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← The search for dark matter -- and what we've found so far

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Showing Revision 8 created 01/09/2020 by Brian Greene.

  1. Do you ever think about what would happen
  2. if the world were a little bit different?
  3. How your life would be different
  4. if you were born 5,000 years from now
  5. instead of today?
  6. How history would be different
  7. if the continents
    were at different latitudes
  8. or how life in the Solar system
    would have developed
  9. if the Sun were 10 percent larger.
  10. Well, playing with these
    kinds of possibilities

  11. is what I get to do for a living
  12. but with the entire universe.
  13. I make model universes in a computer.
  14. Digital universes that have
    different starting points
  15. and are made of different amounts
    of different kinds of material.
  16. And then I compare
    these universes to our own
  17. to see what it is made of
    and how it evolved.
  18. This process of testing models
    with measurements of the sky

  19. has taught us a huge amount
    about our universe so far.
  20. One of the strangest
    things we have learned
  21. is that most of the material
    in the universe
  22. is made of something
    entirely different than you and me.
  23. But without it,
  24. the universe as we know it wouldn't exist.
  25. Everything we can see with telescopes
  26. makes up just about 15 percent
    of the total mass in the universe.
  27. Everything else, 85 percent of it,
  28. doesn't emit or absorb light.
  29. We can't see it with our eyes,
  30. we can't detect it with radio waves
  31. or microwaves or any other kind of light.
  32. But we know it is there
  33. because of its influence
    on what we can see.
  34. It's a little bit like,

  35. if you wanted to map
    the surface of our planet
  36. and everything on it
  37. using this picture of the Earth
    from space at night.
  38. You get some clues
    from where the light is,
  39. but there's a lot that you can't see,
  40. everything from people
    to mountain ranges.
  41. And you have to infer what is there
    from these limited clues.
  42. We call this unseen stuff "dark matter."
  43. Now, a lot of people
    have heard of dark matter,

  44. but even if you have heard of it,
  45. it probably seems abstract,
  46. far away, probably even irrelevant.
  47. Well, the interesting thing is,
  48. dark matter is all around us
  49. and probably right here.
  50. In fact, dark matter particles
  51. are probably going through
    your body right now
  52. as you sit in this room.
  53. Because we are on Earth
  54. and Earth is spinning around the Sun,
  55. and the Sun is hurtling through our galaxy
  56. at about half a million miles per hour.
  57. But dark matter doesn't bump into us,
  58. it just goes right through us.
  59. So how do we figure out more about this?

  60. What is it,
  61. and what does it have to do
    with our existence?
  62. Well, in order to figure out
    how we came to be,
  63. we first need to understand
    how our galaxy came to be.
  64. This is a picture of our galaxy,
    the Milky Way, today.
  65. What did it look like
    10 billion years in the past
  66. or what would it look like
    10 billion years in the future?
  67. What about the stories
  68. of the hundreds of millions
    of other galaxies
  69. that we've already mapped out
    with large surveys of the sky?
  70. How would their histories be different
  71. if the universe was made of something else
  72. or if there was more or less matter in it?
  73. So the interesting thing
    about these model universes
  74. is that they allow us
    to test these possibilities.
  75. Let's go back to the first
    moment of the universe --

  76. just a fraction of a second
    after the big bang.
  77. In this first moment,
  78. there was no matter at all.
  79. The universe was expanding very fast.
  80. And quantum mechanics tells us
  81. that matter is being created and destroyed
  82. all the time, in every moment.
  83. At this time, the universe
    was expanding so fast
  84. that the matter that got created
    couldn't get destroyed.
  85. And thus we think that all of the matter
    was created during this time.
  86. Both the dark matter
  87. and the regular matter
    that makes up you and me.
  88. Now, let's go a little bit further

  89. to a time after the matter was created,
  90. after protons and neutrons formed,
  91. after hydrogen formed,
  92. about 400,000 years after the big bang.
  93. The universe was hot and dense
    and really smooth
  94. but not perfectly smooth.
  95. This image, taken with a space telescope
    called the Planck satellite,
  96. shows us the temperature of the universe
  97. in all directions.
  98. And what we see
  99. is that there were places
    that were a little bit hotter
  100. and denser than others.
  101. The spots in this image
  102. represent places where there was
    more or less mass in the early universe.
  103. Those spots got big because of gravity.

  104. The universe was expanding
    and getting less dense overall
  105. over the last 13.8 billion years.
  106. But gravity worked hard in those spots
  107. where there was a little bit more mass
  108. and pulled more and more mass
    into those regions.
  109. Now, all of this
    is a little hard to imagine,

  110. so let me just show you
    what I am talking about.
  111. Those computer models I mentioned
    allow us to test these ideas,
  112. so let's take a look at one of them.
  113. This movie, made by my research group,
  114. shows us what happened to the universe
    after its earliest moments.
  115. You see the universe
    started out pretty smooth,
  116. but there were some regions
  117. where there was
    a little bit more material.
  118. Gravity turned on
    and brought more and more mass
  119. into those spots that started out
    with a little bit extra.
  120. Over time,
  121. you get enough stuff in one place
  122. that the hydrogen gas,
  123. which was initially well mixed
    with the dark matter,
  124. starts to separate from it,
  125. cool down, form stars,
  126. and you get a small galaxy.
  127. Over time, over billions
    and billions of years,
  128. those small galaxies crash into each other
  129. and merge and grow
    to become larger galaxies,
  130. like our own galaxy, the Milky Way.
  131. Now, what happens
    if you don't have dark matter?

  132. If you don't have dark matter,
  133. those spots never get clumpy enough.
  134. It turns out, you need at least
    a million times the mass of the Sun
  135. in one dense region,
  136. before you can start forming stars.
  137. And without dark matter,
  138. you never get enough stuff in one place.
  139. So here, we're looking
    at two universes, side by side.

  140. In one of them you can see
  141. that things get clumpy quickly.
  142. In that universe,
  143. it's really easy to form galaxies.
  144. In the other universe,
  145. the things that start out
    like small clumps,
  146. they just stay really small.
  147. Not very much happens.
  148. In that universe,
    you wouldn't get our galaxy.
  149. Or any other galaxy.
  150. You wouldn't get the Milky Way,
  151. you wouldn't get the Sun,
  152. you wouldn't get us.
  153. We just couldn't exist in that universe.
  154. OK, so this crazy stuff, dark matter,

  155. it's most of the mass in the universe,
  156. it's going through us right now,
    we wouldn't be here without it.
  157. What is it?
  158. Well, we have no idea.
  159. (Laughter)

  160. But we have a lot of educated guesses,

  161. and a lot of ideas
    for how to find out more.
  162. So, most physicists think
    that dark matter is a particle,
  163. similar in many ways to the subatomic
    particles that we know of,
  164. like protons and neutrons and electrons.
  165. Whatever it is,
  166. it behaves very similarly
    with respect to gravity.
  167. But it doesn't emit or absorb light,
  168. and it goes right through normal matter,
  169. as if it wasn't even there.
  170. We'd like to know what particle it is.
  171. For example, how heavy is it?
  172. Or, does anything at all happen
    if it interacts with normal matter?
  173. Physicists have lots of great ideas
    for what it could be,
  174. they're very creative.
  175. But it's really hard,
  176. because those ideas span a huge range.
  177. It could be as small
    as the smallest subatomic particles,
  178. or it could be as large
    as the mass of 100 Suns.
  179. So, how do we figure out what it is?

  180. Well, physicists and astronomers
  181. have a lot of ways
    to look for dark matter.
  182. One of the things we're doing
    is building sensitive detectors
  183. in deep underground mines,
  184. waiting for the possibility
  185. that a dark matter particle,
    which goes through us and the Earth,
  186. would hit a denser material
  187. and leave behind
    some trace of its passage.
  188. We're looking for dark matter in the sky,
  189. for the possibility
    that dark matter particles
  190. would crash into each other
  191. and create high-energy light
    that we could see
  192. with special gamma-ray telescopes.
  193. We're even trying to make
    dark matter here on Earth,
  194. by smashing particles together
    and looking for what happens,
  195. using the Large Hadron
    Collider in Switzerland.
  196. Now, so far,

  197. all of these experiments
    have taught us a lot
  198. about what dark matter isn't
  199. (Laughter)

  200. but not yet what it is.

  201. There were really good ideas
    that dark matter could have been,
  202. that these experiments would have seen.
  203. And they didn't see them yet,
  204. so we have to keep looking
    and thinking harder.
  205. Now, another way to get a clue
    to what dark matter is

  206. is to study galaxies.
  207. We already talked about
  208. how our galaxy and many other galaxies
    wouldn't even be here
  209. without dark matter.
  210. Those models also make predictions
  211. for many other things about galaxies:
  212. How they're distributed in the universe,
  213. how they move,
  214. how they evolve over time.
  215. And we can test those predictions
    with observations of the sky.
  216. So let me just give you two examples

  217. of these kinds of measurements
    we can make with galaxies.
  218. The first is that we can make
    maps of the universe with galaxies.
  219. I am part of a survey
    called the Dark Energy Survey,
  220. which has made the largest map
    of the universe so far.
  221. We measured the positions and shapes
    of 100 million galaxies
  222. over one-eighth of the sky.
  223. And this map is showing us all the matter
    in this region of the sky,
  224. which is inferred by the light
    distorted from these 100 million galaxies.
  225. The light distorted from all of the matter
  226. that was between those galaxies and us.
  227. The gravity of the matter is strong enough
    to bend the path of light.
  228. And it gives us this image.
  229. So these kinds of maps
  230. can tell us about how much
    dark matter there is,
  231. they also tell us where it is
  232. and how it changes over time.
  233. So we're trying to learn
    about what the universe is made of

  234. on the very largest scales.
  235. It turns out that the tiniest
    galaxies in the universe
  236. provide some of the best clues.
  237. So why is that?
  238. Here are two example simulated universes

  239. with two different kinds of dark matter.
  240. Both of these pictures
    are showing you a region
  241. around a galaxy like the Milky Way.
  242. And you can see that there's a lot
    of other material around it,
  243. little small clumps.
  244. Now, in the image on the right,
  245. dark matter particles are moving slower
    than they are in the one on the left.
  246. If those dark matter particles
    are moving really fast,
  247. then the gravity in small clumps
    is not strong enough
  248. to slow those fast particles down.
  249. And they keep going.
  250. They never collapse
    into these small clumps.
  251. So you end up with fewer of them
    than in the universe on the right.
  252. If you don't have those small clumps,
  253. then you get fewer small galaxies.
  254. If you look up at the southern sky,
  255. you can actually see
    two of these small galaxies,
  256. the largest of the small galaxies
    that are orbiting our Milky Way,
  257. the Large Magellanic Cloud
    and the Small Magellanic Cloud.
  258. In the last several years,

  259. we have detected a whole bunch more
    even smaller galaxies.
  260. This is an example of one of them
  261. that we detected
    with the same dark energy survey
  262. that we used to make maps of the universe.
  263. These really small galaxies,
  264. some of them are extremely small.
  265. Some of them have as few
    as a few hundred stars,
  266. compared to the few hundred
    billion stars in our Milky Way.
  267. So that makes them really hard to find.
  268. But in the last decade,
  269. we've actually found
    a whole bunch more of these.
  270. We now know of 60 of these tiny galaxies
  271. that are orbiting our own Milky Way.
  272. And these little guys
    are a big clue to dark matter.
  273. Because just the existence
    of these galaxies tells us
  274. that dark matter
    can't be moving very fast,
  275. and not much can be happening
    when it runs into normal matter.
  276. In the next several years,

  277. we're going to make much more
    precise maps of the sky.
  278. And those will help refine our movies
  279. of the whole universe
    and the entire galaxy.
  280. Physicists are also making new,
    more sensitive experiments
  281. to try to catch some sign
    of dark matter in their laboratories.
  282. Dark matter is still a huge mystery.

  283. But it's a really exciting time
    to be working on it.
  284. We have really clear evidence it exists.
  285. From the scale of the smallest galaxies
  286. to the scale of the whole universe.
  287. Will we actually find it
    and figure out what it is?
  288. I have no idea.
  289. But it's going to be
    a lot of fun to find out.
  290. We have a lot of possibilities
    for discovery,
  291. and we definitely will learn more
    about what it is doing
  292. and about what it isn't.
  293. Regardless of whether we find
    that particle anytime soon,
  294. I hope I have convinced you
  295. that this mystery is actually
    really close to home.
  296. The search for dark matter
  297. may just be the key to a whole new
    understanding of physics
  298. and our place in the universe.
  299. Thank you.

  300. (Applause)