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00:00The most ambitious map in history is taking shape before our eyes.
00:11And scientists are heading for the edge.
00:17It may be the strangest map you'll ever see.
00:22And it's bigger than you can believe.
00:25It's a map of the entire universe.
00:33There's this whole pattern to the universe that we're starting to map out.
00:37Seeing it really brought home the way the universe actually behaved in a way that all the numbers and equations never quite could.
00:46Cosmologists are making sense of startling discoveries.
00:52Medieval maps would say here be monsters.
00:55They weren't entirely wrong.
00:58They're even building pictures of the invisible.
01:04How do you map something that you can't even see?
01:08Our brains build maps even where our telescopes cannot reach.
01:11This is a map of everything we know.
01:18And it's getting bigger every day.
01:22It kind of hits you how magnificent it is.
01:26It's bigger than we can actually really even imagine.
01:29The universe is so big we may never find the edge.
01:35Mapping the universe is a job for pioneers.
01:38Nick Reisinger is blazing a trail through the American Southwest.
01:40We're now exploring the world.
01:41It's not a miracle that you can't do it.
01:42It's not a dream and has a dream.
01:43I'll talk about it as well.
01:45You're going to be a viewer of lots of mystery.
01:46It's not a dream I can't have a dream.
01:47It's not a dream.
01:48It's not a dream.
01:49It's not a dream.
01:51It's not a dream.
01:53It's not a dream.
01:54Nick Reisinger is blazing a trail through the American Southwest.
02:12You have to be pretty persistent.
02:15No stopping.
02:16You've got to keep going.
02:19Nick wants to put our entire galaxy on the map.
02:24He's on a single-handed mission to photograph the Milky Way.
02:33New Mexico is a great place to take photos.
02:36It's dry, it's high, and there's not a whole lot of city around here.
02:42There's a break in the weather and you get almost a full night in.
02:46Other times you only get, you know, 10% of the night.
02:50But it's all luck of the draw.
02:51It's looking pretty good over there, actually.
03:03In the modern world, few of us have skies dark enough to see the Milky Way.
03:08But Nick plans to show us our home galaxy like we've never seen it before.
03:20I'm trying to give people that broad, big-picture understanding of the entire night sky and where they fit into that.
03:26Our galaxy has nearly half a trillion stars.
03:36Most of them are too dim and distant to see.
03:38But Nick's cameras are more than 2,000 times more sensitive than the naked eye.
03:47If I had known how much work it would be going into it, I probably wouldn't have even started.
03:54My personality is, once you start something, you finish it.
03:57After two years, he's photographed 20 million stars.
04:09By stitching together more than 37,000 separate images.
04:13Some people might be driven crazy by hearing shutters clack all night long,
04:23but it's actually music to my ears because it means they're working.
04:28By combining data from six different cameras,
04:31he's captured something that would tax even the world's most powerful telescopes.
04:35His final image is the highest-definition true-colour map ever made of the Milky Way.
04:50But he hasn't just mapped it.
04:55He's made a hand-held guide to the galaxy.
05:05This is like a window to the sky,
05:13and you can point it in any direction
05:15and be shown exactly what you're looking at.
05:24So here we're looking at the center of our galaxy.
05:26This is our Milky Way.
05:28You can see this bright cluster of many small stars.
05:35The map reveals more features with every level of detail.
05:44As we zoom in here to the center of the galaxy,
05:48I'll point out this dark patch here.
05:50This is the Pipe Nebula,
05:51and it's one of my favorite landmarks to help me orient myself.
05:59But it's the sheer size of the image that reveals its true ambition.
06:05From one side to the other,
06:15it's a hundred thousand light years.
06:17This image is such a big subject,
06:27and it makes you feel so small.
06:30A hundred thousand light years.
06:34It boggles the mind just trying to comprehend
06:36just how vast that is.
06:38But the fact is,
06:44the map of the universe has barely begun.
06:59Anthony Aguirre,
07:00from the University of California in Santa Cruz,
07:03is a theoretical cosmologist.
07:07So he's used to thinking big.
07:10Now to say that we're going to go out and make a map of the universe,
07:13it almost sounds crazy.
07:15It sounds like real hubris, right?
07:17We're going to go map the universe.
07:19And yet,
07:20the universe, as it turns out,
07:22is really amenable to mapping.
07:24But you have to think big and clever.
07:30And that's where the balloons come in.
07:34Because the map of the universe isn't like other maps.
07:39We have to think in a different way.
07:41We can't just go out and look at the universe
07:43and draw things on paper and say,
07:45there's our map of the universe.
07:46The universe is so big
07:48that the laws of physics say we can't see all of it.
07:55It's as if we're at the center of a giant balloon
07:58and we can't see out.
08:02We can only see light.
08:04And light moves at a certain speed.
08:07And so,
08:07as we look farther and farther away,
08:10we're looking at farther and farther back in time
08:12because we're seeing light coming to us from long ago.
08:17But there's only so far we can go back in time.
08:21So there's only so far we can see.
08:27It's called the observable universe.
08:31We can only map what's inside
08:33because the universe is only 13.7 billion years old.
08:39There may well be a lot more universe outside,
08:42but the light hasn't had time to reach us yet.
08:46In the last 20 years,
08:49we've seen this tremendous expansion
08:52both in the amount and in the precision of knowledge
08:55that we have about the observable universe.
08:57This has allowed cosmologists
09:02to make a map of unbelievable scale.
09:08The Milky Way could fit inside
09:1010 million,
09:12million,
09:14million,
09:14million times.
09:18Our entire galaxy is just a dot on the landscape.
09:25In the observable universe,
09:28there are 170 billion galaxies just like it.
09:31one of the most surprising features of the universe is not like making a map of the United States.
09:32It's not like making a map of the United States.
09:33It's an observational fact.
09:36It is an observational fact that it's difficult to be able to be able to achieve a thing.
09:39because it's not like the Mars Earth.
09:39it would be able to achieve that.
09:42It would have to be able to reach the two planets to be able to achieve another planet.
09:45But before she can do that, it's vital to account for one of the most surprising features
09:51of the universe.
09:56Making a map of the whole universe is not like making a map of the United States.
10:01It's an observational fact that if you look at the galaxies around us, and the most distant
10:06galaxies that we can see, they all appear to be moving away from us, and the further
10:10away they are, the faster they're moving away from us.
10:14The galaxies aren't like landmarks on normal maps.
10:19They don't stand still.
10:23Everywhere we look, the most distant galaxies are moving away from us.
10:30This is a strange universe, and the explanation is even stranger.
10:35People want to imagine a central point with everything exploding out from that point, moving
10:41away only from that one central location.
10:45And that's really the wrong picture here.
10:47That makes it sound like we're in a special place, like somehow we're at the center and
10:51everything's moving away from us.
10:53But actually it's not like that.
10:56There's nothing special about our place in the universe.
11:01If we went to another galaxy, we'd see exactly the same thing.
11:06If you went to a distant galaxy, they would have the same perspective.
11:10They would look at all the galaxies around them and see that they were moving away.
11:14You really have to try to imagine that every single point is moving away from every other point.
11:19So no point is special.
11:21No matter where you're standing in the universe, if you look out, you will see galaxies moving
11:25away from you.
11:29Think of it like cities on the map of America.
11:33If you were standing in California, you would see New York moving away from you.
11:38But from the perspective of New York, you would see Boston move away.
11:43And if you were standing in Chicago, you would see New York and California moving away from
11:47you.
11:48So no matter where you're standing, you see everything else moving away from you.
11:54In the observable universe, the galaxies are doing exactly the same thing.
12:01The only explanation for that is that the space itself is stretching, that the universe
12:06itself is getting bigger.
12:08Not that the galaxies are moving on the space, but that the space is getting bigger.
12:15It's as if the whole of America was getting bigger and bigger every day.
12:21So you'd think it would be impossible to keep the map up to date.
12:26But cosmologists take everything into account by using careful measurements of the expansion
12:32rate.
12:34It works like the scale factor on any road map.
12:41So imagine the United States is doubling every day.
12:45You wouldn't want to make a new map every day.
12:47You wouldn't draw a new map.
12:48All you would have to do really is change the legend.
12:52Instead of one mile between tick marks, the next day it would be two miles and the next
12:56day it would be four miles.
12:57And that scale changing on the side in your legend would completely account for the fact
13:02that the states kept doubling.
13:04And so you could keep your originally drawn map.
13:07The map of the observable universe doesn't change, except for the scale factor.
13:18Right now it's 46 billion light years to the edge.
13:24But it's growing all the time.
13:29So while at first this is a little confusing trying to imagine something like a universe
13:34expanding, we realize that by drawing a simple map and by changing the scale on that map,
13:39that we can handle the expansion actually quite simply.
13:42For cosmologists, the expansion of the universe is not a problem.
13:51In fact, it's a gift.
13:54If space is stretching, then the wavelength of light from the galaxies is stretching too.
14:02The greater the distance, the redder the light.
14:06This redshift effect is the mapmaker's vital tool for measuring distance.
14:17And redshift was the key to the next vital stage in mapping the universe.
14:23A survey to pinpoint the exact location of galaxies stretching five and a half billion light years
14:29from Earth.
14:41It started here, in one of the more unusual towns in America.
14:51Welcome to Cloudcroft, New Mexico.
14:58A place where you don't have to be an astronomer to map the universe.
15:05Everyone in town can have a piece of the action.
15:09To us, it's wonderful.
15:13I mean, it's just part of our everyday life.
15:18On a clear night.
15:19My husband will say, well, you're going to be busy tomorrow.
15:23Francis Cope has been working here for two and a half years.
15:27At the last count, she'd mapped a quarter of a million galaxies.
15:33It can be very therapeutic, but mostly, to me personally, it's a sense of fulfillment.
15:42Tracey Norgel trained as a mechanic, then retrained in galactic exploration.
15:50It's neat that you are a part of discovering new galaxies.
15:56It's kind of a good feeling.
15:58Kristina Hunerhoff is a freelance writer.
16:01Mm-hmm.
16:02Mapping the universe helps her wind down.
16:05It's very zen, I think, because you're putting things where they're supposed to be.
16:15They all work with this man.
16:19David Schlegel is a cosmologist from the University of California at Berkeley.
16:25When he first came to town, the map of the universe was almost empty.
16:30The only pictures that we had of the full sky were on photographic plates.
16:34Images taken by the Palomar Sky Survey in the 1950s.
16:38And actually, we were still using that in the 1990s.
16:42That was the best picture that we had of the full sky.
16:46The Palomar Survey was practically a museum piece, photographed on fragile glass negatives.
16:55Even by 1998, only 30,000 galaxies had been placed on the map of the universe.
17:06That's when David joined the Sloan Digital Sky Survey at the nearby Apache Point Observatory.
17:12We had the sense that it was going to be this great thing that was starting,
17:19but it hadn't actually started yet.
17:21What we wanted to do was something much more ambitious
17:25and actually get a map of the million brightest galaxies on the sky.
17:29The task required, measuring the distance and therefore the redshift
17:35for every single one of these galaxies.
17:38Obviously, you need to look at more than one galaxy at a time, so that's the trick.
17:43And if you were a futurist, you'd say,
17:47well, it's the 1990s, we have computers and we have robots.
17:52The folks designing the Sloan, though, decided to take the pragmatic approach
17:57and say, well, we actually want this thing to work.
18:00OK?
18:01Instead of robots, the ingenious system they came up with required a far more human touch.
18:12And they would have to go round the universe, not once, but twice.
18:21It's really doing two maps of the sky.
18:26The first time round, they didn't measure any redshifts.
18:31The telescope simply took photographs.
18:35A map of the sky, but in two dimensions only.
18:40It doesn't give the distance to each galaxy.
18:43Yet.
18:44We actually have, from those images, not very much idea of where these things are
18:50in three-dimensional space.
18:51So, at some level, it's just a pretty picture.
18:55But the next stage was the trick.
18:59They printed the pretty pictures in metal.
19:03Each of these holes corresponds to our two-dimensional location of a galaxy on the sky,
19:08where, if I look at this hole, we have the longitude in this coordinate,
19:13the latitude in this coordinate.
19:15And so, the whole design of the system is to, as efficiently as possible,
19:20get the light from that one galaxy into that specific hole.
19:26The plugging team from town connected every galaxy with a fibre optic cable.
19:33Then fitted the plate back over the telescope.
19:39Second time around, the telescope measures the redshifts
19:42for these specific galaxies alone.
19:48One thousand galaxies on a plate.
19:52Nine plates a night.
19:55And one million galaxies in total,
19:57on a map crafted by human hands.
20:00It's hard to wrap my head around the idea that we're looking at,
20:05you know, with a thousand fibres,
20:07we're looking at a thousand galaxies.
20:09And it's, I have a hard time wrapping my head around it.
20:12The universe is that big.
20:16The Sloan survey is one of the great achievements of precision cosmology.
20:21Redshift measures the distance,
20:28the third and final coordinate for every galaxy.
20:33To make a 3D movie on a colossal scale.
20:42Maybe you've seen things like this in the opening of Star Trek or Star Wars or whatever,
20:46and that all looks great, but it's not real.
20:49This movie, it is the real universe.
20:54Every point of light on the map is a galaxy like the Milky Way.
21:02Cosmologists can now see at a glance how the galaxies are arranged in space.
21:12What these maps let us do is it really allows us to test
21:16all the forces of nature that we know about.
21:19There is structure, really, on all scales.
21:24The galaxies are not just placed at random.
21:26They're bound together by gravity in a vast cosmic web.
21:34This goes on and on, and in fact,
21:36up to the larger scales that we can see,
21:38you can still trace these structures of galaxies.
21:40But the most surprising discovery is what can't be seen.
21:46The gravity due to the stuff that we see, due to, say, the galaxies and the stars, can't do the job.
22:00It's simply not enough stuff to arrange things into the patterns that we see have galaxies spinning in the way that they do.
22:10There's something else there.
22:11There's something beyond the galaxies that we see, the visible matter.
22:14There's some sort of dark matter out there.
22:16Modern cosmology needs a new kind of map maker.
22:22Because most of the universe is hiding in the dark.
22:30We don't know what dark matter is, because it's never been detected on Earth.
22:44We know it must be out there, because its gravity is holding the cosmic web of galaxies together.
22:50But we can't see it, because it doesn't give off light.
23:00Someone has to find it, and put it on the map.
23:09British astronomer Richard Massey is a master of the invisible.
23:20He's a member of a team hunting for dark matter, based at the California Institute of Technology.
23:28So he's a frequent flyer to the city of Los Angeles.
23:33When you're flying over America at night, you see these criss-crossing lanes of streetlights spread out across the continent.
23:44There's obviously some interesting stories going on down there, in between these roads.
23:50In fact, most of the story of what's going on in America is actually happening in those empty spaces that you can't see.
24:01Richard's task is like mapping those apparently empty spaces.
24:07It's as if whole cities were hiding in the dark.
24:12If we're driving across America and trying to map out a new frontier,
24:15we can see mountains and valleys and streams and we can draw them all on a map.
24:20But when we're trying to map out the universe, most of its contents are invisible.
24:25It takes imagination to find your way in a dark universe.
24:35You have to dream up new ways to detect what can't be seen.
24:38One possibility is that if dark matter doesn't give off light, maybe it absorbs light.
24:49Ordinary matter, the stuff that we're made out of, casts a shadow because it absorbs light.
24:56So we can see the ordinary matter in silhouette.
25:02Unfortunately, dark matter doesn't give itself away that easily.
25:09Light just goes straight through it.
25:14Dark matter doesn't interact with light in any way.
25:17So we can't look for its silhouette to map out where it is.
25:22We have to be a bit more ingenious about it.
25:32The solution depends on a very simple idea.
25:38It's like looking at lights in a swimming pool.
25:42The secret to mapping dark matter that you can't see is to look at the light that you can see.
25:51Everything that has mass, including dark matter, actually bends the fabric of space and time that we live in.
25:59And if space is warped, then everything in it is distorted, even the paths of light rays.
26:06The only way that dark matter might reveal itself is through gravity.
26:13According to Einstein's theory of relativity, all matter distorts space, causing light to change direction.
26:22The idea of general relativity bending space and time and deflecting rays of light sounds really complicated.
26:29But actually, you see light rays bending all the time.
26:31When you look into a swimming pool and see that your legs aren't in the right shape, you know that there must be some water in the way.
26:39The distortion of the lights depends on water ripples in the pool, which in turn depend on where the swimmers are at any one moment.
26:49This is great. We're seeing these distorted images of the lights under the pool.
27:00And by looking at the shapes of these, we can work out what the ripples in the water are doing.
27:04The survey team went looking for dark matter in exactly the same way, with a thousand hours of observations on the Hubble Space Telescope.
27:17By looking at distant galaxies halfway across the universe, by looking at their shapes and the distorted images that we see of those, we can work out what ripples are in space between them and us.
27:30And those ripples in space are caused by the dark matter.
27:33The search zone was a thin column of the universe, stretching eight billion light years from Earth.
27:48The team were on the lookout for distortions in the most distant galaxies.
27:52Whenever you see galaxies distorted into these strange, uncharacteristic shapes, you know that there must be something in between them and you, something really massive.
28:04And even if it's invisible, you can still map out where it is by the way that it warps that space-time.
28:10The mapping technique revealed a ghostly, hidden universe.
28:15The light from visible galaxies was recast in new and beautiful forms.
28:22They've become these full rings, distorted, what are known as Einstein rings, whenever there's a big lump of dark matter in front of them.
28:32The lumps become contours on a map of the invisible.
28:36They reveal dark matter as the hidden iceberg beneath the surface of the cosmic ocean.
28:45What we're finding out there in the universe is really weird.
28:50It's equivalent to the idea that only one out of six cities in America actually has any people living in it.
28:56The other five, six of the population are these invisible ghosts that we just can't see.
29:01The survey has transformed the map of the universe.
29:06It suggests that normal visible matter is just a fraction of what's out there.
29:15In the search zone, dark matter outweighs it by six to one.
29:21This is the stuff the universe is really made of.
29:26For cosmologists, the road ahead has become a lot less certain.
29:39Right now, we know the universe is expanding.
29:44But given enough dark matter, it could have a different and very dark future.
29:49It's sensible to conclude, when we look at how that stuff affects the shape of space, that the universe should be expanding, but that it should be slowing down.
30:01Dark matter puts a very heavy foot on the bricks.
30:06Because the more matter there is, the more gravity there is.
30:11Gravity attracts, and so the cosmic expansion should be slowed down by all that attraction.
30:22If there's enough dark matter, the universe will eventually stop expanding altogether, and go into reverse.
30:33Gravity will bring everything back together in a final cataclysmic big crunch.
30:49The question is, when?
30:51The search for the answer began here, on the Berkeley campus of the University of California.
31:04It's a distinctive outpost in the landscape of science, signposted with some of its greatest names.
31:12There's even a car park reserved for Nobel laureates.
31:23Nine prize winners in a row, with five in physics alone.
31:32And it was here, in 1988, that Saul Perlmutter set out to map the deceleration of the universe.
31:38There's nothing you like more than a really good mystery.
31:54I wondered if you could actually measure how much the universe is slowing down.
32:00I thought it was a very exciting possibility that you could make a measurement and find out what the fate of the universe was.
32:06Saul was the leading light behind an international team of physicists and astronomers.
32:17Under his guidance, they embarked on a ten year voyage of exploration, far across the observable universe.
32:24The key was to measure how fast the universe was expanding in the past compared to now.
32:38They planned to map ancient galaxies, 10.8 billion light years from Earth.
32:45But it would take a whole decade to find and analyze what they were looking for.
32:53A candle.
32:54If you want to measure distances across the universe, you would like to be able to use an object that's of known brightness.
33:04We call anything that we know the brightness of a standard candle.
33:09A standard candle always has the same brightness, so you can use it to measure distance very precisely.
33:17The further away it is, the dimmer it will appear in our telescopes.
33:23But candles are elusive objects.
33:25We hunt for what astronomical object could you possibly use that will behave in this very regular way so that you can actually compare the distances.
33:38The galaxies themselves are no good.
33:41They come in many different shapes and sizes.
33:44And at this distance, they're so dim we can barely see them.
33:47We're talking about distances that are even more vast than usual for astronomy.
33:54Now we need to look at some of the most distant objects in the universe, so these have to be very bright objects.
33:59Saul had a very bright idea.
34:06He would find his way by the light of a dying star.
34:11A supernova.
34:12When one of these supernovae explodes, that one star can be as bright as the entire galaxy of a hundred billion other stars.
34:26So this is a remarkably bright single event.
34:31Saul had a special kind of supernova in mind.
34:34A type 1A is triggered when a dying star draws in mass from its neighbour.
34:48Just at the point where there's a critical mass, there will be a runway from a nuclear explosion.
34:59So that means that it's triggered at the same mass every time.
35:02Same mass every time means same brightness every time.
35:09They're perfect standard candles.
35:12But Saul had to find them first.
35:17If you could work with anything else in the world besides a supernova to do your research, you would.
35:23They're just a real pain in the neck to work with.
35:26They're rare, they're random, and they're rapid.
35:32A supernova only burns brightly for three weeks.
35:37And in any given galaxy, they explode without warning roughly once every three hundred years.
35:43With those odds, you can't book valuable time on the world's best telescopes.
35:51It makes a terrible proposal if you say that sometime in the next several hundred years, a type 1A supernova might explode somewhere in this galaxy.
36:00I would like the night of March the 3rd, just in case.
36:03But Saul had a plan to get the odds working in his favor.
36:09With billions of galaxies in the observable universe, there are dozens of supernovae every night.
36:16Saul's team spent six years perfecting a new system for supernovae on demand.
36:26They took snapshots of thousands of galaxies at once, then repeated them two and a half weeks later.
36:35First you don't see a supernova.
36:38You don't see a supernova.
36:42Now you do.
36:45That's very important, that two and a half weeks, because that guarantees that everything you find that's brighter on the second night than the first is on the way up.
37:01We can now guarantee that there would not just be one type 1A supernova, but that there would be a half dozen.
37:08We can now guarantee that there would be one type 1A supernova.
37:12Saul now knew exactly where to point one of the world's most powerful telescopes.
37:17The Keck Observatory in Hawaii.
37:22He was finally ready to measure the deceleration of the universe.
37:29But by late in 1997, the team was getting some very weird results.
37:38The points were not showing up where you would expect.
37:42This was exciting.
37:46The supernovae distance measurements didn't match the predicted deceleration.
37:51We were then faced with the question, okay, what else could be going wrong?
38:00Saul and his team spent five more anxious months eliminating all possible sources of error.
38:06But by January 1998, they were finally ready to go public.
38:09The more we checked, the more we fine-tuned every little step of the calibration, the more the weird result didn't go away.
38:20The weird result has reverberated through the world of science ever since.
38:25In January 2012, Saul Perlmutter won the Nobel Prize for Physics.
38:33And booked a parking space for life.
38:37At the end, we concluded that actually the universe really isn't slowing down.
38:42It's actually speeding up in this expansion.
38:44And that was a big shock.
38:50It's been described as one of the biggest shocks in modern cosmology.
39:01This is a runaway universe.
39:04And everyone's on board.
39:07Whether we like it or not.
39:09We find out that the universe is not just expanding, but that it's getting faster and faster.
39:18The cosmological community, when this result came out, was completely incredulous.
39:22I didn't really believe it when I first heard about it.
39:25I don't even think I paid very much attention to it at the time.
39:28We know the universe doesn't look like this.
39:32There had to be something wrong with these observations.
39:34I thought they would go away.
39:36I really did.
39:39Of course I was wrong.
39:44It's sometimes really fun to be wrong.
39:51Welcome to a very new picture of the universe.
39:58But even the experts can hardly believe it's real.
40:04The most famous force in physics has met its match.
40:09Because the entire universe is defying gravity.
40:17This was saying that there was something that fills the universe and causes an anti-gravity force.
40:22Something that was causing everything to push everything else apart and to make the universe bigger and bigger in an accelerated way.
40:29Gravity acts as a brake, pulling back on the expansion of the universe.
40:36But we now know there's another, more mysterious force with its foot on the gas.
40:41What's doing the pushing, what's that force that's forcing everything apart?
40:46Well we don't know, but we did work out what to call it.
40:50We have a name for it, we call it dark energy.
40:51Cosmologists don't know what dark energy is.
40:57They only know what it does.
41:00Where gravity pulls, dark energy pushes.
41:06You don't see this stuff, you don't see it doing anything directly.
41:15Basically it's sort of this one hit wonder that just does one thing.
41:19It causes an anti-gravity force.
41:21We don't have any other handle on it.
41:23Dark energy is dark matter's dark adversary.
41:26A shadow on the entire universe.
41:34There's dark energy in the galaxy, there's dark energy.
41:37Here on earth there's dark energy passing through us right now.
41:40We're filled with this dark energy.
41:42We don't see it, we don't feel it.
41:45But it's everywhere.
41:46It's kind of just a uniform coloration to our map.
41:5073% of the universe is dark energy.
41:53But you'd never know.
41:54In everyday life this stuff is just hard to detect.
41:59Now it's true that between my two fingers there's an anti-gravity force right now.
42:03But that anti-gravity force is so incredibly minuscule that I'll never ever notice it.
42:08It's only when you get to really large scales that you really see the effect of this stuff.
42:13If I could move my fingers all the way across the universe,
42:15then they'd feel this tremendous push apart due to this dark energy.
42:19In the really big scheme of things, dark matter is fighting a losing battle.
42:28Because there's only so much of it to go around.
42:31If you add more space, if you give more place for those little pieces of matter to be,
42:40then the density of them goes down.
42:42If you just see less of it, it gets diluted.
42:46As the universe expands, dark matter thins out until it can no longer compete with dark energy.
42:53The really crucial thing about how this dark energy behaves is that it doesn't dilute.
43:00When the universe doubles in size, you've got twice as much dark energy.
43:04Make it four times as big, you've just got four times as much dark energy.
43:08Once you get to this cosmological scale, the biggest possible scale,
43:12it becomes the biggest game in town. It becomes the prime player.
43:15Dark energy is on the map.
43:19But cosmologists can't explain it.
43:23Depressing or exciting? I think it's exciting.
43:28As a map maker, this is a strange thing.
43:31We go out, we make this map, we discover this land, we've mapped it out,
43:36and we still don't know what it is.
43:38I love that.
43:39The entire observable universe is saturated in dark energy.
43:57But there's one final set of clues to be found, on its furthest edge.
44:02And it may contain the secrets to the universe beyond.
44:20We're heading off the map, into impossible territory.
44:24The edge of the observable universe is the furthest horizon our telescopes can see.
44:35But for cosmologists like Sean Carroll, that's not enough.
44:40He wants to know the size of the whole universe.
44:44I definitely think it's okay to think about parts of the universe
44:47that we can't observe and can never observe.
44:49We've done a very good job at understanding what the universe looks like in that visible portion.
44:56So now when our imaginations roam, they often sneak outside the visible portion
45:00to ask what might the universe look like beyond our visible horizon.
45:04The universe that we can't see, that's the playground for theorists now.
45:09But if we can't see the rest of the universe, how can we figure out how big it is?
45:16For Jana Levin, it's a similar task to working out the shape and size of the Earth.
45:24But there's a catch.
45:27We know we could step far from the Earth as an astronaut has.
45:31We can look down on it and see from the outside that it was a sphere and it was curved.
45:37You can't step outside of the universe. You have to do everything from inside of space.
45:44Without leaving the Earth, how do you know it's round and therefore has finite size?
45:51It could be completely flat and stretched to infinity in all directions.
45:56One way is to use a simple piece of mathematics.
46:08All you have to do is draw a triangle.
46:11If you're drawing a small enough triangle on the beach, you won't notice the curvature of the Earth.
46:23It'll look like a normal triangle. You'll be able to draw the lines pretty straight
46:28and the interior angles will look like they add up to 180 degrees.
46:32It'll look like the triangle you draw on a flat sheet of paper.
46:35But this isn't a normal triangle because the Earth's surface is curved.
46:42It's just so subtle that the sides of the triangle still look straight.
46:47It would probably be a challenge on the beach to draw it big enough
46:51that you would be able to notice the curvature of the Earth.
46:54The key is to make the curvature more obvious by drawing the biggest triangle you can.
47:00If I draw a triangle big enough that it comes from the North Pole and it wraps all the way around North America,
47:10now it's very obvious that those angles are bigger than 180 degrees
47:16and that the sides of the triangle are not straight lines.
47:22So we can show the Earth is curved and therefore has finite size without leaving it.
47:27And we can find out the shape and size of the universe in exactly the same way.
47:35By looking for triangles of light.
47:42Light will travel in a straight line if the space is flat
47:46and light itself will travel on an arc if the space is curved.
47:48These curves are going to be so subtle, more subtle than the curvature of the Earth.
47:57We really have to look back as far as we possibly can.
48:01And that means the oldest relic we have in the universe.
48:03So that means looking at things like the light left over from the Big Bang.
48:06The early universe was a hot, dense fireball.
48:21When it cooled, a pattern of light emerged at what is now the edge of the observable universe.
48:30This is the cosmic microwave background.
48:34The CMB was discovered in the 1960s.
48:43Throughout his career, Sean Carroll has been able to explore it in greater and greater detail.
48:50Waiting for triangles to emerge.
48:53It takes good technology to do it.
49:00You need better and better receivers.
49:02Less and less noise in your detector.
49:05And ultimately you need satellites to get a really good 360 degree view of the whole cosmic microwave background.
49:11It was NASA's WMAP mission in 2003 that brought the most vital contours into sharp focus.
49:25WMAP for the first time had that resolution.
49:31So when WMAP came out, we could really use those features to make a big triangle measure the geometry of space.
49:37Continents begin to appear, smaller islands, you get a finer resolution of the coastlines and so forth.
49:46The islands are miniscule temperature variations in the early universe.
49:52Less than one hundred thousandth of a degree.
49:58A distinctive feature for making triangles.
50:01These splotches we see in the microwave background appear at all different sizes.
50:15But there is a best size for them to be.
50:18There is a size at which the fluctuations are the strongest.
50:22We know how big they are.
50:24We know how far away they are.
50:26So between us and the size of a feature in the CMB, we can measure a triangle and use that to infer the geometry of space.
50:37The Earth, plus the opposite sides of the island, form the three points of a very long, thin triangle.
50:45The key to measuring whether the universe is flat or curved.
50:50If the universe were positively curved, if the angles inside the triangle added up to greater than 180 degrees, then it would be finite in size.
51:02If the spatial geometry is flat, if the angles inside the triangle add up to 180, then it could go on forever.
51:09The result is one of the greatest triumphs of modern cosmology.
51:18A miracle of precision map making that measures the angles of the triangle to the third decimal place.
51:26And it says that the universe is infinite.
51:30The answer is that Euclid was right.
51:35Space seems to us to be flat as far as we can measure.
51:42That means that the simplest picture of the universe is a universe that's infinite.
51:46We really could live in a universe where there's galaxy after galaxy after galaxy in every direction.
51:52Up, down, sideways, and it never stops.
51:55Cosmologists have found a way to picture the universe in its entirety.
52:01Confirmation of the tremendous power of making maps.
52:05It will never cease to amaze me.
52:08We human beings here on this tiny little rock are able to reach out with our instruments and our brains to understand the whole shebang.
52:17And if an infinite universe isn't big enough for you, then Saul Perlmutter has proved it's still growing.
52:26All the distances are getting bigger every day.
52:30So, it's still infinite, all the same galaxies are there, it's just that we've now pumped more space between every point in this infinite universe.
52:37That's really mind-boggling.
52:39But even this isn't the end of the story.
52:48There may be one final bizarre twist in the road.
52:53Because Anthony Aguirre thinks our universe may not be alone.
53:08Sometimes when I'm headed down the highway and I'm driving, you know, my wife will say, Anthony, you're going 40 on the highway.
53:23And then she knows that I'm thinking about other universes.
53:26He thinks there may be other universes because of the process that created our own.
53:34It's called inflation.
53:38It describes an exponential expansion in the moments after the Big Bang.
53:46At a speed the universe would never repeat again.
53:49Inflation has been a very successful theory in predicting the observed properties of our universe and how our observed universe came into being.
54:03Inflation may have started out as a mathematical theory.
54:10But it has gained acceptance after successful testing against the evidence from the cosmic microwave background.
54:17I was amazed when I saw the results come in from those satellites that reproduced all the bumps and wiggles and all the detailed properties of that microwave background that inflation predicted.
54:32Inflation explains how the observable universe developed.
54:38It was doubling its size over and over and over again in a tiny fraction of a second.
54:41Going from something like a billionth of the size of a proton to something maybe the size of a bubble, a soap bubble.
54:52But inflation didn't stop with our own universe.
54:56Anthony believes it may have happened over and over again.
54:59This is really a side effect.
55:04It's a huge side effect. It's an amazing side effect. But it's a side effect of something we invented already for a different purpose.
55:16It's a process called eternal inflation.
55:18There could be as many as we could imagine.
55:28Anthony's vision of an infinite number of infinite universes may sound far-fetched.
55:36But the search is on to find evidence to support it.
55:45Evidence from the oldest part of our map.
55:51Every once in a while we could have sort of a cosmic collision with another bubble.
55:55It would leave an impact, it would leave a bruise, a disk in the sky on the microwave background radiation that we could look for.
56:11Anthony and his colleagues have simulated what a collision of universes would look like.
56:17A dark bruise, superimposed on the cosmic microwave background.
56:25He doesn't yet have enough data to test it.
56:29But it's a tantalizing glimpse of what the map could reveal with the next generation of satellites.
56:38In principle, I think this scenario with all these bubbles is testable.
56:43We can actually go out and look for them.
56:44This may be the ultimate map of the universe.
56:54We're talking about understanding and testing and theorizing in a scientific way about an infinite number of universes.
57:04It's simultaneously so mind-boggling and yet it's still rigorous science.
57:09We can do mathematics, we can do experiments, we can really test it.
57:12Some day we'll understand the universe so well that we can literally take that map, put it on a little compact disc and put it in our pockets and take it home.
57:26Learn fascinating fun facts from some of Britain's most inspiring educators.
57:41The Teach Me A Lesson podcast with Greg James and Bella Mackey.
57:46Download BBC Sounds to listen.
57:48We'll see you soon as we come.
57:56Thanks.
57:58Thanks for having me.
58:00Thanks for having me.
58:02See you soon as we met in awirtschaft.
58:06It's time for you to learn and learn about any science hours.
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