?

  • We say, the ray of light came this way;

  • the galaxy must be there, OK.

  • Now, suppose I put in the middle a cluster of galaxies --

  • and don't forget the dark matter, OK.

  • Now, if we consider a different ray of light, one going off like this,

  • we now need to take into account

  • what Einstein predicted when he developed general relativity.

  • And that was that the gravitational field, due to mass,

  • will deflect not only the trajectory of particles,

  • but will deflect light itself.

  • So this light ray will not continue in a straight line,

  • but would rather bend and could end up going into our eye.

  • Where will this observer see the galaxy?

  • You can respond. Up, right?

  • We extrapolate backwards and say the galaxy is up here.

  • Is there any other ray of light

  • that could make into the observer's eye from that galaxy?

  • Yes, great. I see people going down like this.

  • So a ray of light could go down, be bent

  • up into the observer's eye,

  • and the observer sees a ray of light here.

  • Now, take into account the fact that we live in

  • a three-dimensional universe, OK,

  • a three-dimensional space.

  • Are there any other rays of light that could make it into the eye?

  • Yes! The rays would lie on a -- I'd like to see -- yeah, on a cone.

  • So there's a whole ray of light -- rays of light on a cone --

  • that will all be bent by that cluster

  • and make it into the observer's eye.

  • If there is a cone of light coming into my eye, what do I see?

  • A circle, a ring. It's called an Einstein ring. Einstein predicted that, OK.

  • Now, it will only be a perfect ring if the source, the deflector

  • and the eyeball, in this case, are all in a perfectly straight line.

  • If they're slightly skewed, we'll see a different image.

  • Now, you can do an experiment tonight over the reception, OK,

  • to figure out what that image will look like.

  • Because it turns out that there is a kind of lens that we can devise,

  • that has the right shape to produce this kind of effect.

  • We call this gravitational lensing.

  • And so, this is your instrument, OK.

  • (Laughter).

  • But ignore the top part.

  • It's the base that I want you to concentrate, OK.

  • So, actually, at home, whenever we break a wineglass,

  • I save the bottom, take it over to the machine shop.

  • We shave it off, and I have a little gravitational lens, OK.

  • So it's got the right shape to produce the lensing.

  • And so the next thing you need to do in your experiment

  • is grab a napkin. I grabbed a piece of graph paper -- I'm a physicist. (Laughter)

  • So, a napkin. Draw a little model galaxy in the middle.

  • And now put the lens over the galaxy,

  • and what you'll find is that you'll see a ring, an Einstein ring.

  • Now, move the base off to the side,

  • and the ring will split up into arcs, OK.

  • And you can put it on top of any image.

  • On the graph paper, you can see

  • how all the lines on the graph paper have been distorted.

  • And again, this is a kind of an accurate model

  • of what happens with the gravitational lensing.

  • OK, so the question is: do we see this in the sky?

  • Do we see arcs in the sky when we look at, say, a cluster of galaxies?

  • And the answer is yes.

  • And so, here's an image from the Hubble Space Telescope.

  • Many of the images you are seeing

  • are earlier from the Hubble Space Telescope.

  • Well, first of all, for the golden shape galaxies --

  • those are the galaxies in the cluster.

  • They're the ones that are embedded in that sea of dark matter

  • that are causing the bending of the light

  • to cause these optical illusions, or mirages, practically,

  • of the background galaxies.

  • So the streaks that you see, all these streaks,

  • are actually distorted images of galaxies that are much further away.

  • So what we can do, then, is based on how much distortion

  • we see in those images, we can calculate how much mass

  • there must be in this cluster.

  • And it's an enormous amount of mass.

  • And also, you can tell by eye, by looking at this,

  • that these arcs are not centered on individual galaxies.

  • They are centered on some more spread out structure,

  • and that is the dark matter

  • in which the cluster is embedded, OK.

  • So this is the closest you can get to kind of seeing

  • at least the effects of the dark matter with your naked eye.

  • OK, so, a quick review then, to see that you're following.

  • So the evidence that we have

  • that a quarter of the universe is dark matter --

  • this gravitationally attracting stuff --

  • is that galaxies, the speed with which stars orbiting galaxies

  • is much too large; it must be embedded in dark matter.

  • The speed with which galaxies within clusters are orbiting is much too large;

  • it must be embedded in dark matter.

  • And we see these gravitational lensing effects, these distortions

  • that say that, again, clusters are embedded in dark matter.

  • OK. So now, let's turn to dark energy.

  • So to understand the evidence for dark energy, we need to discuss something

  • that Stephen Hawking referred to in the previous session.

  • And that is the fact that space itself is expanding.

  • So if we imagine a section of our infinite universe --

  • and so I've put down four spiral galaxies, OK --

  • and imagine that you put down a set of tape measures,

  • so every line on here corresponds to a tape measure,

  • horizontal or vertical, for measuring where things are.

  • If you could do this, what you would find

  • that with each passing day, each passing year,

  • each passing billions of years, OK,

  • the distance between galaxies is getting greater.

  • And it's not because galaxies are moving

  • away from each other through space.

  • They're not necessarily moving through space.

  • They're moving away from each other

  • because space itself is getting bigger, OK.

  • That's what the expansion of the universe or space means.

  • So they're moving further apart.

  • Now, what Stephen Hawking mentioned, as well,

  • is that after the Big Bang, space expanded at a very rapid rate.

  • But because gravitationally attracting matter

  • is embedded in this space,

  • it tends to slow down the expansion of the space, OK.

  • So the expansion slows down with time.

  • So, in the last century, OK, people debated

  • about whether this expansion of space would continue forever;

  • whether it would slow down, you know,

  • will be slowing down, but continue forever;

  • slow down and stop, asymptotically stop;

  • or slow down, stop, and then reverse, so it starts to contract again.

  • So a little over a decade ago,

  • two groups of physicists and astronomers

  • set out to measure the rate at which

  • the expansion of space was slowing down, OK.

  • By how much less is it expanding today,

  • compared to, say, a couple of billion years ago?

  • The startling answer to this question, OK, from these experiments,

  • was that space is expanding at a faster rate today

  • than it was a few billion years ago, OK.

  • So the expansion of space is actually speeding up.

  • This was a completely surprising result.

  • There is no persuasive theoretical argument for why this should happen, OK.

  • No one was predicting ahead of time this is what's going to be found.

  • It was the opposite of what was expected.

  • So we need something to be able to explain that.

  • Now it turns out, in the mathematics,

  • you can put it in as a term that's an energy,

  • but it's a completely different type of energy

  • from anything we've ever seen before.

  • We call it dark energy,

  • and it has this effect of causing space to expand.

  • But we don't have a good motivation

  • for putting it in there at this point, OK.

  • So it's really unexplained as to why we need to put it in.

  • Now, so at this point, then, what I want to really emphasize to you,

  • is that, first of all, dark matter and dark energy

  • are completely different things, OK.

  • There are really two mysteries out there as to what makes