--> Display at 01:00:00:06. --> Display at 01:00:06:04 --> Erase at 01:00:11:19 Funding for this program is provided by Annenberg Media.

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1 --> Display at 01:00:00:06 --> Display at 01:00:06:04 --> Erase at 01:00:11:19 Funding for this program is provided by Annenberg Media. --> Display at 01:00:26:13 NARRATOR: Most of the matter in the universe is not made up --> Display at 01:00:31:01 of the normal matter that makes up stars, gas, --> Display at 01:00:34:23 dust, planets, and people. --> Display at 01:00:37:03 RICK GAITSKELL: It's something else. --> Display at 01:00:38:19 It's known, for want of a better word, --> Display at 01:00:40:22 as dark matter. --> Display at 01:00:42:04 DOUG FINKBEINER: What is dark matter? --> Display at 01:00:44:03 You lose sleep over this stuff. --> Display at 01:00:45:26 NARRATOR: Scientists are determined --> Display at 01:00:48:17 to find out what dark matter is. --> Display at 01:00:51:02 And much of their research --> Display at 01:00:53:06 is pointing them towards a particular --> Display at 01:00:55:01 non-luminous particle.

2 --> Display at 01:00:56:14 GAITSKELL: I think it's fair to say --> Display at 01:00:58:18 that the most favored candidate, which seems to be --> Display at 01:01:01:17 extremely strongly supported by particle theorists --> Display at 01:01:04:24 that we currently have, is what's known --> Display at 01:01:07:13 as the WIMP -- the weakly interacting massive particle. --> Display at 01:01:11:20 So, if here's... --> Display at 01:01:13:12 NARRATOR: Doug Finkbeiner --> Display at 01:01:15:04 of the Harvard-Smithsonian Center for Astrophysics --> Display at 01:01:18:10 may have found evidence of WIMPs --> Display at 01:01:20:10 colliding in the form of an unexplained brightness --> Display at 01:01:23:16 near the center of the Milky Way, --> Display at 01:01:25:24 known as the "Haze." --> Display at 01:01:28:08 FINKBEINER: This is the money plot. --> Display at 01:01:30:05 --> Erase at 01:01:32:16 Do I actually think it's dark matter? --> Display at 01:01:36:27 Well... --> Display at 01:01:38:28

3 just between you and me... [ Laughs ] --> Display at 01:01:41:21 [ Beeping ] --> Display at 01:01:43:23 Here we go. --> Display at 01:01:45:03 NARRATOR: Rick Gaitskell of Brown University is working --> Display at 01:01:48:06 to design an experiment to be assembled --> Display at 01:01:50:17 nearly one mile below the ground, --> Display at 01:01:53:18 in an effort to block out enough background noise --> Display at 01:01:56:12 that they will be able to detect WIMPs --> Display at 01:01:59:01 interacting with normal matter. --> Display at 01:02:00:24 GAITSKELL: We are building --> Display at 01:02:02:10 what will be the most sensitive dark-matter detector --> Display at 01:02:05:07 in the world, and therefore, --> Display at 01:02:06:23 if there is a dark matter signal out there, --> Display at 01:02:09:06 there is every prospect of our seeing it. --> Display at 01:02:12:01 NARRATOR: Two scientists -- one looking to the cosmos,

4 --> Display at 01:02:17:09 the other going deep underground -- --> Display at 01:02:20:09 both with the possibility --> Display at 01:02:21:25 of revealing new information about what the majority --> Display at 01:02:24:27 --> Erase at 01:02:27:11 of the matter in the universe is made of. --> Display at 01:02:35:01 In the early 1930s, while observing --> Display at 01:02:39:05 the 1,000 or so galaxies --> Display at 01:02:41:01 in the Coma Cluster, --> Display at 01:02:42:12 astronomer Fritz Zwicky observed that the galaxies --> Display at 01:02:45:09 at the edge of the cluster were orbiting its center of mass --> Display at 01:02:48:15 much more quickly than predicted, --> Display at 01:02:50:14 unless the cluster contained a lot of matter --> Display at 01:02:53:02 in regions not emitting visible light. --> Display at 01:02:56:22 Zwicky suggested that further study would be in order. --> Display at 01:03:01:12 Zwicky's research lay virtually untouched until the 1970s, --> Display at 01:03:07:09 when astronomers Vera Rubin and Kent Ford,

5 --> Display at 01:03:10:05 while observing gas clouds --> Display at 01:03:11:23 around Andromeda and other galaxies, --> Display at 01:03:14:02 detected similar discrepancies --> Display at 01:03:16:13 between the expected orbital velocities --> Display at 01:03:19:21 and the velocities they observed. --> Display at 01:03:22:16 Rubin and Ford concluded that, when studying a single galaxy, --> Display at 01:03:26:23 astronomers observe only about 1/10 of the total mass needed --> Display at 01:03:30:25 to explain how fast individual objects --> Display at 01:03:33:18 are rotating around the galactic center. --> Display at 01:03:36:14 Scientists now believe the best explanation for this --> Display at 01:03:40:15 and other observational anomalies in our universe --> Display at 01:03:44:08 is that there is some other form --> Display at 01:03:46:09 of invisible, or "dark," matter. --> Display at 01:03:48:09 No one has ever directly detected dark matter. --> Display at 01:03:53:20 But astrophysicist Doug Finkbeiner, --> Display at 01:03:56:09 like Zwicky, Rubin, and Ford before him,

6 --> Display at 01:03:59:04 may have happened upon a significant clue. --> Display at 01:04:02:04 FINKBEINER: Well, it's certainly true that the most interesting --> Display at 01:04:04:23 thing you find is often the thing you weren't looking for. --> Display at 01:04:07:21 I really wasn't looking for dark matter at all. --> Display at 01:04:09:20 I wasn't thinking about dark matter. --> Display at 01:04:11:09 I was looking at the WMAP maps. --> Display at 01:04:14:26 NARRATOR: WMAP, which stands for --> Display at 01:04:17:04 Wilkinson Microwave Anisotropy Probe, --> Display at 01:04:20:09 is a NASA satellite which produced --> Display at 01:04:22:20 the first fine-resolution full-sky map --> Display at 01:04:25:08 of the microwave sky. --> Display at 01:04:28:01 FINKBEINER: This is the sky as WMAP actually observes it. --> Display at 01:04:31:16 You see here a map of the whole sky -- --> Display at 01:04:34:06 red means more emission, blue means less. --> Display at 01:04:37:00 And there's this band across the galactic plane. --> Display at 01:04:41:25

7 Since we're sitting in the disk of a galaxy and we look out, --> Display at 01:04:45:15 we see emission everywhere aligned with that disk, --> Display at 01:04:48:19 especially in the center of the galaxy, here. --> Display at 01:04:51:20 NARRATOR: Like looking through a dirty window, --> Display at 01:04:54:19 foreground emissions from our galaxy -- --> Display at 01:04:57:02 the red stripe across the center of the map -- --> Display at 01:04:59:24 diminish our view of the oldest light in the universe, --> Display at 01:05:03:00 the cosmic microwave background, or CMB. --> Display at 01:05:06:27 Finkbeiner and his colleagues are carefully --> Display at 01:05:10:04 peeling away the foreground signals of the Milky Way --> Display at 01:05:13:27 to build a clearer view of the CMB. --> Display at 01:05:16:04 FINKBEINER: And so I was studying --> Display at 01:05:17:23 these foreground emission mechanisms, --> Display at 01:05:19:05 trying to understand everything we can --> Display at 01:05:21:24 about this emission in the galactic plane, --> Display at 01:05:23:27 while everyone else was looking

8 --> Display at 01:05:26:01 at the extragalactic cosmological signal. --> Display at 01:05:29:00 And so this was really just kind of a sanity check --> Display at 01:05:32:24 to make sure everything made sense. --> Display at 01:05:35:10 NARRATOR: To subtract out the foreground emissions, --> Display at 01:05:38:20 Finkbeiner uses other full-sky maps --> Display at 01:05:41:04 taken at different wavelengths by different surveys. --> Display at 01:05:43:27 By using the information on these maps, he can calculate --> Display at 01:05:47:26 the equivalent microwave emissions on the WMAP --> Display at 01:05:50:18 and subtract them out. --> Display at 01:05:53:21 FINKBEINER: This is a map of infrared emission --> Display at 01:05:56:02 from dust grains all over the sky. --> Display at 01:05:58:21 So this is a full-sky survey, and you can see --> Display at 01:06:01:06 there's dust, not just in the galactic plane, --> Display at 01:06:03:20 where it appeared in the WMAP maps, --> Display at 01:06:05:25 but at high galactic latitudes, away from the plane.

9 --> Display at 01:06:08:11 So there's dust everywhere. --> Display at 01:06:10:21 So this map of dust allows us --> Display at 01:06:12:13 to subtract the microwave dust emission from WMAP. --> Display at 01:06:15:18 NARRATOR: In addition to removing the dust emissions, --> Display at 01:06:18:22 Finkbeiner also needed to remove emissions from ionized gas. --> Display at 01:06:22:24 FINKBEINER: This is a map of ionized gas. --> Display at 01:06:25:19 We can use this --> Display at 01:06:27:07 to estimate the microwave emission from ionized gas, --> Display at 01:06:30:17 which is the other most important component, --> Display at 01:06:33:13 and subtract that from the WMAP maps. --> Display at 01:06:35:23 NARRATOR: Once Finkbeiner subtracts out --> Display at 01:06:38:11 the known emissions of dust and gas from WMAP, he is left --> Display at 01:06:41:28 with this image, in which most of the emission --> Display at 01:06:44:28 in the Milky Way is masked out. --> Display at 01:06:47:03 But there is one more major emission source --> Display at 01:06:50:12 that needs to be accounted for --

10 --> Display at 01:06:52:21 synchrotron radiation. --> Display at 01:06:56:17 Synchrotron radiation is electromagnetic radiation --> Display at 01:06:59:26 that is generated as charged particles --> Display at 01:07:02:13 moving near the speed of light --> Display at 01:07:04:16 spiral around the lines of a magnetic field. --> Display at 01:07:07:12 FINKBEINER: And so these charges --> Display at 01:07:10:18 spinning around the magnetic field, they're essentially --> Display at 01:07:13:02 vibrating as they go around the loop, --> Display at 01:07:15:02 --> Erase at 01:07:17:26 and those vibrating charges are making photons. --> Display at 01:07:20:15 NARRATOR: To account for synchrotron radiation, --> Display at 01:07:23:23 Finkbeiner uses the Haslam map, --> Display at 01:07:25:27 --> Erase at 01:07:29:03 which is an all-sky survey of radio emissions. --> Display at 01:07:32:07 FINKBEINER: Synchrotron emission can produce --> Display at 01:07:34:16 electromagnetic radiation of nearly any frequency. --> Display at 01:07:37:28

11 It can be visible light, x-rays, it can be anything. --> Display at 01:07:41:09 But the Haslam map is supposed to trace --> Display at 01:07:43:13 the ordinary synchrotron emission --> Display at 01:07:45:07 from ordinary mechanisms like supernova shocks. --> Display at 01:07:48:03 So now we've taken out the dust. --> Display at 01:07:50:03 We've taken out the ionized gas. --> Display at 01:07:51:19 We also have to remove the synchrotron emission. --> Display at 01:07:54:14 NARRATOR: By subtracting out the synchrotron emission, --> Display at 01:07:57:22 Finkbeiner thought that all of the foreground emissions on WMAP --> Display at 01:08:01:18 would be accounted for, --> Display at 01:08:03:02 and expected to see a zero foreground signal. --> Display at 01:08:05:27 Instead, this is what he found. --> Display at 01:08:10:16 FINKBEINER: So one of the great things about science --> Display at 01:08:13:14 is when you're surprised, and this was definitely --> Display at 01:08:16:07 a case where I expected the WMAP foregrounds to look --> Display at 01:08:20:00 like the sum of gas, dust,

12 and synchrotron, and they don't. --> Display at 01:08:24:05 Here, we have this excess does not really look --> Display at 01:08:27:08 like the Haslam map we just looked at -- in particular, --> Display at 01:08:30:27 there's too much down here --> Display at 01:08:32:28 inside this box. --> Display at 01:08:35:10 And we call that the WMAP Haze, and it's a mystery. --> Display at 01:08:41:16 So I didn't think it could have to do with dust. --> Display at 01:08:43:15 So my first thought was ionized gas, but there are reasons --> Display at 01:08:46:25 that that's impossible -- the gas would have to be --> Display at 01:08:49:22 very hot to be able to explain --> Display at 01:08:51:20 why we didn't see it in the ionized gas map. --> Display at 01:08:54:09 One possible explanation for this excess is --> Display at 01:08:57:10 synchrotron emission from dark matter annihilation. --> Display at 01:09:00:26 NARRATOR: Annihilation is what happens --> Display at 01:09:02:29 when a particle meets its antimatter partner. --> Display at 01:09:06:11 For example, when an electron --> Display at 01:09:08:10

13 and its antimatter partner, the positron, collide, --> Display at 01:09:11:22 they annihilate, creating new particles. --> Display at 01:09:15:25 Dark matter particles could be their own antimatter partners. --> Display at 01:09:19:18 Therefore, annihilation could occur --> Display at 01:09:22:09 if two dark matter particles collide. --> Display at 01:09:25:12 This is a dark matter particle. --> Display at 01:09:27:23 NARRATOR: Greg Dobler is a post-doctoral researcher --> Display at 01:09:30:29 working with Doug Finkbeiner to better understand --> Display at 01:09:33:03 the WMAP Haze. --> Display at 01:09:35:10 These are two dark matter particles that we can't see --> Display at 01:09:37:23 at all, they don't interact with anything. --> Display at 01:09:40:23 Eventually, two of them come in and they hit each other here, --> Display at 01:09:45:05 and they annihilate through some process that we don't know. --> Display at 01:09:49:15 And in the end, there's e-plus and e-minus, --> Display at 01:09:51:27 electrons and positrons. --> Display at 01:09:53:07 And these electrons and positrons,

14 --> Display at 01:09:54:24 as soon as they're produced, they're in a magnetic field. --> Display at 01:09:57:07 And so they start spiraling around the magnetic field, --> Display at 01:09:59:20 producing synchrotron as soon as that happens. --> Display at 01:10:01:24 And that synchrotron radiation is what we think we see --> Display at 01:10:05:00 in the Haze. --> Display at 01:10:08:03 FINKBEINER: Now, you might wonder --> Display at 01:10:10:18 why this synchrotron is different from the synchrotron --> Display at 01:10:12:24 in the Haslam map that we already subtracted. --> Display at 01:10:14:20 It's very important that this has a harder spectrum. --> Display at 01:10:18:22 NARRATOR: The Haslam map measures synchrotron radiation --> Display at 01:10:23:02 at a radio frequency. --> Display at 01:10:24:25 From this data, Finkbeiner can infer the brightness --> Display at 01:10:28:06 of ordinary synchrotron radiation, such as supernovae, --> Display at 01:10:31:22 across the full microwave spectrum. --> Display at 01:10:34:06 Finkbeiner compares this level of brightness, or spectrum, --> Display at 01:10:38:04 with WMAP, which measured microwave emissions --> Display at 01:10:41:10 at five different frequencies

15 in the higher range. --> Display at 01:10:44:24 At these frequencies, WMAP shows a higher intensity --> Display at 01:10:48:22 of synchrotron radiation, or a harder spectrum, --> Display at 01:10:52:11 that cannot be explained by the Haslam map. --> Display at 01:10:55:23 FINKBEINER: And so, if you have --> Display at 01:10:57:21 a harder spectrum from something, --> Display at 01:10:59:23 it can appear in the WMAP frequencies but not be --> Display at 01:11:02:19 very bright at much lower frequencies in the Haslam map. --> Display at 01:11:05:25 And that's why subtracting off the Haslam map, we think, --> Display at 01:11:09:08 removes the ordinary synchrotron from supernovae, --> Display at 01:11:12:17 but would not remove as much --> Display at 01:11:14:13 the synchrotron from dark matter. --> Display at 01:11:16:24 NARRATOR: The next step for Finkbeiner in investigating --> Display at 01:11:20:22 if dark matter annihilation could be responsible --> Display at 01:11:24:06 for the Haze was to test if a simple model --> Display at 01:11:27:10 could produce something like he was seeing.

16 --> Display at 01:11:29:13 He started by considering a candidate for dark matter --> Display at 01:11:33:04 called a WIMP. --> Display at 01:11:34:27 FINKBEINER: So a WIMP is --> Display at 01:11:36:13 a really cool idea about what the dark matter could be. --> Display at 01:11:39:12 WIMP stands for weakly interacting massive particle, --> Display at 01:11:42:29 where "weakly" here is technical jargon, so it refers --> Display at 01:11:45:28 to the weak nuclear force, as opposed to --> Display at 01:11:48:09 the strong nuclear force or electromagnetism or gravity. --> Display at 01:11:51:13 Those are the forces of nature. --> Display at 01:11:52:28 So weakly interacting, massive -- as if it's -- --> Display at 01:11:56:26 you know, it's not really light, it's massive, --> Display at 01:12:00:24 much more massive than a proton, say. --> Display at 01:12:03:16 And then particle. --> Display at 01:12:05:15 If this is really just in the... --> Display at 01:12:07:07 NARRATOR: In order to calculate --> Display at 01:12:08:27 if WIMP annihilation could produce the Haze,

17 --> Display at 01:12:11:04 Finkbeiner and Dobler need to know the density of dark matter --> Display at 01:12:15:03 throughout the galaxy --> Display at 01:12:17:02 and the rate at which these particles --> Display at 01:12:18:18 are annihilating. --> Display at 01:12:20:17 DOBLER: On average, annihilation --> Display at 01:12:23:00 never happens in the universe -- it happens very, very rarely. --> Display at 01:12:27:03 The dark matter particles, in order to annihilate, --> Display at 01:12:32:09 have to get very close to each other. --> Display at 01:12:33:27 In certain regions, like towards the galactic center, --> Display at 01:12:36:23 where the dark matter density is thought to be very high, --> Display at 01:12:39:25 the idea is that this can happen much more often. --> Display at 01:12:42:23 Still not very often, but more often. --> Display at 01:12:45:00 And one of the reasons we think about dark matter is because --> Display at 01:12:48:06 if you take your simplest idea for what --> Display at 01:12:50:13 the dark matter particle is, --> Display at 01:12:51:29 and your simplest idea for how it's distributed in the galaxy, --> Display at 01:12:56:15

18 the resulting synchrotron emission that you get --> Display at 01:13:00:15 is roughly consistent, within a factor of 10 or so, --> Display at 01:13:04:22 with what we observe from WMAP. --> Display at 01:13:07:16 So in the end, what you would expect to see would be --> Display at 01:13:10:16 a glowing ball of synchrotron towards the galactic center. --> Display at 01:13:13:25 FINKBEINER: So the Haze is not proof of anything. --> Display at 01:13:19:23 We are not looking at a picture of dark matter here. --> Display at 01:13:22:23 Dark matter is dark, so you can't see it. --> Display at 01:13:24:16 This is a picture of microwaves from electrons and positrons --> Display at 01:13:29:16 that might come from dark matter annihilation. --> Display at 01:13:33:16 NARRATOR: Finkbeiner and his colleagues will continue --> Display at 01:13:37:13 testing the Haze model with new data. --> Display at 01:13:40:09 A new full-sky survey from the Planck satellite will give --> Display at 01:13:43:28 a much more detailed look at microwave emissions than WMAP. --> Display at 01:13:47:21 FINKBEINER: So then you'd like this to fall off... --> Display at 01:13:50:17 NARRATOR: And already,

19 they are analyzing data --> Display at 01:13:52:08 from the Fermi Gamma-ray Telescope, which has provided --> Display at 01:13:55:13 the first full-sky gamma-ray picture of the universe. --> Display at 01:13:59:02 Extracting the foreground emissions from the Fermi data --> Display at 01:14:02:09 is the next step for Finkbeiner --> Display at 01:14:04:02 in investigating if dark matter annihilation --> Display at 01:14:06:19 could be responsible for the Haze. --> Display at 01:14:08:22 FINKBEINER: In these maps, what you see --> Display at 01:14:11:00 is a little white fuzziness at the center. --> Display at 01:14:13:13 So, if there is dark matter annihilation going on, --> Display at 01:14:16:23 you would expect something to look like this. --> Display at 01:14:19:25 But there may be other explanations. --> Display at 01:14:21:22 NARRATOR: But even as Finkbeiner examines --> Display at 01:14:25:10 the latest images from telescopes that might give --> Display at 01:14:28:09 more definitive evidence that dark matter is annihilating --> Display at 01:14:30:15 at the center of the galaxy, other experiments are still

20 --> Display at 01:14:34:01 necessary to confirm its existence. --> Display at 01:14:36:22 FINKBEINER: Approaching this problem --> Display at 01:14:38:09 of dark matter from astrophysics first --> Display at 01:14:40:26 is kind of putting the cart ahead of the horse. --> Display at 01:14:43:27 [ Beeping ] --> Display at 01:14:45:29 Right, well, I think the next big thing to happen --> Display at 01:14:50:06 is underground direct detection experiments. --> Display at 01:14:53:00 If they show WIMPs actually scattering, --> Display at 01:14:55:19 --> Erase at 01:14:59:09 then we can really start to piece together the whole story. --> Display at 01:15:01:04 NARRATOR: One such experiment is being conducted --> Display at 01:15:03:22 at the abandoned Home State gold mine --> Display at 01:15:05:27 near Deadwood, South Dakota. --> Display at 01:15:08:17 Rick Gaitskell of Brown University is one member --> Display at 01:15:12:07 of a team of scientists and researchers --> Display at 01:15:15:06 attempting to strike

21 particle-physics gold. --> Display at 01:15:18:12 He is overseeing a massive operation -- --> Display at 01:15:22:06 actually, two massive operations -- --> Display at 01:15:24:17 one above ground and one 4,850 feet below, --> Display at 01:15:29:26 both working on the construction --> Display at 01:15:32:14 of the Large Underground Xenon Detector, or LUX Detector, --> Display at 01:15:37:08 which, hopefully, could detect, for the first time ever, --> Display at 01:15:40:14 a dark matter particle. --> Display at 01:15:43:12 GAITSKELL: We are building what will be the most sensitive --> Display at 01:15:46:08 dark matter detector in the world, and therefore, --> Display at 01:15:49:01 there is every prospect of us, if there is --> Display at 01:15:51:09 a dark matter signal out there, of us seeing it. --> Display at 01:15:53:24 NARRATOR: Above, they are assembling the detector itself, --> Display at 01:15:58:12 calibrating and testing the instrumentation, --> Display at 01:16:01:05 before the whole thing is moved nearly a mile below ground, --> Display at 01:16:07:21 where they are preparing

22 an abandoned mine --> Display at 01:16:11:02 to become a sophisticated particle physics laboratory. --> Display at 01:16:15:19 For Gaitskell, detecting dark matter particles --> Display at 01:16:19:14 has been a career-long endeavor. --> Display at 01:16:22:14 This is an appalling admission, because it means --> Display at 01:16:24:16 for the last 20 years, I've been working the same field. --> Display at 01:16:26:19 I actually went -- I'd done a lot of reading, --> Display at 01:16:30:28 was aware -- or became aware of this question --> Display at 01:16:33:04 of the missing mass of the universe, --> Display at 01:16:35:00 the dark matter problem, and went straight in --> Display at 01:16:37:06 as a graduate student, developing --> Display at 01:16:41:02 a detector in order to look directly for dark matter. --> Display at 01:16:44:11 And that, in fact, is where --> Display at 01:16:45:29 I find myself still, 20 years later. --> Display at 01:16:48:06 And at the time I went into the field, we -- --> Display at 01:16:50:25 based on the theories we had at that time,

23 --> Display at 01:16:53:00 we thought we would solve this problem --> Display at 01:16:55:00 within five years, at the outside. --> Display at 01:16:58:02 NARRATOR: What has eluded Gaitskell for decades --> Display at 01:17:01:13 may be the same hypothetical class of matter --> Display at 01:17:04:16 that Doug Finkbeiner suspects is responsible for the Haze -- --> Display at 01:17:08:14 the WIMP. --> Display at 01:17:11:22 WIMPs, if they exist, are invisible particles --> Display at 01:17:14:23 with no charge, that rarely interact with ordinary matter. --> Display at 01:17:18:14 They are expected to have --> Display at 01:17:20: to 1,000 times the mass of a proton. --> Display at 01:17:24:08 WIMPs are a good candidate for dark matter, because they --> Display at 01:17:27:15 fit naturally into a model of how our universe formed --> Display at 01:17:30:17 into its current structure of galaxy clusters and galaxies. --> Display at 01:17:34:09 GAITSKELL: So, in order to have --> Display at 01:17:36:13 the tremendous wealth of structure we see today --> Display at 01:17:39:12

24 in the form of galaxies and galaxy clusters, --> Display at 01:17:42:07 the quantity of models that we have derived --> Display at 01:17:45:14 say that you need a thing which is very ready --> Display at 01:17:48:28 to sort of fall in and make structure, --> Display at 01:17:52:09 as gravity worked its process over the billions of years. --> Display at 01:17:57:00 NARRATOR: Structure formed in the early universe --> Display at 01:17:59:29 as matter fell into slightly over-dense regions --> Display at 01:18:02:28 under the influence of gravity. --> Display at 01:18:04:24 WIMPs, because they move relatively slowly, --> Display at 01:18:07:20 compared to the speed of light, and are also massive, --> Display at 01:18:11:03 would naturally fall into these over-dense regions, --> Display at 01:18:14:13 building up the structures we observe in our universe today. --> Display at 01:18:18:11 The problem with WIMPs is that we can't see them. --> Display at 01:18:22:26 GAITSKELL: Because these WIMP particles --> Display at 01:18:25:04 are neutral and, in fact, as the acronym indicates, --> Display at 01:18:29:07 interact only very weakly

25 with other particles, --> Display at 01:18:33:02 it turns out that it is very possible for this room --> Display at 01:18:37:01 to be filled with hundreds of millions of particles --> Display at 01:18:41:13 that we're simply not aware of --> Display at 01:18:43:25 because they are so weakly interacting --> Display at 01:18:46:06 and interact so rarely with conventional matter. --> Display at 01:18:49:08 Gaitskell's goal is to document these rare interactions --> Display at 01:18:53:04 between WIMPs and conventional matter, --> Display at 01:18:56:19 a task made more challenging by other --> Display at 01:18:59:08 more commonly occurring signals --> Display at 01:19:00:27 that can lead to a false detection -- --> Display at 01:19:02:29 which is why the experiment is using the Earth --> Display at 01:19:05:26 as a natural shield and being conducted underground. --> Display at 01:19:09:20 One of the major signals that they are trying to block --> Display at 01:19:13:20 is the one produced by cosmic rays. --> Display at 01:19:16:13

26 Cosmic rays are very high-energy particles --> Display at 01:19:19:04 coming in from space. --> Display at 01:19:20:16 GAITSKELL: Standing here at the surface, --> Display at 01:19:22:22 if I hold out my hand, there are three to four cosmic rays --> Display at 01:19:26:13 traveling through it every second. --> Display at 01:19:28:25 If we were to try to operate --> Display at 01:19:30:11 the dark matter detector at the surface, --> Display at 01:19:32:20 these -- the signal deposited from these particles --> Display at 01:19:35:11 would completely screen out the dark matter signals. --> Display at 01:19:38:05 NARRATOR: But as you travel down to the depths of 4,850 feet, --> Display at 01:19:44:15 an elevator ride through nearly a mile of rock, --> Display at 01:19:47:18 that takes over 15 minutes to complete, --> Display at 01:19:50:07 this changes dramatically. --> Display at 01:19:53:17 As we stand here now, fewer than two or three cosmic rays --> Display at 01:19:57:22 are going through my hand every year. --> Display at 01:20:00:19 That substantial reduction in cosmic rays allows us

27 --> Display at 01:20:03:27 to eliminate one of the multitude of backgrounds --> Display at 01:20:07:00 that we have to beat down in order to be able --> Display at 01:20:09:28 to finally discover or see --> Display at 01:20:11:18 the dark matter events which are extremely rare. --> Display at 01:20:14:21 NARRATOR: But the cosmic rays are not the only --> Display at 01:20:17:26 background signal they are concerned with. --> Display at 01:20:20:25 Once underground, the natural radioactivity from people, --> Display at 01:20:24:08 the walls of the laboratory, and the rock itself --> Display at 01:20:27:09 are enough to make the detector go off --> Display at 01:20:29:20 hundreds of times a second. --> Display at 01:20:31:15 To eliminate this radioactivity, --> Display at 01:20:33:13 Gaitskell and his team will immerse the detector --> Display at 01:20:36:12 into an 8-meter-diameter tank of highly purified water. --> Display at 01:20:41:19 This multilayered strategy of putting the detector --> Display at 01:20:45:04 below ground and immersing it in water allows for

28 --> Display at 01:20:48:17 an extremely low background event rate in their detector. --> Display at 01:20:53:07 It is one thing to eliminate background signals. --> Display at 01:20:56:21 Gaitskell now has to figure out how to detect --> Display at 01:20:59:29 what has thus far been undetectable. --> Display at 01:21:02:23 GAITSKELL: The challenge with WIMPs is, when they interact --> Display at 01:21:06:14 with conventional material, they're moving --> Display at 01:21:09:16 at hundreds of kilometers a second, --> Display at 01:21:12:06 they weigh about the same as an atom, --> Display at 01:21:15:18 but that is just a single particle, --> Display at 01:21:17:17 and when it interacts with a target, --> Display at 01:21:19:18 the amount of energy that's deposited is extremely small. --> Display at 01:21:23:13 So our challenge was to find a material that, even though --> Display at 01:21:27:16 the volume of our detector was going to be very large, --> Display at 01:21:31:14 that we could somehow see the deposition --> Display at 01:21:33:16 of a small amount of energy in the middle of that detector.

29 --> Display at 01:21:36:11 NARRATOR: The material they chose is xenon. --> Display at 01:21:40:15 Xenon is less radioactive than other materials they could use, --> Display at 01:21:44:17 so it has a lower probability of false detections. --> Display at 01:21:47:20 It also is more sensitive --> Display at 01:21:49:14 at low energies, so it is better able --> Display at 01:21:51:26 to detect small interactions. --> Display at 01:21:53:26 The LUX team chose to use xenon in its liquid form. --> Display at 01:21:58:05 GAITSKELL: While we could have done --> Display at 01:21:59:26 the experiment with xenon gas, --> Display at 01:22:01:14 the probability of the dark matter particles, the WIMPs, --> Display at 01:22:04:28 interacting with our target is directly proportional --> Display at 01:22:08:05 to the amount of mass of the target. --> Display at 01:22:11:14 The more mass you can get, the more sensitive --> Display at 01:22:14:20 our detector is for searching for these WIMPs. --> Display at 01:22:17:27 We realized that, in order

30 to sort of most effectively get --> Display at 01:22:21:01 the most amount of mass in the smallest amount of space, --> Display at 01:22:24:03 we found that going directly to a liquid xenon --> Display at 01:22:27:06 gives us a significant increase in the amount of mass --> Display at 01:22:30:21 that we see within a certain volume. --> Display at 01:22:33:28 Are you able to get live signal? --> Display at 01:22:35:24 MAN: Yeah, by 1:00 p.m. --> Display at 01:22:37:17 NARRATOR: Another consideration --> Display at 01:22:39:02 for Gaitskell was the volume of liquid to use. --> Display at 01:22:41:29 GAITSKELL: One of the things we realized --> Display at 01:22:44:18 is that, if you take a large volume --> Display at 01:22:46:29 of xenon, one of the -- in order to look for very rare events, --> Display at 01:22:51:12 one of the great strengths this kind of detector has --> Display at 01:22:55:10 is that the outer layers of xenon --> Display at 01:22:58:08 shield or stop particles -- conventional background -- --> Display at 01:23:02:22 from getting into the central region.

31 --> Display at 01:23:05:28 So we are now constructing a detector --> Display at 01:23:08:11 which is nearly a third of a ton in mass. --> Display at 01:23:10:23 And by doing that, it means we can, for the first time, --> Display at 01:23:15:07 really get a level of absence of radioactivity in the xenon, --> Display at 01:23:18:19 in the core of the xenon, --> Display at 01:23:20:00 that will simply be unparalleled --> Display at 01:23:21:13 in previous experiments that have taken place. --> Display at 01:23:23:22 NARRATOR: Even with a detector that is as quiet --> Display at 01:23:27:07 as any that have come before, detecting a dark matter particle --> Display at 01:23:31:15 is not guaranteed. --> Display at 01:23:33:03 Most WIMPs, because they are neutral --> Display at 01:23:35:16 and are weakly interacting, will pass right through --> Display at 01:23:38:24 the liquid xenon without any interaction. --> Display at 01:23:41:16 What Gaitskell hypothesizes is that, occasionally, --> Display at 01:23:44:29 one will have

32 a very low-energy interaction --> Display at 01:23:48:00 with one of the nuclei in the atoms of the liquid xenon. --> Display at 01:23:51:24 GAITSKELL: The nucleus recoils and emits scintillation light, --> Display at 01:23:55:09 which we detect using photosensitive detectors --> Display at 01:23:58:01 at the boundaries of the container. --> Display at 01:24:01:01 NARRATOR: The photosensitive detectors --> Display at 01:24:04:13 are photomultiplier tubes, or PMTs. --> Display at 01:24:07: of them will be arranged above and below the liquid. --> Display at 01:24:13:02 At ground level, they are preparing and testing --> Display at 01:24:16:13 each individual tube for use in the detector. --> Display at 01:24:19:08 MAN: So this is what one of our PMTs looks like, --> Display at 01:24:22:15 and that's where light is detected. --> Display at 01:24:24:11 We can even measure a single photon. --> Display at 01:24:26:29 So imagine a xenon atom over here -- we have our detector. --> Display at 01:24:29:18 Here is the photomultiplier tube looking into the xenon. --> Display at 01:24:32:08 Particle comes in and gives us a flash of light.

33 --> Display at 01:24:34:26 And that flash of light, we can detect --> Display at 01:24:37:24 with these PMTs once it hits the photocathode, --> Display at 01:24:40:13 which is essentially a sensitive material that likes --> Display at 01:24:43:10 to emit an electron when a photon interacts with it. --> Display at 01:24:46:07 And that electron will give us --> Display at 01:24:48:07 a signal we can actually measure. --> Display at 01:24:50:18 NARRATOR: The signature signal of a WIMP --> Display at 01:24:53:03 is actually not one signal, but two. --> Display at 01:24:55:12 When a WIMP interacts with the xenon, --> Display at 01:24:57:27 it not only causes an initial burst of light, --> Display at 01:25:01:00 but it also ionizes the xenon atoms, --> Display at 01:25:03:15 causing electrons to be released, --> Display at 01:25:05:25 which creates another signal just microseconds later. --> Display at 01:25:09:10 The two signals will look like this on the detector.

34 --> Display at 01:25:14:19 So, in real time, the detector is quite quiet. --> Display at 01:25:17:00 There's no signal in the detector. --> Display at 01:25:18:26 And then the WIMP interacts in the xenon, --> Display at 01:25:21:15 and you get this primary scintillation pulse. --> Display at 01:25:24:10 And then the electrons are also ionized at the site, --> Display at 01:25:27:24 and you get this large, --> Display at 01:25:29:09 called the S2 pulse, the ionization signal. --> Display at 01:25:31:15 NARRATOR: This signal is easy to distinguish --> Display at 01:25:34:25 from one created by a cosmic ray. --> Display at 01:25:39:05 So the cosmic ray in the detector will have this long --> Display at 01:25:42:22 track of light, and the detector goes up like a Christmas tree. --> Display at 01:25:46:13 It's very easy to distinguish --> Display at 01:25:49:06 WIMP-like low-energy events from cosmic rays. --> Display at 01:25:53:01 MAN: Channel 1. --> Display at 01:25:54:21 NARRATOR: Gaitskell's team will continue --> Display at 01:25:57:05

35 calibrating each individual PMT by simulating WIMP events --> Display at 01:26:01:04 with LED lights. --> Display at 01:26:03:11 Eventually, they will have the entire detector --> Display at 01:26:06:07 functioning at ground level. --> Display at 01:26:08:03 MAN: That's 4,100 feet. --> Display at 01:26:10:02 NARRATOR: Then they will disassemble the whole thing --> Display at 01:26:14:11 and bring it down, piece by piece, --> Display at 01:26:17:10 4,850 feet below ground, where they will reassemble it --> Display at 01:26:22:00 and start the experiment. --> Display at 01:26:24:17 GAITSKELL: We could be seeing our first event --> Display at 01:26:26:29 in the first few days of operation of the detector, --> Display at 01:26:29:20 but we may have to wait many months --> Display at 01:26:31:19 in order to see a dark matter event. --> Display at 01:26:34:11 I think when the first few of those WIMP events come in, --> Display at 01:26:38:06 it'll certainly be an opportunity -- --> Display at 01:26:40:00 we're going to have to pinch ourselves,

36 --> Display at 01:26:41:28 after so many years of searching. --> Display at 01:26:43:19 At that point, we will begin to be able --> Display at 01:26:46:06 to tell something for the first time --> Display at 01:26:48:18 about the dark matter WIMP events themselves. --> Display at 01:26:51:14 We'll be able to tell something about their abundance, --> Display at 01:26:54:14 about their mass, which will allow us to plug this --> Display at 01:26:57:08 back into our overall model of cosmology and better understand --> Display at 01:27:00:22 how the universe and our galaxy are put together. --> Display at 01:27:03:19 NARRATOR: While Rick Gaitskell waits for data --> Display at 01:27:06:28 supporting the existence of WIMPs, --> Display at 01:27:09:17 Doug Finkbeiner examines --> Display at 01:27:11:28 the latest images from telescopes... --> Display at 01:27:13:25 FINKBEINER: True, but this is brighter. --> Display at 01:27:15:19 NARRATOR:...that might give more definitive evidence --> Display at 01:27:18:05 that dark matter is annihilating at the center of the galaxy. --> Display at 01:27:23:22 Both physicists may contribute important research

37 --> Display at 01:27:27:00 to help answer the question, --> Display at 01:27:28:25 --> Erase at 01:27:32:08 "What is most of the matter in the universe made of?" --> Display at 01:28:06:18 --> Erase at 01:28:11:23 Funding for this program is provided by Annenberg Media. --> Display at 01:28:14:25 For information about this --> Display at 01:28:16:19 and other Annenberg Media programs, --> Display at 01:28:18:27 call LEARNER --> Display at 01:28:20:17 and visit us at --> Display at 01:28:24:14