TED@NAS
Risa Wechsler: The search for dark matter -- and what we've found so far
Filmed:
Readability: 3.5
397,764 views
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.
Risa Wechsler - Astrophysicist, cosmologist
Risa Wechsler uses computer simulations of the entire universe to explore questions about our existence on the largest scales. Full bio
Risa Wechsler uses computer simulations of the entire universe to explore questions about our existence on the largest scales. Full bio
Double-click the English transcript below to play the video.
00:12
Do you ever think about what would happen
0
958
1953
00:14
if the world were a little bit different?
1
2935
2583
00:17
How your life would be different
2
5542
1559
00:19
if you were born 5,000 years from now
3
7125
2643
00:21
instead of today?
4
9792
1291
00:23
How history would be different
5
11958
1518
00:25
if the continents
were at different latitudes
were at different latitudes
6
13500
2143
00:27
or how life in the Solar system
would have developed
would have developed
7
15667
3934
00:31
if the Sun were 10 percent larger.
8
19625
2542
00:35
Well, playing with these
kinds of possibilities
kinds of possibilities
9
23292
2892
00:38
is what I get to do for a living
10
26208
1601
00:39
but with the entire universe.
11
27833
2167
00:43
I make model universes in a computer.
12
31042
2809
00:45
Digital universes that have
different starting points
different starting points
13
33875
2934
00:48
and are made of different amounts
of different kinds of material.
of different kinds of material.
14
36833
4601
00:53
And then I compare
these universes to our own
these universes to our own
15
41458
3310
00:56
to see what it is made of
and how it evolved.
and how it evolved.
16
44792
3708
01:02
This process of testing models
with measurements of the sky
with measurements of the sky
17
50542
4351
01:06
has taught us a huge amount
about our universe so far.
about our universe so far.
18
54917
2875
01:10
One of the strangest
things we have learned
things we have learned
19
58625
2226
01:12
is that most of the material
in the universe
in the universe
20
60875
2559
01:15
is made of something
entirely different than you and me.
entirely different than you and me.
21
63458
3917
01:20
But without it,
22
68750
2726
01:23
the universe as we know it wouldn't exist.
23
71500
3500
01:28
Everything we can see with telescopes
24
76958
3601
01:32
makes up just about 15 percent
of the total mass in the universe.
of the total mass in the universe.
25
80583
4375
01:38
Everything else, 85 percent of it,
26
86000
4393
01:42
doesn't emit or absorb light.
27
90417
2333
01:45
We can't see it with our eyes,
28
93708
2351
01:48
we can't detect it with radio waves
29
96083
3518
01:51
or microwaves or any other kind of light.
30
99625
2417
01:55
But we know it is there
31
103083
1935
01:57
because of its influence
on what we can see.
on what we can see.
32
105042
2375
02:01
It's a little bit like,
33
109208
1268
02:02
if you wanted to map
the surface of our planet
the surface of our planet
34
110500
2184
02:04
and everything on it
35
112708
1976
02:06
using this picture of the Earth
from space at night.
from space at night.
36
114708
3667
02:11
You get some clues
from where the light is,
from where the light is,
37
119708
2476
02:14
but there's a lot that you can't see,
38
122208
1810
02:16
everything from people
to mountain ranges.
to mountain ranges.
39
124042
3642
02:19
And you have to infer what is there
from these limited clues.
from these limited clues.
40
127708
4459
02:25
We call this unseen stuff "dark matter."
41
133958
2917
02:29
Now, a lot of people
have heard of dark matter,
have heard of dark matter,
42
137708
2893
02:32
but even if you have heard of it,
43
140625
2309
02:34
it probably seems abstract,
44
142958
3018
02:38
far away, probably even irrelevant.
45
146000
2958
02:42
Well, the interesting thing is,
46
150625
2809
02:45
dark matter is all around us
47
153458
2518
02:48
and probably right here.
48
156000
2934
02:50
In fact, dark matter particles
49
158958
1935
02:52
are probably going through
your body right now
your body right now
50
160917
3142
02:56
as you sit in this room.
51
164083
1976
02:58
Because we are on Earth
52
166083
1351
02:59
and Earth is spinning around the Sun,
53
167458
1851
03:01
and the Sun is hurtling through our galaxy
54
169333
2935
03:04
at about half a million miles per hour.
55
172292
3226
03:07
But dark matter doesn't bump into us,
56
175542
1809
03:09
it just goes right through us.
57
177375
1500
03:12
So how do we figure out more about this?
58
180583
4435
03:17
What is it,
59
185042
1267
03:18
and what does it have to do
with our existence?
with our existence?
60
186333
2435
03:20
Well, in order to figure out
how we came to be,
how we came to be,
61
188792
4767
03:25
we first need to understand
how our galaxy came to be.
how our galaxy came to be.
62
193583
3976
03:29
This is a picture of our galaxy,
the Milky Way, today.
the Milky Way, today.
63
197583
4476
03:34
What did it look like
10 billion years in the past
10 billion years in the past
64
202083
2351
03:36
or what would it look like
10 billion years in the future?
10 billion years in the future?
65
204458
3417
03:40
What about the stories
66
208917
1434
03:42
of the hundreds of millions
of other galaxies
of other galaxies
67
210375
2643
03:45
that we've already mapped out
with large surveys of the sky?
with large surveys of the sky?
68
213042
4142
03:49
How would their histories be different
69
217208
2226
03:51
if the universe was made of something else
70
219458
2893
03:54
or if there was more or less matter in it?
71
222375
3101
03:57
So the interesting thing
about these model universes
about these model universes
72
225500
3851
04:01
is that they allow us
to test these possibilities.
to test these possibilities.
73
229375
2792
04:05
Let's go back to the first
moment of the universe --
moment of the universe --
74
233500
5167
04:11
just a fraction of a second
after the big bang.
after the big bang.
75
239542
4791
04:17
In this first moment,
76
245875
1934
04:19
there was no matter at all.
77
247833
2101
04:21
The universe was expanding very fast.
78
249958
3476
04:25
And quantum mechanics tells us
79
253458
2768
04:28
that matter is being created and destroyed
80
256250
2726
04:31
all the time, in every moment.
81
259000
2000
04:34
At this time, the universe
was expanding so fast
was expanding so fast
82
262357
2286
04:36
that the matter that got created
couldn't get destroyed.
couldn't get destroyed.
83
264667
3250
04:40
And thus we think that all of the matter
was created during this time.
was created during this time.
84
268833
4810
04:45
Both the dark matter
85
273667
2184
04:47
and the regular matter
that makes up you and me.
that makes up you and me.
86
275875
3083
04:52
Now, let's go a little bit further
87
280167
2434
04:54
to a time after the matter was created,
88
282625
2476
04:57
after protons and neutrons formed,
89
285125
2309
04:59
after hydrogen formed,
90
287458
1726
05:01
about 400,000 years after the big bang.
91
289208
4018
05:05
The universe was hot and dense
and really smooth
and really smooth
92
293250
4268
05:09
but not perfectly smooth.
93
297542
2726
05:12
This image, taken with a space telescope
called the Planck satellite,
called the Planck satellite,
94
300292
4767
05:17
shows us the temperature of the universe
95
305083
2476
05:19
in all directions.
96
307583
1375
05:22
And what we see
97
310000
1393
05:23
is that there were places
that were a little bit hotter
that were a little bit hotter
98
311417
3017
05:26
and denser than others.
99
314458
2393
05:28
The spots in this image
100
316875
1684
05:30
represent places where there was
more or less mass in the early universe.
more or less mass in the early universe.
101
318583
4917
05:37
Those spots got big because of gravity.
102
325125
3333
05:41
The universe was expanding
and getting less dense overall
and getting less dense overall
103
329500
4434
05:45
over the last 13.8 billion years.
104
333958
3685
05:49
But gravity worked hard in those spots
105
337667
2851
05:52
where there was a little bit more mass
106
340542
2142
05:54
and pulled more and more mass
into those regions.
into those regions.
107
342708
4310
05:59
Now, all of this
is a little hard to imagine,
is a little hard to imagine,
108
347042
2767
06:01
so let me just show you
what I am talking about.
what I am talking about.
109
349833
3351
06:05
Those computer models I mentioned
allow us to test these ideas,
allow us to test these ideas,
110
353208
3560
06:08
so let's take a look at one of them.
111
356792
1791
06:11
This movie, made by my research group,
112
359625
2934
06:14
shows us what happened to the universe
after its earliest moments.
after its earliest moments.
113
362583
4209
06:20
You see the universe
started out pretty smooth,
started out pretty smooth,
114
368375
2893
06:23
but there were some regions
115
371292
1434
06:24
where there was
a little bit more material.
a little bit more material.
116
372750
3059
06:27
Gravity turned on
and brought more and more mass
and brought more and more mass
117
375833
3226
06:31
into those spots that started out
with a little bit extra.
with a little bit extra.
118
379083
4042
06:36
Over time,
119
384167
1434
06:37
you get enough stuff in one place
120
385625
2851
06:40
that the hydrogen gas,
121
388500
1268
06:41
which was initially well mixed
with the dark matter,
with the dark matter,
122
389792
3017
06:44
starts to separate from it,
123
392833
1810
06:46
cool down, form stars,
124
394667
2559
06:49
and you get a small galaxy.
125
397250
2434
06:51
Over time, over billions
and billions of years,
and billions of years,
126
399708
3018
06:54
those small galaxies crash into each other
127
402750
2434
06:57
and merge and grow
to become larger galaxies,
to become larger galaxies,
128
405208
3018
07:00
like our own galaxy, the Milky Way.
129
408250
2333
07:03
Now, what happens
if you don't have dark matter?
if you don't have dark matter?
130
411875
3583
07:08
If you don't have dark matter,
131
416292
1892
07:10
those spots never get clumpy enough.
132
418208
2959
07:14
It turns out, you need at least
a million times the mass of the Sun
a million times the mass of the Sun
133
422000
5143
07:19
in one dense region,
134
427167
1642
07:20
before you can start forming stars.
135
428833
2518
07:23
And without dark matter,
136
431375
1643
07:25
you never get enough stuff in one place.
137
433042
3767
07:28
So here, we're looking
at two universes, side by side.
at two universes, side by side.
138
436833
4601
07:33
In one of them you can see
139
441458
2476
07:35
that things get clumpy quickly.
140
443958
3018
07:39
In that universe,
141
447000
1268
07:40
it's really easy to form galaxies.
142
448292
2333
07:43
In the other universe,
143
451458
1476
07:44
the things that start out
like small clumps,
like small clumps,
144
452958
2518
07:47
they just stay really small.
145
455500
1893
07:49
Not very much happens.
146
457417
2101
07:51
In that universe,
you wouldn't get our galaxy.
you wouldn't get our galaxy.
147
459542
3351
07:54
Or any other galaxy.
148
462917
1684
07:56
You wouldn't get the Milky Way,
149
464625
1851
07:58
you wouldn't get the Sun,
150
466500
1809
08:00
you wouldn't get us.
151
468333
1726
08:02
We just couldn't exist in that universe.
152
470083
2667
08:07
OK, so this crazy stuff, dark matter,
153
475083
3310
08:10
it's most of the mass in the universe,
154
478417
1810
08:12
it's going through us right now,
we wouldn't be here without it.
we wouldn't be here without it.
155
480251
3100
08:15
What is it?
156
483375
1250
08:17
Well, we have no idea.
157
485625
1268
08:18
(Laughter)
158
486917
1375
08:21
But we have a lot of educated guesses,
159
489208
3060
08:24
and a lot of ideas
for how to find out more.
for how to find out more.
160
492292
3267
08:27
So, most physicists think
that dark matter is a particle,
that dark matter is a particle,
161
495583
4518
08:32
similar in many ways to the subatomic
particles that we know of,
particles that we know of,
162
500125
3059
08:35
like protons and neutrons and electrons.
163
503208
3018
08:38
Whatever it is,
164
506250
1268
08:39
it behaves very similarly
with respect to gravity.
with respect to gravity.
165
507542
3875
08:44
But it doesn't emit or absorb light,
166
512333
3185
08:47
and it goes right through normal matter,
167
515542
1934
08:49
as if it wasn't even there.
168
517500
1792
08:52
We'd like to know what particle it is.
169
520375
2393
08:54
For example, how heavy is it?
170
522792
2476
08:57
Or, does anything at all happen
if it interacts with normal matter?
if it interacts with normal matter?
171
525292
5059
09:02
Physicists have lots of great ideas
for what it could be,
for what it could be,
172
530375
2809
09:05
they're very creative.
173
533208
2018
09:07
But it's really hard,
174
535250
1809
09:09
because those ideas span a huge range.
175
537083
3685
09:12
It could be as small
as the smallest subatomic particles,
as the smallest subatomic particles,
176
540792
3226
09:16
or it could be as large
as the mass of 100 Suns.
as the mass of 100 Suns.
177
544042
3583
09:20
So, how do we figure out what it is?
178
548958
3685
09:24
Well, physicists and astronomers
179
552667
1809
09:26
have a lot of ways
to look for dark matter.
to look for dark matter.
180
554500
3476
09:30
One of the things we're doing
is building sensitive detectors
is building sensitive detectors
181
558000
4184
09:34
in deep underground mines,
182
562208
2935
09:37
waiting for the possibility
183
565167
2851
09:40
that a dark matter particle,
which goes through us and the Earth,
which goes through us and the Earth,
184
568042
3851
09:43
would hit a denser material
185
571917
2309
09:46
and leave behind
some trace of its passage.
some trace of its passage.
186
574250
2917
09:50
We're looking for dark matter in the sky,
187
578000
2976
09:53
for the possibility
that dark matter particles
that dark matter particles
188
581000
2184
09:55
would crash into each other
189
583208
1393
09:56
and create high-energy light
that we could see
that we could see
190
584625
2976
09:59
with special gamma-ray telescopes.
191
587625
2976
10:02
We're even trying to make
dark matter here on Earth,
dark matter here on Earth,
192
590625
3726
10:06
by smashing particles together
and looking for what happens,
and looking for what happens,
193
594375
4559
10:10
using the Large Hadron
Collider in Switzerland.
Collider in Switzerland.
194
598958
3084
10:15
Now, so far,
195
603083
2185
10:17
all of these experiments
have taught us a lot
have taught us a lot
196
605292
3267
10:20
about what dark matter isn't
197
608583
1643
10:22
(Laughter)
198
610250
1184
10:23
but not yet what it is.
199
611458
1542
10:25
There were really good ideas
that dark matter could have been,
that dark matter could have been,
200
613875
3143
10:29
that these experiments would have seen.
201
617042
2059
10:31
And they didn't see them yet,
202
619125
1393
10:32
so we have to keep looking
and thinking harder.
and thinking harder.
203
620542
2791
10:37
Now, another way to get a clue
to what dark matter is
to what dark matter is
204
625458
4810
10:42
is to study galaxies.
205
630292
2267
10:44
We already talked about
206
632583
1435
10:46
how our galaxy and many other galaxies
wouldn't even be here
wouldn't even be here
207
634042
3559
10:49
without dark matter.
208
637625
1851
10:51
Those models also make predictions
209
639500
2351
10:53
for many other things about galaxies:
210
641875
2059
10:55
How they're distributed in the universe,
211
643958
1905
10:57
how they move,
212
645887
1256
10:59
how they evolve over time.
213
647167
2017
11:01
And we can test those predictions
with observations of the sky.
with observations of the sky.
214
649208
4810
11:06
So let me just give you two examples
215
654042
2809
11:08
of these kinds of measurements
we can make with galaxies.
we can make with galaxies.
216
656875
3250
11:13
The first is that we can make
maps of the universe with galaxies.
maps of the universe with galaxies.
217
661208
4893
11:18
I am part of a survey
called the Dark Energy Survey,
called the Dark Energy Survey,
218
666125
2726
11:20
which has made the largest map
of the universe so far.
of the universe so far.
219
668875
3208
11:25
We measured the positions and shapes
of 100 million galaxies
of 100 million galaxies
220
673167
5434
11:30
over one-eighth of the sky.
221
678625
2292
11:34
And this map is showing us all the matter
in this region of the sky,
in this region of the sky,
222
682458
5435
11:39
which is inferred by the light
distorted from these 100 million galaxies.
distorted from these 100 million galaxies.
223
687917
5625
11:46
The light distorted from all of the matter
224
694708
3060
11:49
that was between those galaxies and us.
225
697792
3208
11:54
The gravity of the matter is strong enough
to bend the path of light.
to bend the path of light.
226
702333
4643
11:59
And it gives us this image.
227
707000
3667
12:04
So these kinds of maps
228
712250
1434
12:05
can tell us about how much
dark matter there is,
dark matter there is,
229
713708
3060
12:08
they also tell us where it is
230
716792
2267
12:11
and how it changes over time.
231
719083
1917
12:14
So we're trying to learn
about what the universe is made of
about what the universe is made of
232
722583
4185
12:18
on the very largest scales.
233
726792
2309
12:21
It turns out that the tiniest
galaxies in the universe
galaxies in the universe
234
729125
4726
12:25
provide some of the best clues.
235
733875
3143
12:29
So why is that?
236
737042
1250
12:31
Here are two example simulated universes
237
739375
3434
12:34
with two different kinds of dark matter.
238
742833
2601
12:37
Both of these pictures
are showing you a region
are showing you a region
239
745458
2226
12:39
around a galaxy like the Milky Way.
240
747708
2893
12:42
And you can see that there's a lot
of other material around it,
of other material around it,
241
750625
2976
12:45
little small clumps.
242
753625
1684
12:47
Now, in the image on the right,
243
755333
2185
12:49
dark matter particles are moving slower
than they are in the one on the left.
than they are in the one on the left.
244
757542
5184
12:54
If those dark matter particles
are moving really fast,
are moving really fast,
245
762750
3101
12:57
then the gravity in small clumps
is not strong enough
is not strong enough
246
765875
2809
13:00
to slow those fast particles down.
247
768708
2435
13:03
And they keep going.
248
771167
1267
13:04
They never collapse
into these small clumps.
into these small clumps.
249
772458
2476
13:06
So you end up with fewer of them
than in the universe on the right.
than in the universe on the right.
250
774958
4768
13:11
If you don't have those small clumps,
251
779750
1934
13:13
then you get fewer small galaxies.
252
781708
2709
13:18
If you look up at the southern sky,
253
786208
1685
13:19
you can actually see
two of these small galaxies,
two of these small galaxies,
254
787917
3351
13:23
the largest of the small galaxies
that are orbiting our Milky Way,
that are orbiting our Milky Way,
255
791292
3476
13:26
the Large Magellanic Cloud
and the Small Magellanic Cloud.
and the Small Magellanic Cloud.
256
794792
3000
13:30
In the last several years,
257
798958
1268
13:32
we have detected a whole bunch more
even smaller galaxies.
even smaller galaxies.
258
800250
3351
13:35
This is an example of one of them
259
803625
1809
13:37
that we detected
with the same dark energy survey
with the same dark energy survey
260
805458
2935
13:40
that we used to make maps of the universe.
261
808417
3101
13:43
These really small galaxies,
262
811542
2226
13:45
some of them are extremely small.
263
813792
1958
13:48
Some of them have as few
as a few hundred stars,
as a few hundred stars,
264
816458
2935
13:51
compared to the few hundred
billion stars in our Milky Way.
billion stars in our Milky Way.
265
819417
3642
13:55
So that makes them really hard to find.
266
823083
2810
13:57
But in the last decade,
267
825917
1892
13:59
we've actually found
a whole bunch more of these.
a whole bunch more of these.
268
827833
2685
14:02
We now know of 60 of these tiny galaxies
269
830542
2726
14:05
that are orbiting our own Milky Way.
270
833292
3017
14:08
And these little guys
are a big clue to dark matter.
are a big clue to dark matter.
271
836333
3584
14:12
Because just the existence
of these galaxies tells us
of these galaxies tells us
272
840833
3810
14:16
that dark matter
can't be moving very fast,
can't be moving very fast,
273
844667
2809
14:19
and not much can be happening
when it runs into normal matter.
when it runs into normal matter.
274
847500
3917
14:24
In the next several years,
275
852583
1435
14:26
we're going to make much more
precise maps of the sky.
precise maps of the sky.
276
854042
3666
14:31
And those will help refine our movies
277
859292
2434
14:33
of the whole universe
and the entire galaxy.
and the entire galaxy.
278
861750
3000
14:37
Physicists are also making new,
more sensitive experiments
more sensitive experiments
279
865708
4060
14:41
to try to catch some sign
of dark matter in their laboratories.
of dark matter in their laboratories.
280
869792
3833
14:47
Dark matter is still a huge mystery.
281
875042
3267
14:50
But it's a really exciting time
to be working on it.
to be working on it.
282
878333
3250
14:54
We have really clear evidence it exists.
283
882750
2351
14:57
From the scale of the smallest galaxies
284
885125
2393
14:59
to the scale of the whole universe.
285
887542
2583
15:03
Will we actually find it
and figure out what it is?
and figure out what it is?
286
891458
3500
15:08
I have no idea.
287
896542
1767
15:10
But it's going to be
a lot of fun to find out.
a lot of fun to find out.
288
898333
2935
15:13
We have a lot of possibilities
for discovery,
for discovery,
289
901292
2392
15:15
and we definitely will learn more
about what it is doing
about what it is doing
290
903708
3476
15:19
and about what it isn't.
291
907208
1584
15:21
Regardless of whether we find
that particle anytime soon,
that particle anytime soon,
292
909833
3060
15:24
I hope I have convinced you
293
912917
1726
15:26
that this mystery is actually
really close to home.
really close to home.
294
914667
4059
15:30
The search for dark matter
295
918750
1518
15:32
may just be the key to a whole new
understanding of physics
understanding of physics
296
920292
3309
15:35
and our place in the universe.
297
923625
1851
15:37
Thank you.
298
925500
1268
15:38
(Applause)
299
926792
3750
ABOUT THE SPEAKER
Risa Wechsler - Astrophysicist, cosmologistRisa Wechsler uses computer simulations of the entire universe to explore questions about our existence on the largest scales.
Why you should listen
Risa Wechsler is the director of the Kavli Institute for Particle Astrophysics and Cosmology and a professor of physics at Stanford University. She is currently playing a leading role in mapping out tens of billions of galaxies over the last 13 billion years. Her research combines these maps with computer simulations of the entire universe to understand how galaxies form and to figure out the nature of dark matter and dark energy.
Wechsler is a fellow of the American Physical Society as well as a former NASA Hubble fellow, and has written about and discussed science in numerous public venues, from Teen Vogue to the BBC.
Risa Wechsler | Speaker | TED.com