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  • Do you ever think about what would happen

  • if the world were a little bit different?

  • How your life would be different

  • if you were born 5,000 years from now

  • instead of today?

  • How history would be different

  • if the continents were at different latitudes

  • or how life in the Solar system would have developed

  • if the Sun were 10 percent larger.

  • Well, playing with these kinds of possibilities

  • is what I get to do for a living

  • but with the entire universe.

  • I make model universes in a computer.

  • Digital universes that have different starting points

  • and are made of different amounts of different kinds of material.

  • And then I compare these universes to our own

  • to see what it is made of and how it evolved.

  • This process of testing models with measurements of the sky

  • has taught us a huge amount about our universe so far.

  • One of the strangest things we have learned

  • is that most of the material in the universe

  • is made of something entirely different than you and me.

  • But without it,

  • the universe as we know it wouldn't exist.

  • Everything we can see with telescopes

  • makes up just about 15 percent of the total mass in the universe.

  • Everything else, 85 percent of it,

  • doesn't emit or absorb light.

  • We can't see it with our eyes,

  • we can't detect it with radio waves

  • or microwaves or any other kind of light.

  • But we know it is there

  • because of its influence on what we can see.

  • It's a little bit like,

  • if you wanted to map the surface of our planet

  • and everything on it

  • using this picture of the Earth from space at night.

  • You get some clues from where the light is,

  • but there's a lot that you can't see,

  • everything from people to mountain ranges.

  • And you have to infer what is there from these limited clues.

  • We call this unseen stuff "dark matter."

  • Now, a lot of people have heard of dark matter,

  • but even if you have heard of it,

  • it probably seems abstract,

  • far away, probably even irrelevant.

  • Well, the interesting thing is,

  • dark matter is all around us

  • and probably right here.

  • In fact, dark matter particles

  • are probably going through your body right now

  • as you sit in this room.

  • Because we are on Earth

  • and Earth is spinning around the Sun,

  • and the Sun is hurtling through our galaxy

  • at about half a million miles per hour.

  • But dark matter doesn't bump into us,

  • it just goes right through us.

  • So how do we figure out more about this?

  • What is it,

  • and what does it have to do with our existence?

  • Well, in order to figure out how we came to be,

  • we first need to understand how our galaxy came to be.

  • This is a picture of our galaxy, the Milky Way, today.

  • What did it look like 10 billion years in the past

  • or what would it look like 10 billion years in the future?

  • What about the stories

  • of the hundreds of millions of other galaxies

  • that we've already mapped out with large surveys of the sky?

  • How would their histories be different

  • if the universe was made of something else

  • or if there was more or less matter in it?

  • So the interesting thing about these model universes

  • is that they allow us to test these possibilities.

  • Let's go back to the first moment of the universe --

  • just a fraction of a second after the big bang.

  • In this first moment,

  • there was no matter at all.

  • The universe was expanding very fast.

  • And quantum mechanics tells us

  • that matter is being created and destroyed

  • all the time, in every moment.

  • At this time, the universe was expanding so fast

  • that the matter that got created couldn't get destroyed.

  • And thus we think that all of the matter was created during this time.

  • Both the dark matter

  • and the regular matter that makes up you and me.

  • Now, let's go a little bit further

  • to a time after the matter was created,

  • after protons and neutrons formed,

  • after hydrogen formed,

  • about 400,000 years after the big bang.

  • The universe was hot and dense and really smooth

  • but not perfectly smooth.

  • This image, taken with a space telescope called the Planck satellite,

  • shows us the temperature of the universe

  • in all directions.

  • And what we see

  • is that there were places that were a little bit hotter

  • and denser than others.

  • The spots in this image

  • represent places where there was more or less mass in the early universe.

  • Those spots got big because of gravity.

  • The universe was expanding and getting less dense overall

  • over the last 13.8 billion years.

  • But gravity worked hard in those spots

  • where there was a little bit more mass

  • and pulled more and more mass into those regions.

  • Now, all of this is a little hard to imagine,

  • so let me just show you what I am talking about.

  • Those computer models I mentioned allow us to test these ideas,

  • so let's take a look at one of them.

  • This movie, made by my research group,

  • shows us what happened to the universe after its earliest moments.

  • You see the universe started out pretty smooth,

  • but there were some regions

  • where there was a little bit more material.

  • Gravity turned on and brought more and more mass

  • into those spots that started out with a little bit extra.

  • Over time,

  • you get enough stuff in one place

  • that the hydrogen gas,

  • which was initially well mixed with the dark matter,

  • starts to separate from it,

  • cool down, form stars,

  • and you get a small galaxy.

  • Over time, over billions and billions of years,

  • those small galaxies crash into each other

  • and merge and grow to become larger galaxies,

  • like our own galaxy, the Milky Way.

  • Now, what happens if you don't have dark matter?

  • If you don't have dark matter,

  • those spots never get clumpy enough.

  • It turns out, you need at least a million times the mass of the Sun

  • in one dense region,

  • before you can start forming stars.

  • And without dark matter,

  • you never get enough stuff in one place.

  • So here, we're looking at two universes, side by side.

  • In one of them you can see

  • that things get clumpy quickly.

  • In that universe,

  • it's really easy to form galaxies.

  • In the other universe,

  • the things that start out like small clumps,

  • they just stay really small.

  • Not very much happens.

  • In that universe, you wouldn't get our galaxy.

  • Or any other galaxy.

  • You wouldn't get the Milky Way,

  • you wouldn't get the Sun,

  • you wouldn't get us.

  • We just couldn't exist in that universe.

  • OK, so this crazy stuff, dark matter,

  • it's most of the mass in the universe,

  • it's going through us right now, we wouldn't be here without it.

  • What is it?

  • Well, we have no idea.

  • But we have a lot of educated guesses,

  • and a lot of ideas for how to find out more.

  • So, most physicists think that dark matter is a particle,

  • similar in many ways to the subatomic particles that we know of,

  • like protons and neutrons and electrons.

  • Whatever it is,

  • it behaves very similarly with respect to gravity.

  • But it doesn't emit or absorb light,

  • and it goes right through normal matter,

  • as if it wasn't even there.

  • We'd like to know what particle it is.

  • For example, how heavy is it?

  • Or, does anything at all happen if it interacts with normal matter?

  • Physicists have lots of great ideas for what it could be,

  • they're very creative.

  • But it's really hard,

  • because those ideas span a huge range.

  • It could be as small as the smallest subatomic particles,

  • or it could be as large as the mass of 100 Suns.

  • So, how do we figure out what it is?

  • Well, physicists and astronomers

  • have a lot of ways to look for dark matter.

  • One of the things we're doing is building sensitive detectors

  • in deep underground mines,

  • waiting for the possibility

  • that a dark matter particle, which goes through us and the Earth,

  • would hit a denser material

  • and leave behind some trace of its passage.

  • We're looking for dark matter in the sky,

  • for the possibility that dark matter particles

  • would crash into each other

  • and create high-energy light that we could see

  • with special gamma-ray telescopes.

  • We're even trying to make dark matter here on Earth,

  • by smashing particles together and looking for what happens,

  • using the Large Hadron Collider in Switzerland.

  • Now, so far,

  • all of these experiments have taught us a lot

  • about what dark matter isn't

  • but not yet what it is.

  • There were really good ideas that dark matter could have been,

  • that these experiments would have seen.

  • And they didn't see them yet,

  • so we have to keep looking and thinking harder.

  • Now, another way to get a clue to what dark matter is

  • is to study galaxies.

  • We already talked about

  • how our galaxy and many other galaxies wouldn't even be here

  • without dark matter.

  • Those models also make predictions

  • for many other things about galaxies:

  • How they're distributed in the universe,

  • how they move,

  • how they evolve over time.

  • And we can test those predictions with observations of the sky.

  • So let me just give you two examples

  • of these kinds of measurements we can make with galaxies.

  • The first is that we can make maps of the universe with galaxies.

  • I am part of a survey called the Dark Energy Survey,

  • which has made the largest map of the universe so far.

  • We measured the positions and shapes of 100 million galaxies

  • over one-eighth of the sky.

  • And this map is showing us all the matter in this region of the sky,

  • which is inferred by the light distorted from these 100 million galaxies.

  • The light distorted from all of the matter

  • that was between those galaxies and us.

  • The gravity of the matter is strong enough to bend the path of light.

  • And it gives us this image.

  • So these kinds of maps

  • can tell us about how much dark matter there is,

  • they also tell us where it is

  • and how it changes over time.

  • So we're trying to learn about what the universe is made of

  • on the very largest scales.

  • It turns out that the tiniest galaxies in the universe

  • provide some of the best clues.

  • So why is that?

  • Here are two example simulated universes

  • with two different kinds of dark matter.

  • Both of these pictures are showing you a region

  • around a galaxy like the Milky Way.

  • And you can see that there's a lot of other material around it,

  • little small clumps.

  • Now, in the image on the right,

  • dark matter particles are moving slower than they are in the one on the left.

  • If those dark matter particles are moving really fast,

  • then the gravity in small clumps is not strong enough

  • to slow those fast particles down.

  • And they keep going.

  • They never collapse into these small clumps.