The search for dark matter -- and what we've found so far
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0:01 - 0:03Do you ever think about what would happen
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0:03 - 0:06if the world were a little bit different?
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0:06 - 0:07How your life would be different
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0:07 - 0:10if you were born 5,000 years from now
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0:10 - 0:11instead of today?
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0:12 - 0:13How history would be different
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0:14 - 0:16if the continents
were at different latitudes -
0:16 - 0:20or how life in the Solar system
would have developed -
0:20 - 0:22if the Sun were 10 percent larger.
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0:23 - 0:26Well, playing with these
kinds of possibilities -
0:26 - 0:28is what I get to do for a living
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0:28 - 0:30but with the entire universe.
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0:31 - 0:34I make model universes in a computer.
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0:34 - 0:37Digital universes that have
different starting points -
0:37 - 0:41and are made of different amounts
of different kinds of material. -
0:41 - 0:45And then I compare
these universes to our own -
0:45 - 0:48to see what it is made of
and how it evolved. -
0:51 - 0:55This process of testing models
with measurements of the sky -
0:55 - 0:58has taught us a huge amount
about our universe so far. -
0:59 - 1:01One of the strangest
things we have learned -
1:01 - 1:03is that most of the material
in the universe -
1:03 - 1:07is made of something
entirely different than you and me. -
1:09 - 1:11But without it,
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1:12 - 1:15the universe as we know it wouldn't exist.
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1:17 - 1:21Everything we can see with telescopes
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1:21 - 1:25makes up just about 15 percent
of the total mass in the universe. -
1:26 - 1:30Everything else, 85 percent of it,
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1:30 - 1:33doesn't emit or absorb light.
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1:34 - 1:36We can't see it with our eyes,
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1:36 - 1:40we can't detect it with radio waves
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1:40 - 1:42or microwaves or any other kind of light.
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1:43 - 1:45But we know it is there
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1:45 - 1:47because of its influence
on what we can see. -
1:49 - 1:50It's a little bit like,
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1:50 - 1:53if you wanted to map
the surface of our planet -
1:53 - 1:55and everything on it
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1:55 - 1:58using this picture of the Earth
from space at night. -
2:00 - 2:02You get some clues
from where the light is, -
2:02 - 2:04but there's a lot that you can't see,
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2:04 - 2:08everything from people
to mountain ranges. -
2:08 - 2:12And you have to infer what is there
from these limited clues. -
2:14 - 2:17We call this unseen stuff "dark matter."
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2:18 - 2:21Now, a lot of people
have heard of dark matter, -
2:21 - 2:23but even if you have heard of it,
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2:23 - 2:26it probably seems abstract,
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2:26 - 2:29far away, probably even irrelevant.
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2:31 - 2:33Well, the interesting thing is,
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2:33 - 2:36dark matter is all around us
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2:36 - 2:39and probably right here.
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2:39 - 2:41In fact, dark matter particles
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2:41 - 2:44are probably going through
your body right now -
2:44 - 2:46as you sit in this room.
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2:46 - 2:47Because we are on Earth
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2:47 - 2:49and Earth is spinning around the Sun,
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2:49 - 2:52and the Sun is hurtling through our galaxy
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2:52 - 2:56at about half a million miles per hour.
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2:56 - 2:57But dark matter doesn't bump into us,
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2:57 - 2:59it just goes right through us.
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3:01 - 3:05So how do we figure out more about this?
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3:05 - 3:06What is it,
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3:06 - 3:09and what does it have to do
with our existence? -
3:09 - 3:14Well, in order to figure out
how we came to be, -
3:14 - 3:18we first need to understand
how our galaxy came to be. -
3:18 - 3:22This is a picture of our galaxy,
the Milky Way, today. -
3:22 - 3:24What did it look like
10 billion years in the past -
3:24 - 3:28or what would it look like
10 billion years in the future? -
3:29 - 3:30What about the stories
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3:30 - 3:33of the hundreds of millions
of other galaxies -
3:33 - 3:37that we've already mapped out
with large surveys of the sky? -
3:37 - 3:39How would their histories be different
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3:39 - 3:42if the universe was made of something else
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3:42 - 3:45or if there was more or less matter in it?
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3:46 - 3:49So the interesting thing
about these model universes -
3:49 - 3:52is that they allow us
to test these possibilities. -
3:54 - 3:59Let's go back to the first
moment of the universe -- -
4:00 - 4:04just a fraction of a second
after the big bang. -
4:06 - 4:08In this first moment,
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4:08 - 4:10there was no matter at all.
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4:10 - 4:13The universe was expanding very fast.
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4:13 - 4:16And quantum mechanics tells us
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4:16 - 4:19that matter is being created and destroyed
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4:19 - 4:21all the time, in every moment.
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4:22 - 4:25At this time, the universe
was expanding so fast -
4:25 - 4:28that the matter that got created
couldn't get destroyed. -
4:29 - 4:34And thus we think that all of the matter
was created during this time. -
4:34 - 4:36Both the dark matter
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4:36 - 4:39and the regular matter
that makes up you and me. -
4:40 - 4:43Now, let's go a little bit further
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4:43 - 4:45to a time after the matter was created,
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4:45 - 4:47after protons and neutrons formed,
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4:47 - 4:49after hydrogen formed,
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4:49 - 4:53about 400,000 years after the big bang.
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4:53 - 4:58The universe was hot and dense
and really smooth -
4:58 - 5:00but not perfectly smooth.
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5:00 - 5:05This image, taken with a space telescope
called the Planck satellite, -
5:05 - 5:08shows us the temperature of the universe
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5:08 - 5:09in all directions.
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5:10 - 5:11And what we see
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5:11 - 5:14is that there were places
that were a little bit hotter -
5:14 - 5:17and denser than others.
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5:17 - 5:19The spots in this image
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5:19 - 5:24represent places where there was
more or less mass in the early universe. -
5:25 - 5:28Those spots got big because of gravity.
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5:30 - 5:34The universe was expanding
and getting less dense overall -
5:34 - 5:38over the last 13.8 billion years.
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5:38 - 5:41But gravity worked hard in those spots
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5:41 - 5:43where there was a little bit more mass
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5:43 - 5:47and pulled more and more mass
into those regions. -
5:47 - 5:50Now, all of this
is a little hard to imagine, -
5:50 - 5:53so let me just show you
what I am talking about. -
5:53 - 5:57Those computer models I mentioned
allow us to test these ideas, -
5:57 - 5:59so let's take a look at one of them.
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6:00 - 6:03This movie, made by my research group,
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6:03 - 6:07shows us what happened to the universe
after its earliest moments. -
6:08 - 6:11You see the universe
started out pretty smooth, -
6:11 - 6:13but there were some regions
-
6:13 - 6:16where there was
a little bit more material. -
6:16 - 6:19Gravity turned on
and brought more and more mass -
6:19 - 6:23into those spots that started out
with a little bit extra. -
6:24 - 6:26Over time,
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6:26 - 6:28you get enough stuff in one place
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6:28 - 6:30that the hydrogen gas,
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6:30 - 6:33which was initially well mixed
with the dark matter, -
6:33 - 6:35starts to separate from it,
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6:35 - 6:37cool down, form stars,
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6:37 - 6:40and you get a small galaxy.
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6:40 - 6:43Over time, over billions
and billions of years, -
6:43 - 6:45those small galaxies crash into each other
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6:45 - 6:48and merge and grow
to become larger galaxies, -
6:48 - 6:51like our own galaxy, the Milky Way.
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6:52 - 6:55Now, what happens
if you don't have dark matter? -
6:56 - 6:58If you don't have dark matter,
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6:58 - 7:01those spots never get clumpy enough.
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7:02 - 7:07It turns out, you need at least
a million times the mass of the Sun -
7:07 - 7:09in one dense region,
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7:09 - 7:11before you can start forming stars.
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7:11 - 7:13And without dark matter,
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7:13 - 7:17you never get enough stuff in one place.
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7:17 - 7:21So here, we're looking
at two universes, side by side. -
7:21 - 7:24In one of them you can see
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7:24 - 7:27that things get clumpy quickly.
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7:27 - 7:28In that universe,
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7:28 - 7:31it's really easy to form galaxies.
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7:31 - 7:33In the other universe,
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7:33 - 7:35the things that start out
like small clumps, -
7:36 - 7:37they just stay really small.
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7:37 - 7:40Not very much happens.
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7:40 - 7:43In that universe,
you wouldn't get our galaxy. -
7:43 - 7:45Or any other galaxy.
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7:45 - 7:46You wouldn't get the Milky Way,
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7:46 - 7:48you wouldn't get the Sun,
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7:48 - 7:50you wouldn't get us.
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7:50 - 7:53We just couldn't exist in that universe.
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7:55 - 7:58OK, so this crazy stuff, dark matter,
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7:58 - 8:00it's most of the mass in the universe,
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8:00 - 8:03it's going through us right now,
we wouldn't be here without it. -
8:03 - 8:05What is it?
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8:06 - 8:07Well, we have no idea.
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8:07 - 8:08(Laughter)
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8:09 - 8:12But we have a lot of educated guesses,
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8:12 - 8:16and a lot of ideas
for how to find out more. -
8:16 - 8:20So, most physicists think
that dark matter is a particle, -
8:20 - 8:23similar in many ways to the subatomic
particles that we know of, -
8:23 - 8:26like protons and neutrons and electrons.
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8:26 - 8:28Whatever it is,
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8:28 - 8:31it behaves very similarly
with respect to gravity. -
8:32 - 8:36But it doesn't emit or absorb light,
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8:36 - 8:37and it goes right through normal matter,
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8:38 - 8:39as if it wasn't even there.
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8:40 - 8:43We'd like to know what particle it is.
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8:43 - 8:45For example, how heavy is it?
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8:45 - 8:50Or, does anything at all happen
if it interacts with normal matter? -
8:50 - 8:53Physicists have lots of great ideas
for what it could be, -
8:53 - 8:55they're very creative.
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8:55 - 8:57But it's really hard,
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8:57 - 9:01because those ideas span a huge range.
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9:01 - 9:04It could be as small
as the smallest subatomic particles, -
9:04 - 9:08or it could be as large
as the mass of 100 Suns. -
9:09 - 9:13So, how do we figure out what it is?
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9:13 - 9:14Well, physicists and astronomers
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9:14 - 9:18have a lot of ways
to look for dark matter. -
9:18 - 9:22One of the things we're doing
is building sensitive detectors -
9:22 - 9:25in deep underground mines,
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9:25 - 9:28waiting for the possibility
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9:28 - 9:32that a dark matter particle,
which goes through us and the Earth, -
9:32 - 9:34would hit a denser material
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9:34 - 9:37and leave behind
some trace of its passage. -
9:38 - 9:41We're looking for dark matter in the sky,
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9:41 - 9:43for the possibility
that dark matter particles -
9:43 - 9:45would crash into each other
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9:45 - 9:48and create high-energy light
that we could see -
9:48 - 9:51with special gamma-ray telescopes.
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9:51 - 9:54We're even trying to make
dark matter here on Earth, -
9:54 - 9:59by smashing particles together
and looking for what happens, -
9:59 - 10:02using the Large Hadron
Collider in Switzerland. -
10:03 - 10:05Now, so far,
-
10:05 - 10:09all of these experiments
have taught us a lot -
10:09 - 10:10about what dark matter isn't
-
10:10 - 10:11(Laughter)
-
10:11 - 10:13but not yet what it is.
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10:14 - 10:17There were really good ideas
that dark matter could have been, -
10:17 - 10:19that these experiments would have seen.
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10:19 - 10:21And they didn't see them yet,
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10:21 - 10:23so we have to keep looking
and thinking harder. -
10:25 - 10:30Now, another way to get a clue
to what dark matter is -
10:30 - 10:33is to study galaxies.
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10:33 - 10:34We already talked about
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10:34 - 10:38how our galaxy and many other galaxies
wouldn't even be here -
10:38 - 10:39without dark matter.
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10:40 - 10:42Those models also make predictions
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10:42 - 10:44for many other things about galaxies:
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10:44 - 10:46How they're distributed in the universe,
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10:46 - 10:47how they move,
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10:47 - 10:49how they evolve over time.
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10:49 - 10:54And we can test those predictions
with observations of the sky. -
10:54 - 10:57So let me just give you two examples
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10:57 - 11:00of these kinds of measurements
we can make with galaxies. -
11:01 - 11:06The first is that we can make
maps of the universe with galaxies. -
11:06 - 11:09I am part of a survey
called the Dark Energy Survey, -
11:09 - 11:12which has made the largest map
of the universe so far. -
11:13 - 11:19We measured the positions and shapes
of 100 million galaxies -
11:19 - 11:21over one-eighth of the sky.
-
11:22 - 11:28And this map is showing us all the matter
in this region of the sky, -
11:28 - 11:34which is inferred by the light
distorted from these 100 million galaxies. -
11:35 - 11:38The light distorted from all of the matter
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11:38 - 11:41that was between those galaxies and us.
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11:42 - 11:47The gravity of the matter is strong enough
to bend the path of light. -
11:47 - 11:51And it gives us this image.
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11:52 - 11:54So these kinds of maps
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11:54 - 11:57can tell us about how much
dark matter there is, -
11:57 - 11:59they also tell us where it is
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11:59 - 12:01and how it changes over time.
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12:03 - 12:07So we're trying to learn
about what the universe is made of -
12:07 - 12:09on the very largest scales.
-
12:09 - 12:14It turns out that the tiniest
galaxies in the universe -
12:14 - 12:17provide some of the best clues.
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12:17 - 12:18So why is that?
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12:19 - 12:23Here are two example simulated universes
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12:23 - 12:25with two different kinds of dark matter.
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12:25 - 12:28Both of these pictures
are showing you a region -
12:28 - 12:31around a galaxy like the Milky Way.
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12:31 - 12:34And you can see that there's a lot
of other material around it, -
12:34 - 12:35little small clumps.
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12:35 - 12:38Now, in the image on the right,
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12:38 - 12:43dark matter particles are moving slower
than they are in the one on the left. -
12:43 - 12:46If those dark matter particles
are moving really fast, -
12:46 - 12:49then the gravity in small clumps
is not strong enough -
12:49 - 12:51to slow those fast particles down.
-
12:51 - 12:52And they keep going.
-
12:52 - 12:55They never collapse
into these small clumps. -
12:55 - 13:00So you end up with fewer of them
than in the universe on the right. -
13:00 - 13:02If you don't have those small clumps,
-
13:02 - 13:04then you get fewer small galaxies.
-
13:06 - 13:08If you look up at the southern sky,
-
13:08 - 13:11you can actually see
two of these small galaxies, -
13:11 - 13:15the largest of the small galaxies
that are orbiting our Milky Way, -
13:15 - 13:18the Large Magellanic Cloud
and the Small Magellanic Cloud. -
13:19 - 13:20In the last several years,
-
13:20 - 13:24we have detected a whole bunch more
even smaller galaxies. -
13:24 - 13:25This is an example of one of them
-
13:25 - 13:28that we detected
with the same dark energy survey -
13:28 - 13:32that we used to make maps of the universe.
-
13:32 - 13:34These really small galaxies,
-
13:34 - 13:36some of them are extremely small.
-
13:36 - 13:39Some of them have as few
as a few hundred stars, -
13:39 - 13:43compared to the few hundred
billion stars in our Milky Way. -
13:43 - 13:46So that makes them really hard to find.
-
13:46 - 13:48But in the last decade,
-
13:48 - 13:51we've actually found
a whole bunch more of these. -
13:51 - 13:53We now know of 60 of these tiny galaxies
-
13:53 - 13:56that are orbiting our own Milky Way.
-
13:56 - 14:00And these little guys
are a big clue to dark matter. -
14:01 - 14:05Because just the existence
of these galaxies tells us -
14:05 - 14:07that dark matter
can't be moving very fast, -
14:08 - 14:11and not much can be happening
when it runs into normal matter. -
14:13 - 14:14In the next several years,
-
14:14 - 14:18we're going to make much more
precise maps of the sky. -
14:19 - 14:22And those will help refine our movies
-
14:22 - 14:25of the whole universe
and the entire galaxy. -
14:26 - 14:30Physicists are also making new,
more sensitive experiments -
14:30 - 14:34to try to catch some sign
of dark matter in their laboratories. -
14:35 - 14:38Dark matter is still a huge mystery.
-
14:38 - 14:42But it's a really exciting time
to be working on it. -
14:43 - 14:45We have really clear evidence it exists.
-
14:45 - 14:48From the scale of the smallest galaxies
-
14:48 - 14:50to the scale of the whole universe.
-
14:51 - 14:55Will we actually find it
and figure out what it is? -
14:57 - 14:58I have no idea.
-
14:58 - 15:01But it's going to be
a lot of fun to find out. -
15:01 - 15:04We have a lot of possibilities
for discovery, -
15:04 - 15:07and we definitely will learn more
about what it is doing -
15:07 - 15:09and about what it isn't.
-
15:10 - 15:13Regardless of whether we find
that particle anytime soon, -
15:13 - 15:15I hope I have convinced you
-
15:15 - 15:19that this mystery is actually
really close to home. -
15:19 - 15:20The search for dark matter
-
15:20 - 15:24may just be the key to a whole new
understanding of physics -
15:24 - 15:25and our place in the universe.
-
15:26 - 15:27Thank you.
-
15:27 - 15:31(Applause)
- Title:
- The search for dark matter -- and what we've found so far
- Speaker:
- Risa Wechsler
- Description:
-
Roughly 85 percent of mass in the universe is "dark matter" -- mysterious material that can't be directly observed but has an immense influence on the cosmos. What exactly is this strange stuff, and what does it have to do with our existence? Astrophysicist Risa Wechsler explores why dark matter may be the key to understanding how the universe formed -- and shares how physicists in labs around the world are coming up with creative ways to study it.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TEDTalks
- Duration:
- 15:43
Brian Greene edited English subtitles for The search for dark matter -- and what we've found so far | ||
Oliver Friedman edited English subtitles for The search for dark matter -- and what we've found so far | ||
Erin Gregory approved English subtitles for The search for dark matter -- and what we've found so far | ||
Erin Gregory edited English subtitles for The search for dark matter -- and what we've found so far | ||
Erin Gregory edited English subtitles for The search for dark matter -- and what we've found so far | ||
Joanna Pietrulewicz accepted English subtitles for The search for dark matter -- and what we've found so far | ||
Joanna Pietrulewicz edited English subtitles for The search for dark matter -- and what we've found so far | ||
Joanna Pietrulewicz edited English subtitles for The search for dark matter -- and what we've found so far |