WEBVTT

00:00:00.958 --> 00:00:02.911
Do you ever think about what would happen

00:00:02.935 --> 00:00:05.518
if the world were a little bit different?

00:00:05.542 --> 00:00:07.101
How your life would be different

00:00:07.125 --> 00:00:09.768
if you were born 5,000 years from now

00:00:09.792 --> 00:00:11.083
instead of today?

00:00:11.958 --> 00:00:13.476
How history would be different

00:00:13.500 --> 00:00:15.643
if the continents
were at different latitudes

00:00:15.667 --> 00:00:19.601
or how life in the Solar system
would have developed

00:00:19.625 --> 00:00:22.167
if the Sun were 10 percent larger.

NOTE Paragraph

00:00:23.292 --> 00:00:26.184
Well, playing with these
kinds of possibilities

00:00:26.208 --> 00:00:27.809
is what I get to do for a living

00:00:27.833 --> 00:00:30.000
but with the entire universe.

00:00:31.042 --> 00:00:33.851
I make model universes in a computer.

00:00:33.875 --> 00:00:36.809
Digital universes that have
different starting points

00:00:36.833 --> 00:00:41.434
and are made of different amounts
of different kinds of material.

00:00:41.458 --> 00:00:44.768
And then I compare
these universes to our own

00:00:44.792 --> 00:00:48.500
to see what it is made of
and how it evolved.

NOTE Paragraph

00:00:50.542 --> 00:00:54.893
This process of testing models
with measurements of the sky

00:00:54.917 --> 00:00:57.792
has taught us a huge amount
about our universe so far.

00:00:58.625 --> 00:01:00.851
One of the strangest
things we have learned

00:01:00.875 --> 00:01:03.434
is that most of the material
in the universe

00:01:03.458 --> 00:01:07.375
is made of something
entirely different than you and me.

00:01:08.750 --> 00:01:11.476
But without it,

00:01:11.500 --> 00:01:15.000
the universe as we know it wouldn't exist.

00:01:16.958 --> 00:01:20.559
Everything we can see with telescopes

00:01:20.583 --> 00:01:24.958
makes up just about 15 percent
of the total mass in the universe.

00:01:26.000 --> 00:01:30.393
Everything else, 85 percent of it,

00:01:30.417 --> 00:01:32.750
doesn't emit or absorb light.

00:01:33.708 --> 00:01:36.059
We can't see it with our eyes,

00:01:36.083 --> 00:01:39.601
we can't detect it with radio waves

00:01:39.625 --> 00:01:42.042
or microwaves or any other kind of light.

00:01:43.083 --> 00:01:45.018
But we know it is there

00:01:45.042 --> 00:01:47.417
because of its influence
on what we can see.

NOTE Paragraph

00:01:49.208 --> 00:01:50.476
It's a little bit like,

00:01:50.500 --> 00:01:52.684
if you wanted to map
the surface of our planet

00:01:52.708 --> 00:01:54.684
and everything on it

00:01:54.708 --> 00:01:58.375
using this picture of the Earth
from space at night.

00:01:59.708 --> 00:02:02.184
You get some clues
from where the light is,

00:02:02.208 --> 00:02:04.018
but there's a lot that you can't see,

00:02:04.042 --> 00:02:07.684
everything from people
to mountain ranges.

00:02:07.708 --> 00:02:12.167
And you have to infer what is there
from these limited clues.

00:02:13.958 --> 00:02:16.875
We call this unseen stuff "dark matter."

NOTE Paragraph

00:02:17.708 --> 00:02:20.601
Now, a lot of people
have heard of dark matter,

00:02:20.625 --> 00:02:22.934
but even if you have heard of it,

00:02:22.958 --> 00:02:25.976
it probably seems abstract,

00:02:26.000 --> 00:02:28.958
far away, probably even irrelevant.

00:02:30.625 --> 00:02:33.434
Well, the interesting thing is,

00:02:33.458 --> 00:02:35.976
dark matter is all around us

00:02:36.000 --> 00:02:38.934
and probably right here.

00:02:38.958 --> 00:02:40.893
In fact, dark matter particles

00:02:40.917 --> 00:02:44.059
are probably going through
your body right now

00:02:44.083 --> 00:02:46.059
as you sit in this room.

00:02:46.083 --> 00:02:47.434
Because we are on Earth

00:02:47.458 --> 00:02:49.309
and Earth is spinning around the Sun,

00:02:49.333 --> 00:02:52.268
and the Sun is hurtling through our galaxy

00:02:52.292 --> 00:02:55.518
at about half a million miles per hour.

00:02:55.542 --> 00:02:57.351
But dark matter doesn't bump into us,

00:02:57.375 --> 00:02:58.875
it just goes right through us.

NOTE Paragraph

00:03:00.583 --> 00:03:05.018
So how do we figure out more about this?

00:03:05.042 --> 00:03:06.309
What is it,

00:03:06.333 --> 00:03:08.768
and what does it have to do
with our existence?

00:03:08.792 --> 00:03:13.559
Well, in order to figure out
how we came to be,

00:03:13.583 --> 00:03:17.559
we first need to understand
how our galaxy came to be.

00:03:17.583 --> 00:03:22.059
This is a picture of our galaxy,
the Milky Way, today.

00:03:22.083 --> 00:03:24.434
What did it look like
10 billion years in the past

00:03:24.458 --> 00:03:27.875
or what would it look like
10 billion years in the future?

00:03:28.917 --> 00:03:30.351
What about the stories

00:03:30.375 --> 00:03:33.018
of the hundreds of millions
of other galaxies

00:03:33.042 --> 00:03:37.184
that we've already mapped out
with large surveys of the sky?

00:03:37.208 --> 00:03:39.434
How would their histories be different

00:03:39.458 --> 00:03:42.351
if the universe was made of something else

00:03:42.375 --> 00:03:45.476
or if there was more or less matter in it?

00:03:45.500 --> 00:03:49.351
So the interesting thing
about these model universes

00:03:49.375 --> 00:03:52.167
is that they allow us
to test these possibilities.

NOTE Paragraph

00:03:53.500 --> 00:03:58.667
Let's go back to the first
moment of the universe --

00:03:59.542 --> 00:04:04.333
just a fraction of a second
after the big bang.

00:04:05.875 --> 00:04:07.809
In this first moment,

00:04:07.833 --> 00:04:09.934
there was no matter at all.

00:04:09.958 --> 00:04:13.434
The universe was expanding very fast.

00:04:13.458 --> 00:04:16.226
And quantum mechanics tells us

00:04:16.250 --> 00:04:18.976
that matter is being created and destroyed

00:04:19.000 --> 00:04:21.000
all the time, in every moment.

00:04:22.357 --> 00:04:24.643
At this time, the universe
was expanding so fast

00:04:24.667 --> 00:04:27.917
that the matter that got created
couldn't get destroyed.

00:04:28.833 --> 00:04:33.643
And thus we think that all of the matter
was created during this time.

00:04:33.667 --> 00:04:35.851
Both the dark matter

00:04:35.875 --> 00:04:38.958
and the regular matter
that makes up you and me.

NOTE Paragraph

00:04:40.167 --> 00:04:42.601
Now, let's go a little bit further

00:04:42.625 --> 00:04:45.101
to a time after the matter was created,

00:04:45.125 --> 00:04:47.434
after protons and neutrons formed,

00:04:47.458 --> 00:04:49.184
after hydrogen formed,

00:04:49.208 --> 00:04:53.226
about 400,000 years after the big bang.

00:04:53.250 --> 00:04:57.518
The universe was hot and dense
and really smooth

00:04:57.542 --> 00:05:00.268
but not perfectly smooth.

00:05:00.292 --> 00:05:05.059
This image, taken with a space telescope
called the Planck satellite,

00:05:05.083 --> 00:05:07.559
shows us the temperature of the universe

00:05:07.583 --> 00:05:08.958
in all directions.

00:05:10.000 --> 00:05:11.393
And what we see

00:05:11.417 --> 00:05:14.434
is that there were places
that were a little bit hotter

00:05:14.458 --> 00:05:16.851
and denser than others.

00:05:16.875 --> 00:05:18.559
The spots in this image

00:05:18.583 --> 00:05:23.500
represent places where there was
more or less mass in the early universe.

NOTE Paragraph

00:05:25.125 --> 00:05:28.458
Those spots got big because of gravity.

00:05:29.500 --> 00:05:33.934
The universe was expanding
and getting less dense overall

00:05:33.958 --> 00:05:37.643
over the last 13.8 billion years.

00:05:37.667 --> 00:05:40.518
But gravity worked hard in those spots

00:05:40.542 --> 00:05:42.684
where there was a little bit more mass

00:05:42.708 --> 00:05:47.018
and pulled more and more mass
into those regions.

NOTE Paragraph

00:05:47.042 --> 00:05:49.809
Now, all of this
is a little hard to imagine,

00:05:49.833 --> 00:05:53.184
so let me just show you
what I am talking about.

00:05:53.208 --> 00:05:56.768
Those computer models I mentioned
allow us to test these ideas,

00:05:56.792 --> 00:05:58.583
so let's take a look at one of them.

00:05:59.625 --> 00:06:02.559
This movie, made by my research group,

00:06:02.583 --> 00:06:06.792
shows us what happened to the universe
after its earliest moments.

00:06:08.375 --> 00:06:11.268
You see the universe
started out pretty smooth,

00:06:11.292 --> 00:06:12.726
but there were some regions

00:06:12.750 --> 00:06:15.809
where there was
a little bit more material.

00:06:15.833 --> 00:06:19.059
Gravity turned on
and brought more and more mass

00:06:19.083 --> 00:06:23.125
into those spots that started out
with a little bit extra.

00:06:24.167 --> 00:06:25.601
Over time,

00:06:25.625 --> 00:06:28.476
you get enough stuff in one place

00:06:28.500 --> 00:06:29.768
that the hydrogen gas,

00:06:29.792 --> 00:06:32.809
which was initially well mixed
with the dark matter,

00:06:32.833 --> 00:06:34.643
starts to separate from it,

00:06:34.667 --> 00:06:37.226
cool down, form stars,

00:06:37.250 --> 00:06:39.684
and you get a small galaxy.

00:06:39.708 --> 00:06:42.726
Over time, over billions
and billions of years,

00:06:42.750 --> 00:06:45.184
those small galaxies crash into each other

00:06:45.208 --> 00:06:48.226
and merge and grow
to become larger galaxies,

00:06:48.250 --> 00:06:50.583
like our own galaxy, the Milky Way.

NOTE Paragraph

00:06:51.875 --> 00:06:55.458
Now, what happens
if you don't have dark matter?

00:06:56.292 --> 00:06:58.184
If you don't have dark matter,

00:06:58.208 --> 00:07:01.167
those spots never get clumpy enough.

00:07:02.000 --> 00:07:07.143
It turns out, you need at least
a million times the mass of the Sun

00:07:07.167 --> 00:07:08.809
in one dense region,

00:07:08.833 --> 00:07:11.351
before you can start forming stars.

00:07:11.375 --> 00:07:13.018
And without dark matter,

00:07:13.042 --> 00:07:16.809
you never get enough stuff in one place.

NOTE Paragraph

00:07:16.833 --> 00:07:21.434
So here, we're looking
at two universes, side by side.

00:07:21.458 --> 00:07:23.934
In one of them you can see

00:07:23.958 --> 00:07:26.976
that things get clumpy quickly.

00:07:27.000 --> 00:07:28.268
In that universe,

00:07:28.292 --> 00:07:30.625
it's really easy to form galaxies.

00:07:31.458 --> 00:07:32.934
In the other universe,

00:07:32.958 --> 00:07:35.476
the things that start out
like small clumps,

00:07:35.500 --> 00:07:37.393
they just stay really small.

00:07:37.417 --> 00:07:39.518
Not very much happens.

00:07:39.542 --> 00:07:42.893
In that universe,
you wouldn't get our galaxy.

00:07:42.917 --> 00:07:44.601
Or any other galaxy.

00:07:44.625 --> 00:07:46.476
You wouldn't get the Milky Way,

00:07:46.500 --> 00:07:48.309
you wouldn't get the Sun,

00:07:48.333 --> 00:07:50.059
you wouldn't get us.

00:07:50.083 --> 00:07:52.750
We just couldn't exist in that universe.

NOTE Paragraph

00:07:55.083 --> 00:07:58.393
OK, so this crazy stuff, dark matter,

00:07:58.417 --> 00:08:00.227
it's most of the mass in the universe,

00:08:00.251 --> 00:08:03.351
it's going through us right now,
we wouldn't be here without it.

00:08:03.375 --> 00:08:04.625
What is it?

00:08:05.625 --> 00:08:06.893
Well, we have no idea.

NOTE Paragraph

00:08:06.917 --> 00:08:08.292
(Laughter)

NOTE Paragraph

00:08:09.208 --> 00:08:12.268
But we have a lot of educated guesses,

00:08:12.292 --> 00:08:15.559
and a lot of ideas
for how to find out more.

00:08:15.583 --> 00:08:20.101
So, most physicists think
that dark matter is a particle,

00:08:20.125 --> 00:08:23.184
similar in many ways to the subatomic
particles that we know of,

00:08:23.208 --> 00:08:26.226
like protons and neutrons and electrons.

00:08:26.250 --> 00:08:27.518
Whatever it is,

00:08:27.542 --> 00:08:31.417
it behaves very similarly
with respect to gravity.

00:08:32.333 --> 00:08:35.518
But it doesn't emit or absorb light,

00:08:35.542 --> 00:08:37.476
and it goes right through normal matter,

00:08:37.500 --> 00:08:39.292
as if it wasn't even there.

00:08:40.375 --> 00:08:42.768
We'd like to know what particle it is.

00:08:42.792 --> 00:08:45.268
For example, how heavy is it?

00:08:45.292 --> 00:08:50.351
Or, does anything at all happen
if it interacts with normal matter?

00:08:50.375 --> 00:08:53.184
Physicists have lots of great ideas
for what it could be,

00:08:53.208 --> 00:08:55.226
they're very creative.

00:08:55.250 --> 00:08:57.059
But it's really hard,

00:08:57.083 --> 00:09:00.768
because those ideas span a huge range.

00:09:00.792 --> 00:09:04.018
It could be as small
as the smallest subatomic particles,

00:09:04.042 --> 00:09:07.625
or it could be as large
as the mass of 100 Suns.

NOTE Paragraph

00:09:08.958 --> 00:09:12.643
So, how do we figure out what it is?

00:09:12.667 --> 00:09:14.476
Well, physicists and astronomers

00:09:14.500 --> 00:09:17.976
have a lot of ways
to look for dark matter.

00:09:18.000 --> 00:09:22.184
One of the things we're doing
is building sensitive detectors

00:09:22.208 --> 00:09:25.143
in deep underground mines,

00:09:25.167 --> 00:09:28.018
waiting for the possibility

00:09:28.042 --> 00:09:31.893
that a dark matter particle,
which goes through us and the Earth,

00:09:31.917 --> 00:09:34.226
would hit a denser material

00:09:34.250 --> 00:09:37.167
and leave behind
some trace of its passage.

00:09:38.000 --> 00:09:40.976
We're looking for dark matter in the sky,

00:09:41.000 --> 00:09:43.184
for the possibility
that dark matter particles

00:09:43.208 --> 00:09:44.601
would crash into each other

00:09:44.625 --> 00:09:47.601
and create high-energy light
that we could see

00:09:47.625 --> 00:09:50.601
with special gamma-ray telescopes.

00:09:50.625 --> 00:09:54.351
We're even trying to make
dark matter here on Earth,

00:09:54.375 --> 00:09:58.934
by smashing particles together
and looking for what happens,

00:09:58.958 --> 00:10:02.042
using the Large Hadron
Collider in Switzerland.

NOTE Paragraph

00:10:03.083 --> 00:10:05.268
Now, so far,

00:10:05.292 --> 00:10:08.559
all of these experiments
have taught us a lot

00:10:08.583 --> 00:10:10.226
about what dark matter isn't

NOTE Paragraph

00:10:10.250 --> 00:10:11.434
(Laughter)

NOTE Paragraph

00:10:11.458 --> 00:10:13.000
but not yet what it is.

00:10:13.875 --> 00:10:17.018
There were really good ideas
that dark matter could have been,

00:10:17.042 --> 00:10:19.101
that these experiments would have seen.

00:10:19.125 --> 00:10:20.518
And they didn't see them yet,

00:10:20.542 --> 00:10:23.333
so we have to keep looking
and thinking harder.

NOTE Paragraph

00:10:25.458 --> 00:10:30.268
Now, another way to get a clue
to what dark matter is

00:10:30.292 --> 00:10:32.559
is to study galaxies.

00:10:32.583 --> 00:10:34.018
We already talked about

00:10:34.042 --> 00:10:37.601
how our galaxy and many other galaxies
wouldn't even be here

00:10:37.625 --> 00:10:39.476
without dark matter.

00:10:39.500 --> 00:10:41.851
Those models also make predictions

00:10:41.875 --> 00:10:43.934
for many other things about galaxies:

00:10:43.958 --> 00:10:45.863
How they're distributed in the universe,

00:10:45.887 --> 00:10:47.143
how they move,

00:10:47.167 --> 00:10:49.184
how they evolve over time.

00:10:49.208 --> 00:10:54.018
And we can test those predictions
with observations of the sky.

NOTE Paragraph

00:10:54.042 --> 00:10:56.851
So let me just give you two examples

00:10:56.875 --> 00:11:00.125
of these kinds of measurements
we can make with galaxies.

00:11:01.208 --> 00:11:06.101
The first is that we can make
maps of the universe with galaxies.

00:11:06.125 --> 00:11:08.851
I am part of a survey
called the Dark Energy Survey,

00:11:08.875 --> 00:11:12.083
which has made the largest map
of the universe so far.

00:11:13.167 --> 00:11:18.601
We measured the positions and shapes
of 100 million galaxies

00:11:18.625 --> 00:11:20.917
over one-eighth of the sky.

00:11:22.458 --> 00:11:27.893
And this map is showing us all the matter
in this region of the sky,

00:11:27.917 --> 00:11:33.542
which is inferred by the light
distorted from these 100 million galaxies.

00:11:34.708 --> 00:11:37.768
The light distorted from all of the matter

00:11:37.792 --> 00:11:41.000
that was between those galaxies and us.

00:11:42.333 --> 00:11:46.976
The gravity of the matter is strong enough
to bend the path of light.

00:11:47.000 --> 00:11:50.667
And it gives us this image.

00:11:52.250 --> 00:11:53.684
So these kinds of maps

00:11:53.708 --> 00:11:56.768
can tell us about how much
dark matter there is,

00:11:56.792 --> 00:11:59.059
they also tell us where it is

00:11:59.083 --> 00:12:01.000
and how it changes over time.

NOTE Paragraph

00:12:02.583 --> 00:12:06.768
So we're trying to learn
about what the universe is made of

00:12:06.792 --> 00:12:09.101
on the very largest scales.

00:12:09.125 --> 00:12:13.851
It turns out that the tiniest
galaxies in the universe

00:12:13.875 --> 00:12:17.018
provide some of the best clues.

00:12:17.042 --> 00:12:18.292
So why is that?

NOTE Paragraph

00:12:19.375 --> 00:12:22.809
Here are two example simulated universes

00:12:22.833 --> 00:12:25.434
with two different kinds of dark matter.

00:12:25.458 --> 00:12:27.684
Both of these pictures
are showing you a region

00:12:27.708 --> 00:12:30.601
around a galaxy like the Milky Way.

00:12:30.625 --> 00:12:33.601
And you can see that there's a lot
of other material around it,

00:12:33.625 --> 00:12:35.309
little small clumps.

00:12:35.333 --> 00:12:37.518
Now, in the image on the right,

00:12:37.542 --> 00:12:42.726
dark matter particles are moving slower
than they are in the one on the left.

00:12:42.750 --> 00:12:45.851
If those dark matter particles
are moving really fast,

00:12:45.875 --> 00:12:48.684
then the gravity in small clumps
is not strong enough

00:12:48.708 --> 00:12:51.143
to slow those fast particles down.

00:12:51.167 --> 00:12:52.434
And they keep going.

00:12:52.458 --> 00:12:54.934
They never collapse
into these small clumps.

00:12:54.958 --> 00:12:59.726
So you end up with fewer of them
than in the universe on the right.

00:12:59.750 --> 00:13:01.684
If you don't have those small clumps,

00:13:01.708 --> 00:13:04.417
then you get fewer small galaxies.

00:13:06.208 --> 00:13:07.893
If you look up at the southern sky,

00:13:07.917 --> 00:13:11.268
you can actually see
two of these small galaxies,

00:13:11.292 --> 00:13:14.768
the largest of the small galaxies
that are orbiting our Milky Way,

00:13:14.792 --> 00:13:17.792
the Large Magellanic Cloud
and the Small Magellanic Cloud.

NOTE Paragraph

00:13:18.958 --> 00:13:20.226
In the last several years,

00:13:20.250 --> 00:13:23.601
we have detected a whole bunch more
even smaller galaxies.

00:13:23.625 --> 00:13:25.434
This is an example of one of them

00:13:25.458 --> 00:13:28.393
that we detected
with the same dark energy survey

00:13:28.417 --> 00:13:31.518
that we used to make maps of the universe.

00:13:31.542 --> 00:13:33.768
These really small galaxies,

00:13:33.792 --> 00:13:35.750
some of them are extremely small.

00:13:36.458 --> 00:13:39.393
Some of them have as few
as a few hundred stars,

00:13:39.417 --> 00:13:43.059
compared to the few hundred
billion stars in our Milky Way.

00:13:43.083 --> 00:13:45.893
So that makes them really hard to find.

00:13:45.917 --> 00:13:47.809
But in the last decade,

00:13:47.833 --> 00:13:50.518
we've actually found
a whole bunch more of these.

00:13:50.542 --> 00:13:53.268
We now know of 60 of these tiny galaxies

00:13:53.292 --> 00:13:56.309
that are orbiting our own Milky Way.

00:13:56.333 --> 00:13:59.917
And these little guys
are a big clue to dark matter.

00:14:00.833 --> 00:14:04.643
Because just the existence
of these galaxies tells us

00:14:04.667 --> 00:14:07.476
that dark matter
can't be moving very fast,

00:14:07.500 --> 00:14:11.417
and not much can be happening
when it runs into normal matter.

NOTE Paragraph

00:14:12.583 --> 00:14:14.018
In the next several years,

00:14:14.042 --> 00:14:17.708
we're going to make much more
precise maps of the sky.

00:14:19.292 --> 00:14:21.726
And those will help refine our movies

00:14:21.750 --> 00:14:24.750
of the whole universe
and the entire galaxy.

00:14:25.708 --> 00:14:29.768
Physicists are also making new,
more sensitive experiments

00:14:29.792 --> 00:14:33.625
to try to catch some sign
of dark matter in their laboratories.

NOTE Paragraph

00:14:35.042 --> 00:14:38.309
Dark matter is still a huge mystery.

00:14:38.333 --> 00:14:41.583
But it's a really exciting time
to be working on it.

00:14:42.750 --> 00:14:45.101
We have really clear evidence it exists.

00:14:45.125 --> 00:14:47.518
From the scale of the smallest galaxies

00:14:47.542 --> 00:14:50.125
to the scale of the whole universe.

00:14:51.458 --> 00:14:54.958
Will we actually find it
and figure out what it is?

00:14:56.542 --> 00:14:58.309
I have no idea.

00:14:58.333 --> 00:15:01.268
But it's going to be
a lot of fun to find out.

00:15:01.292 --> 00:15:03.684
We have a lot of possibilities
for discovery,

00:15:03.708 --> 00:15:07.184
and we definitely will learn more
about what it is doing

00:15:07.208 --> 00:15:08.792
and about what it isn't.

00:15:09.833 --> 00:15:12.893
Regardless of whether we find
that particle anytime soon,

00:15:12.917 --> 00:15:14.643
I hope I have convinced you

00:15:14.667 --> 00:15:18.726
that this mystery is actually
really close to home.

00:15:18.750 --> 00:15:20.268
The search for dark matter

00:15:20.292 --> 00:15:23.601
may just be the key to a whole new
understanding of physics

00:15:23.625 --> 00:15:25.476
and our place in the universe.

NOTE Paragraph

00:15:25.500 --> 00:15:26.768
Thank you.

NOTE Paragraph

00:15:26.792 --> 00:15:30.542
(Applause)