0:00:16 > 0:00:22As the sun dips below the horizon, its light begins to fade.
0:00:22 > 0:00:27Night falls and our world descends into darkness.
0:00:42 > 0:00:46Today, in our street-lit towns and cities,
0:00:46 > 0:00:49we rarely experience true darkness.
0:00:49 > 0:00:52But without our eyes to guide us,
0:00:52 > 0:00:55the world becomes a much more mysterious place.
0:01:12 > 0:01:14I can't see anything now,
0:01:14 > 0:01:17but strangely I can still sense the presence of the trees
0:01:17 > 0:01:19enveloping me in the gloom.
0:01:20 > 0:01:24I can't see them, but I know there's something out there.
0:01:31 > 0:01:35And in the same way, as we've explored the cosmos,
0:01:35 > 0:01:38we've come to realise we can only see
0:01:38 > 0:01:40the merest hint of what's out there.
0:01:43 > 0:01:48Our best estimate is that more than 99% of the universe
0:01:48 > 0:01:52lies hidden in the dark, invisible to our telescopes
0:01:52 > 0:01:54and beyond our comprehension.
0:01:57 > 0:02:00This film is the story of how we went from thinking
0:02:00 > 0:02:04we were close to a complete understanding of the universe,
0:02:04 > 0:02:08to realising we'd seen almost none of it,
0:02:08 > 0:02:12and the extraordinary quest to uncover what's really out there
0:02:12 > 0:02:14in the dark.
0:02:17 > 0:02:21It's perhaps the most important undertaking in science,
0:02:21 > 0:02:25because our universe was forged in darkness.
0:02:25 > 0:02:29And darkness will one day tear it apart.
0:02:49 > 0:02:53For centuries, scientists have used light to build up a seemingly
0:02:53 > 0:02:56comprehensive picture of the universe.
0:02:58 > 0:03:01We'd discovered that the Earth was just one planet
0:03:01 > 0:03:02in orbit around the sun.
0:03:03 > 0:03:06And that the sun was itself a star,
0:03:06 > 0:03:09made of the same stuff as the billions upon billions
0:03:09 > 0:03:14of stars that light up a vast - perhaps endless - cosmos.
0:03:26 > 0:03:29But there was one niggling problem that had remained unsolved
0:03:29 > 0:03:32for over 400 years, and it was this -
0:03:32 > 0:03:36with so many stars out there, why was there any darkness at all?
0:03:38 > 0:03:42The story of the dark begins with this simple question.
0:03:44 > 0:03:47And at its heart lies a deep paradox.
0:03:49 > 0:03:53In the forest, no matter what direction I point my torch,
0:03:53 > 0:03:56the beam will always hit the trunk of a tree.
0:03:57 > 0:04:00And just as everywhere I look I see a tree,
0:04:00 > 0:04:03if the universe is sufficiently large,
0:04:03 > 0:04:08then every line of sight from Earth should end in a star.
0:04:08 > 0:04:10The night sky shouldn't be black at all,
0:04:10 > 0:04:13it should be ablaze with starlight.
0:04:16 > 0:04:19First posed in the 1570s,
0:04:19 > 0:04:23this question would become known as Olbers' Paradox.
0:04:24 > 0:04:29One possible solution was that the Earth was surrounded
0:04:29 > 0:04:34by dark stuff that obscured our view of the stars behind.
0:04:34 > 0:04:39But it was soon realised that these dark clouds would absorb
0:04:39 > 0:04:43the light from the stars, heat up and eventually glow
0:04:43 > 0:04:46with the same brightness as the stars they obscured.
0:04:52 > 0:04:57The paradox was only satisfactorily explained in the 20th century.
0:05:00 > 0:05:03The answer - the reason it gets dark at night
0:05:03 > 0:05:06is because the universe had a beginning.
0:05:06 > 0:05:11It began with the big bang 13.8 billion years ago, and so
0:05:11 > 0:05:16we only see those stars whose light has had time to reach us since then.
0:05:16 > 0:05:19The sky is dark because light from the most distant stars
0:05:19 > 0:05:21hasn't got here yet.
0:05:29 > 0:05:34No mysterious stuff was needed to block out the light.
0:05:34 > 0:05:38The dark spaces that starlight had yet to reach were empty,
0:05:38 > 0:05:41and cosmologists could sleep easy at night.
0:05:43 > 0:05:46But before long, we began to see hints that there might be
0:05:46 > 0:05:49more out there than meets the eye,
0:05:49 > 0:05:52that the shadowy recesses of empty space
0:05:52 > 0:05:55might not be so empty after all.
0:05:57 > 0:06:00The first clues had in fact begun to emerge from the gloom
0:06:00 > 0:06:04some 200 years ago,
0:06:04 > 0:06:07not in the depths of the universe, but in our own back yard.
0:06:19 > 0:06:23The invention of the telescope in the 17th century had allowed us
0:06:23 > 0:06:27to see the dimmest light from the deepest reaches of the solar system.
0:06:28 > 0:06:34And in 1781, it had revealed a seventh planet, Uranus,
0:06:34 > 0:06:37the first to be found since ancient times.
0:06:38 > 0:06:42But there was something odd about this new planet.
0:06:42 > 0:06:46Astronomers found that as time passed, Uranus's actual position
0:06:46 > 0:06:50was drifting further and further away from the position
0:06:50 > 0:06:53the laws of gravity predicted it should be at.
0:06:53 > 0:06:57One explanation was that the laws themselves were wrong,
0:06:57 > 0:06:59but working at the Paris Observatory,
0:06:59 > 0:07:02one man came up with a different solution.
0:07:02 > 0:07:04There was something else out there,
0:07:04 > 0:07:08something we couldn't see that was interfering with Uranus's orbit.
0:07:19 > 0:07:23In 1846, the mathematician Urbain Le Verrier
0:07:23 > 0:07:28was employed at the observatory to calculate the orbits of comets
0:07:28 > 0:07:30as they wandered through the solar system...
0:07:33 > 0:07:36..and predict when they would light up the night sky.
0:07:40 > 0:07:43Le Verrier has been described as having an almost pathological
0:07:43 > 0:07:48need to impose order on everything and everyone around him,
0:07:48 > 0:07:51and to have made no allowances for human error or frailty.
0:07:53 > 0:07:56When asked what he was like, a colleague remarked,
0:07:56 > 0:07:58"I do not know whether Monsieur Le Verrier is actually
0:07:58 > 0:08:01"the most detestable man in France,
0:08:01 > 0:08:04"but I am quite certain that he is the most detested."
0:08:06 > 0:08:09But he was undoubtedly a mathematical genius,
0:08:09 > 0:08:11and he was as harsh on himself as he was on others.
0:08:14 > 0:08:17And because he was a mathematician, he set about finding the object
0:08:17 > 0:08:22he thought was influencing Uranus not by scouring the skies
0:08:22 > 0:08:27with a telescope, but by determining its position through calculation.
0:08:36 > 0:08:41These are Le Verrier's original hand-written notes from 1846.
0:08:41 > 0:08:42This one is called
0:08:42 > 0:08:45"Searches of the disturbing body. Second approximation."
0:08:50 > 0:08:52It contains page after page
0:08:52 > 0:08:55of complicated mathematical calculations.
0:09:05 > 0:09:08What Le Verrier was attempting was quite different
0:09:08 > 0:09:10to what was normally done in astronomy,
0:09:10 > 0:09:12where you know where an object is -
0:09:12 > 0:09:17say a star or planet or comet - you then use the laws of gravity
0:09:17 > 0:09:22to explain its effects on nearby objects.
0:09:22 > 0:09:25Here, he didn't know where his disturbing body was.
0:09:25 > 0:09:30All he had to go by was the effect it had on the orbit of Uranus.
0:09:30 > 0:09:34So he made some starting assumptions about its position, and then
0:09:34 > 0:09:39carried out a calculation to predict the effect it would have on Uranus.
0:09:39 > 0:09:42He then compared that with what had been observed.
0:09:42 > 0:09:45When the two didn't match, he went back and adjusted
0:09:45 > 0:09:48his starting assumptions and repeated the calculation.
0:09:50 > 0:09:53He did this again and again
0:09:53 > 0:09:55until his prediction matched the observation.
0:10:01 > 0:10:04On the 31st of August, 1846,
0:10:04 > 0:10:07after three months of painstaking work,
0:10:07 > 0:10:11Le Verrier presented his results to the French Academy.
0:10:11 > 0:10:13He announced that his calculations had revealed
0:10:13 > 0:10:16what he believed was a new planet,
0:10:16 > 0:10:20and, crucially, that he had the co-ordinates in the night sky
0:10:20 > 0:10:22that showed where it could be found.
0:10:27 > 0:10:30And yet, despite this, he was unable to persuade
0:10:30 > 0:10:33any French astronomers to search for his planet.
0:10:38 > 0:10:41Eventually, Le Verrier sent his calculations
0:10:41 > 0:10:44to Johann Galle at the Berlin Observatory.
0:10:44 > 0:10:47His letter arrived on the 23rd of September,
0:10:47 > 0:10:50and the new planet was found the same evening
0:10:50 > 0:10:54within one degree of Le Verrier's predicted location.
0:10:54 > 0:10:57His calculations were so precise,
0:10:57 > 0:11:00it took Galle less than an hour to find it.
0:11:08 > 0:11:11Le Verrier and Galle had discovered the planet Neptune.
0:11:12 > 0:11:17A vast ice giant, 17 times heavier than the Earth
0:11:17 > 0:11:20and nearly 60 times its volume,
0:11:20 > 0:11:24lurking in the shadows some 4 billion kilometres from the sun.
0:11:30 > 0:11:33Neptune had been hard to find
0:11:33 > 0:11:36not because there was anything inherently mysterious about it.
0:11:36 > 0:11:40It's dark simply because it's so far from the sun,
0:11:40 > 0:11:42there's precious little light to illuminate it.
0:11:44 > 0:11:46And outside our solar system,
0:11:46 > 0:11:50this lack of illumination is an even bigger problem.
0:11:52 > 0:11:56And it means even more stuff is hidden in the dark.
0:11:58 > 0:12:04Stars are thought to contain just 11% of the atoms in the universe.
0:12:04 > 0:12:08The rest - clouds of gas and dust, planets, dead stars -
0:12:08 > 0:12:12we can't see, because they give off hardly any light.
0:12:13 > 0:12:17The dark spaces between the stars aren't empty at all.
0:12:17 > 0:12:20In fact, they contain the vast majority
0:12:20 > 0:12:22of the stuff that's out there.
0:12:26 > 0:12:30Up until the middle of the 20th century, most astronomers
0:12:30 > 0:12:36believed that, although they couldn't see nearly 90% of it,
0:12:36 > 0:12:39the universe was still, theoretically at least,
0:12:39 > 0:12:40entirely visible.
0:12:43 > 0:12:45But that was about to change.
0:12:49 > 0:12:51Welcome to White Sands Missile Range.
0:12:58 > 0:13:03In 1964, NASA scientists fitted an Aerobee rocket
0:13:03 > 0:13:06with an X-ray detector...
0:13:06 > 0:13:07'..two, one...'
0:13:09 > 0:13:12..and blasted it to the edge of space.
0:13:15 > 0:13:18High above the X-ray-absorbing layers of the atmosphere,
0:13:18 > 0:13:21the detector spotted something extremely bright
0:13:21 > 0:13:24in the constellation of Cygnus.
0:13:28 > 0:13:32The young British astronomer Paul Murdin was fascinated by this
0:13:32 > 0:13:36mysterious X-ray source, known as Cygnus X-1.
0:13:36 > 0:13:38And when he joined the Royal Greenwich Observatory
0:13:38 > 0:13:40in the summer of 1971,
0:13:40 > 0:13:43he was given with the perfect opportunity to discover what it was.
0:13:45 > 0:13:48It was known that X-rays were produced
0:13:48 > 0:13:53when gas was heated to temperatures upwards of a million degrees.
0:13:53 > 0:13:54DOORBELL CHIMES
0:13:54 > 0:13:56Hello, Paul!
0:13:56 > 0:14:00'But no-one knew for sure what could produce such extreme
0:14:00 > 0:14:02'conditions out in space.'
0:14:03 > 0:14:06What was it about X-ray sources that interested you?
0:14:06 > 0:14:10Celestial X-ray sources had just been discovered.
0:14:10 > 0:14:13They were places in the sky where X-rays came from.
0:14:13 > 0:14:14It's a very energetic radiation,
0:14:14 > 0:14:17it means something really powerful is happening there.
0:14:17 > 0:14:21I mean, the X-rays are a flag which the star is waving at you,
0:14:21 > 0:14:24saying, "Look at me, look at me, look at me - I'm really interesting."
0:14:27 > 0:14:30But when Paul trained his optical telescope on the source,
0:14:30 > 0:14:34all he saw was an ordinary, everyday star,
0:14:34 > 0:14:37nowhere near hot enough to produce X-rays.
0:14:39 > 0:14:42Most stars are in systems where there's two stars,
0:14:42 > 0:14:45three stars, even five stars or many more.
0:14:45 > 0:14:49It's really unusual to have a star like our sun that's on its own.
0:14:49 > 0:14:51I decided therefore that I'd try
0:14:51 > 0:14:54and look for evidence on the star that I could see, that there
0:14:54 > 0:14:58was another star nearby and that they were circling one another.
0:14:58 > 0:15:02By recording its motion night after night, Paul discovered
0:15:02 > 0:15:08the star was orbiting an invisible partner, once every 5.6 days.
0:15:08 > 0:15:12What you can calculate, once you know the period of a binary star,
0:15:12 > 0:15:18is the mass of the system and the mass of the component parts of it.
0:15:18 > 0:15:20And so, that was the thing to do next.
0:15:20 > 0:15:23And then, maybe within an hour,
0:15:23 > 0:15:27I knew that the star which I couldn't see
0:15:27 > 0:15:31was four solar masses or more.
0:15:33 > 0:15:37Something that heavy so close to the star he could see
0:15:37 > 0:15:40would strip material from its outer layers, the immense
0:15:40 > 0:15:46frictional forces heating the gas to such an extent it produced X-rays.
0:15:49 > 0:15:54But physicists only knew of one object that could be that massive
0:15:54 > 0:15:57and yet remain completely invisible.
0:15:57 > 0:16:00It was something that had only ever existed in theory.
0:16:02 > 0:16:05Paul Murdin had discovered the first black hole.
0:16:10 > 0:16:12I was just... I was just elated.
0:16:12 > 0:16:17And I had to get up from my desk and walk about a bit to calm down.
0:16:17 > 0:16:19My pulse raced.
0:16:19 > 0:16:23I knew it was big, but I was also a little bit frightened of it,
0:16:23 > 0:16:24so I knew I had to check it very carefully
0:16:24 > 0:16:30and go through it all again and check what I was doing.
0:16:30 > 0:16:31But it was... It was a great hour
0:16:31 > 0:16:35and I couldn't really do any serious work for the rest of the day.
0:16:36 > 0:16:39And I felt... I felt really happy with myself, actually.
0:16:43 > 0:16:45Thanks to Paul Murdin,
0:16:45 > 0:16:50the universe now had a new and profoundly dark inhabitant.
0:16:53 > 0:16:55Black holes are so incredibly dense,
0:16:55 > 0:17:00their gravity warps the fabric of space and time around them
0:17:00 > 0:17:05to such an extent that nothing, not even light, can escape.
0:17:13 > 0:17:15As you approach a black hole,
0:17:15 > 0:17:17an observer watching you from a distance
0:17:17 > 0:17:21will see the light coming from you getting redder and redder.
0:17:21 > 0:17:24And you will appear to be moving in slow motion
0:17:24 > 0:17:28as the immense gravitational field of the black hole
0:17:28 > 0:17:31stretches both space and time.
0:17:31 > 0:17:35And then, as you pass through the event horizon,
0:17:35 > 0:17:39the point of no return that marks the edge of a black hole,
0:17:39 > 0:17:44you simply disappear, lost from the universe for ever.
0:17:54 > 0:17:58Black holes are objects that would remain dark
0:17:58 > 0:18:01no matter how much light you shone on them.
0:18:05 > 0:18:07Through their effects on other things,
0:18:07 > 0:18:10we've now discovered dozens of black holes in our own galaxy,
0:18:10 > 0:18:14and estimate there must be billions upon billions
0:18:14 > 0:18:16of them throughout the universe.
0:18:16 > 0:18:19Including huge, supermassive black holes
0:18:19 > 0:18:22millions of times the mass of the sun
0:18:22 > 0:18:25at the heart of nearly every galaxy.
0:18:34 > 0:18:36As strange as black holes are,
0:18:36 > 0:18:40they were at least something we'd expected to find.
0:18:42 > 0:18:44We had theories that predicted their existence
0:18:44 > 0:18:46and described their properties.
0:18:49 > 0:18:51But since the 1930s,
0:18:51 > 0:18:56astronomers had seen disturbing hints of something much stranger.
0:18:58 > 0:19:04Stuff that was both completely invisible and completely unexpected.
0:19:09 > 0:19:10FAINT WHISPERING
0:19:14 > 0:19:18As a child, Vera Rubin spent hours awake at night
0:19:18 > 0:19:21staring out of the window above her bed,
0:19:21 > 0:19:24gazing at the stars as they moved across the sky.
0:19:28 > 0:19:31Then, in her 30s and a mother herself,
0:19:31 > 0:19:34she decided to realise her childhood dream
0:19:34 > 0:19:37and embark on a career as an astronomer.
0:19:37 > 0:19:39FAINT WHISPERING
0:19:47 > 0:19:52In the mid 1960s, the hottest topic in astronomy was quasars.
0:19:54 > 0:19:57But the field was extremely crowded
0:19:57 > 0:20:01and because the biggest telescopes that were needed to study them
0:20:01 > 0:20:03were often in the remotest parts of the world,
0:20:03 > 0:20:08working on quasars meant a lot of time spent away from home.
0:20:08 > 0:20:10So Vera needed to find a research topic
0:20:10 > 0:20:14that was more compatible with being a working mum,
0:20:14 > 0:20:17and a smaller field where she could really make her mark.
0:20:24 > 0:20:29So she began a project measuring the way stars move within galaxies
0:20:29 > 0:20:31like our own Milky Way.
0:20:56 > 0:20:59- Whoa! - HE LAUGHS
0:21:05 > 0:21:10Everything in a galaxy is on the move and rotating.
0:21:10 > 0:21:13In one minute, the Earth travels
0:21:13 > 0:21:16nearly 2,000 kilometres around the sun.
0:21:19 > 0:21:23But in that same time, the sun and the entire solar system
0:21:23 > 0:21:28travel 12,000 kilometres around the centre of the Milky Way galaxy.
0:21:31 > 0:21:33Ah!
0:21:33 > 0:21:35I'm not liking this!
0:21:42 > 0:21:45If you think this is spinning fast, think about this.
0:21:45 > 0:21:50The Earth is travelling around the sun at 108,000 kilometres an hour.
0:21:52 > 0:21:55Ha! And the sun and the entire solar system
0:21:55 > 0:21:59are travelling at 720,000 kilometres an hour
0:21:59 > 0:22:01around the centre of the galaxy.
0:22:01 > 0:22:03HE LAUGHS
0:22:07 > 0:22:09Can we stop it now?
0:22:20 > 0:22:22That's done me in, that really has.
0:22:22 > 0:22:24Thanks very much.
0:22:30 > 0:22:33But when Vera Rubin measured the speed of stars
0:22:33 > 0:22:35orbiting the centre of the Andromeda Galaxy,
0:22:35 > 0:22:38she found something deeply puzzling.
0:22:42 > 0:22:45If I plot a graph of the speed
0:22:45 > 0:22:48at which planets in our solar system orbit the sun
0:22:48 > 0:22:51against their distance from the sun,
0:22:51 > 0:22:57I find that the closest planet, Mercury, orbits the fastest.
0:22:57 > 0:23:03It's then followed by Venus, Earth, Mars and so on.
0:23:03 > 0:23:07The further out you go...
0:23:07 > 0:23:09the slower the orbit.
0:23:09 > 0:23:13In fact, Neptune moves so slowly relative to the other planets
0:23:13 > 0:23:16and has so far to go in orbit around the sun,
0:23:16 > 0:23:19that it's only completed one full circuit
0:23:19 > 0:23:21since it was discovered 167 years ago.
0:23:23 > 0:23:28Now, if I plot the same graph again of speed against distance,
0:23:28 > 0:23:31but this time, the speed
0:23:31 > 0:23:34at which the stars orbit the centre of a galaxy
0:23:34 > 0:23:36against their distance from the centre,
0:23:36 > 0:23:39I'd expect to see for the outer stars,
0:23:39 > 0:23:43that the speed drops off with distance, as it did for the planets.
0:23:43 > 0:23:46But when Vera Rubin plotted her data,
0:23:46 > 0:23:49she found that the further out you went,
0:23:49 > 0:23:52the speed of the stars didn't drop off,
0:23:52 > 0:23:54it remained roughly the same.
0:23:56 > 0:23:59The planets move more slowly the further out they are
0:23:59 > 0:24:01because the further you go,
0:24:01 > 0:24:04the weaker the sun's gravitational field becomes.
0:24:04 > 0:24:10So anything moving too fast would simply fly off into outer space.
0:24:10 > 0:24:13But Vera Rubin's result for galaxies
0:24:13 > 0:24:15suggested there must be an extra source of gravity
0:24:15 > 0:24:19holding all those fast-moving stars in their orbits.
0:24:23 > 0:24:25This extra gravity was needed
0:24:25 > 0:24:29because when astronomers added up the gravitational pull
0:24:29 > 0:24:33of all the dark things they thought might be lurking in the galaxy,
0:24:33 > 0:24:37planets, clouds of dust, even black holes,
0:24:37 > 0:24:40it always came out about ten times less
0:24:40 > 0:24:42than that needed to account for
0:24:42 > 0:24:44the stellar speeds Vera Rubin had measured.
0:24:46 > 0:24:49There were two possible explanations.
0:24:49 > 0:24:52Either Einstein's theory of gravity was wrong,
0:24:52 > 0:24:56or galaxies were full of a completely new kind of stuff.
0:24:56 > 0:24:58Something that wasn't made of atoms,
0:24:58 > 0:25:01was completely invisible and very heavy.
0:25:01 > 0:25:03A new form of dark matter.
0:25:03 > 0:25:06Something astronomers named... dark matter.
0:25:21 > 0:25:23Unsurprisingly, rather than accept
0:25:23 > 0:25:28that galaxies were full of some mysterious unseen stuff,
0:25:28 > 0:25:31some physicists once again thought tweaking the laws of gravity
0:25:31 > 0:25:33might be the simplest solution.
0:25:36 > 0:25:40That was until astronomers captured an astonishing image.
0:25:41 > 0:25:43For me, this is one of the most
0:25:43 > 0:25:45amazing pictures in modern astronomy.
0:25:45 > 0:25:50It's an image of a cluster of galaxies called the Bullet Cluster.
0:25:50 > 0:25:54It gets its name from this bullet-shaped cloud of gas,
0:25:54 > 0:25:58which is actually a shockwave caused by the collision
0:25:58 > 0:26:03not of just clouds of gas or stars or even whole galaxies,
0:26:03 > 0:26:06but clusters of galaxies coming together
0:26:06 > 0:26:11and passing through each other at 10-million kilometres an hour.
0:26:14 > 0:26:18It almost gives me vertigo trying to imagine the immensity of the scale.
0:26:21 > 0:26:23But it's not the magnitude of the collision
0:26:23 > 0:26:26that makes this image so important.
0:26:26 > 0:26:29It's what it did to the clusters' constituent parts.
0:26:30 > 0:26:33As the clusters came together,
0:26:33 > 0:26:35the stars and planets in the galaxies
0:26:35 > 0:26:37pretty much passed through each other
0:26:37 > 0:26:40because although they're big, the distances between them are so vast
0:26:40 > 0:26:45that the chances of any two stars colliding is actually very small.
0:26:45 > 0:26:48But that doesn't apply to the dust and gas
0:26:48 > 0:26:53that makes up 90% by mass of all the stuff we can see in a galaxy.
0:26:53 > 0:26:57When these collide, they create a huge, hot cloud -
0:26:57 > 0:27:00these two pink regions in the centre of the image.
0:27:01 > 0:27:05But if most of the mass is trapped here in the clouds,
0:27:05 > 0:27:09then you'd expect most of the gravity to be centred there, too.
0:27:09 > 0:27:11But that's not what you see.
0:27:11 > 0:27:15These outer blue regions show where light has been bent round
0:27:15 > 0:27:19as gravity warps the fabric of space itself.
0:27:19 > 0:27:23That means most of the gravity is centred out here,
0:27:23 > 0:27:25rather than in the middle.
0:27:25 > 0:27:28The simplest way to explain this
0:27:28 > 0:27:30is that it wasn't just stars and planets
0:27:30 > 0:27:33that passed through as the clusters collided,
0:27:33 > 0:27:35something else did, too.
0:27:35 > 0:27:38Something massive, yet invisible.
0:27:38 > 0:27:41This image is the best evidence we have yet
0:27:41 > 0:27:43for the existence of dark matter.
0:27:51 > 0:27:55It's now generally accepted that dark matter is real,
0:27:55 > 0:27:59which means there's far more stuff in the universe than we'd thought.
0:28:01 > 0:28:02In fact, there's four times
0:28:02 > 0:28:06as much dark matter as there is normal matter.
0:28:08 > 0:28:13And so vast swathes of the universe are not just unseen,
0:28:13 > 0:28:16they're fundamentally unseeable.
0:28:18 > 0:28:20The reason dark matter is so elusive
0:28:20 > 0:28:24is because it doesn't reflect light and it doesn't emit light.
0:28:24 > 0:28:27So we can't see it.
0:28:27 > 0:28:31And worse than that, what gives normal matter its solidity
0:28:31 > 0:28:33is the electromagnetic force.
0:28:33 > 0:28:36And dark matter particles don't feel that force,
0:28:36 > 0:28:39so they just pass straight through matter.
0:28:39 > 0:28:43The only hope we have is if they hit an atomic nucleus head-on.
0:28:43 > 0:28:47And even if they do, that's really hard to detect.
0:28:51 > 0:28:54And so the hunt for dark matter
0:28:54 > 0:28:59has turned from the incredibly large to the unimaginably small.
0:29:04 > 0:29:07From scouring the skies with telescopes
0:29:07 > 0:29:10to detectors buried deep underground.
0:29:11 > 0:29:13When it comes to the search for dark matter,
0:29:13 > 0:29:17the place I'm going to is pretty much the centre of the universe.
0:29:24 > 0:29:27The Gran Sasso National Laboratory
0:29:27 > 0:29:31lies beneath almost a kilometre and a half of solid rock.
0:29:34 > 0:29:36And can only be reached through a tunnel
0:29:36 > 0:29:39cut deep into the Italian Apennines.
0:29:44 > 0:29:47The reason you'd build a laboratory underneath a mountain
0:29:47 > 0:29:52is because our planet is constantly being bombarded by cosmic rays.
0:29:52 > 0:29:54These collide with the upper atmosphere,
0:29:54 > 0:29:57creating a cascade of particles
0:29:57 > 0:30:00that shower down onto the surface of the Earth.
0:30:00 > 0:30:05The rock above me effectively forms a 1400-metre-thick roof
0:30:05 > 0:30:08that absorbs most of these particles,
0:30:08 > 0:30:12shielding and protecting the equipment below.
0:30:12 > 0:30:14But crucially for dark-matter hunters,
0:30:14 > 0:30:17it passes straight through normal matter,
0:30:17 > 0:30:18straight through the rock,
0:30:18 > 0:30:20and the hope is, into their detectors.
0:30:26 > 0:30:28Oh!
0:30:28 > 0:30:30It looks like a Bond villain's evil lair.
0:31:00 > 0:31:03Gran Sasso is the world's largest underground laboratory.
0:31:09 > 0:31:11And for the last ten years,
0:31:11 > 0:31:15it's been home to dark matter scientists like Dr Chamkaur Ghag,
0:31:15 > 0:31:18who works on DarkSide-50,
0:31:18 > 0:31:21one of five dark matter experiments based here.
0:31:31 > 0:31:34- So hairnet.- Hairnet.
0:31:34 > 0:31:36Or head net, in my case.
0:31:38 > 0:31:43- Milligram levels of dust can destroy the experiment.- Right.
0:31:59 > 0:32:02That looks very impressive.
0:32:02 > 0:32:03- Yep.- Very sci-fi.
0:32:08 > 0:32:10So tell me, how does the experiment work?
0:32:10 > 0:32:13Well, the entire experiment is configured like a Russian doll,
0:32:13 > 0:32:16where the first outer layer is the mountain itself,
0:32:16 > 0:32:19protecting the experiment from radiation from space.
0:32:19 > 0:32:22Then we have this tank that we're standing in.
0:32:22 > 0:32:25And this tank is going to be flooded full of water.
0:32:25 > 0:32:27What, the whole cylinder?
0:32:27 > 0:32:29Absolutely. This is all completely filled to the brim.
0:32:29 > 0:32:33About 750 cubic metres of water will fill this thing
0:32:33 > 0:32:37to stop radiation coming from the laboratory and the rock around us.
0:32:37 > 0:32:40That's protecting this huge metal sphere right here,
0:32:40 > 0:32:42which is the final layer of protection
0:32:42 > 0:32:45before we get to DarkSide itself, which is inside there right now.
0:32:45 > 0:32:47That's the detector, that's the heart of the experiment.
0:32:47 > 0:32:50That's the thing that will be detecting dark matter.
0:32:52 > 0:32:55- You haven't got a light switch up there.- No.
0:32:55 > 0:32:57I'm going to get up there and have a look.
0:33:04 > 0:33:06DarkSide-50 is designed to detect
0:33:06 > 0:33:09a new class of fundamental particles
0:33:09 > 0:33:12called weakly interacting massive particles.
0:33:14 > 0:33:17Predicted by theory, it's thought that these WIMPs
0:33:17 > 0:33:21might be the stuff of which dark matter is made.
0:33:24 > 0:33:27So that metal sphere in the centre, that's DarkSide?
0:33:27 > 0:33:30That's right. That's a detector full of 150kg of liquid argon.
0:33:30 > 0:33:32Dark matter particles should be
0:33:32 > 0:33:34streaming through the detector all the time,
0:33:34 > 0:33:36but most of them just go straight through
0:33:36 > 0:33:39because they're very weakly interacting particles.
0:33:39 > 0:33:42If we're lucky, one will collide with the nucleus of an argon atom,
0:33:42 > 0:33:45producing flashes of light that the detector will pick up.
0:33:51 > 0:33:54DarkSide is yet to begin its search,
0:33:54 > 0:33:57but elsewhere in the laboratory's labyrinth of tunnels,
0:33:57 > 0:34:00they're already seeing tantalising hints.
0:34:03 > 0:34:05This is the XENON100 experiment that's already running
0:34:05 > 0:34:07and taking data and has been for a while.
0:34:07 > 0:34:11It's the most sensitive dark matter detector in the world right now.
0:34:11 > 0:34:15And this is a live feed of dark matter data coming in right now.
0:34:15 > 0:34:18So, what exactly... What sort of signal or shape are you looking for?
0:34:18 > 0:34:21Well, what we're looking for is an initial flash of light
0:34:21 > 0:34:23which will be a very sharp peak like this,
0:34:23 > 0:34:26followed by a much larger peak like that one,
0:34:26 > 0:34:30which is light being generated in a gas layer
0:34:30 > 0:34:32on top of the liquid xenon.
0:34:32 > 0:34:34Oh. That could be a good one as well, actually.
0:34:34 > 0:34:35There you go.
0:34:35 > 0:34:38So any one of those events, those spikes,
0:34:38 > 0:34:39could be a dark matter particle?
0:34:39 > 0:34:42That's right. Any one of these events
0:34:42 > 0:34:45could be the signature of dark matter
0:34:45 > 0:34:46interacting in XENON100.
0:34:46 > 0:34:49It's just we won't know for sure until the data's been analysed.
0:34:51 > 0:34:53Because it's so sensitive,
0:34:53 > 0:34:56the overwhelming majority of the spikes
0:34:56 > 0:34:59are due to radiation emitted by the metal
0:34:59 > 0:35:01that makes up the detector itself.
0:35:06 > 0:35:09But the hope is experiments like this
0:35:09 > 0:35:12will definitively detect dark matter particles
0:35:12 > 0:35:14within the next ten years.
0:35:21 > 0:35:25Today, we think that dark matter not only exists,
0:35:25 > 0:35:28but that it is a vital part of our universe,
0:35:28 > 0:35:32because without it, the world that we can see wouldn't exist
0:35:32 > 0:35:36and that's because dark matter not only holds galaxies together,
0:35:36 > 0:35:41it's dark matter that brought the clouds of gas together
0:35:41 > 0:35:46to form the galaxies in which stars could ignite in the first place.
0:35:54 > 0:35:57Dark matter has gone from being a curious quirk
0:35:57 > 0:36:01of the way stars move around the fringes of galaxies
0:36:01 > 0:36:04to the reason there are stars and galaxies at all.
0:36:10 > 0:36:14But in the late 1990s, scientists attempting
0:36:14 > 0:36:17to measure exactly how much dark matter there was
0:36:17 > 0:36:20made an astonishing discovery.
0:36:20 > 0:36:23There was something even more mysterious
0:36:23 > 0:36:25and even more elusive out there.
0:36:27 > 0:36:29And to understand what that is,
0:36:29 > 0:36:34you have to go back to the very beginning of everything.
0:36:34 > 0:36:36The universe began with a gigantic fireball.
0:36:44 > 0:36:4913.8 billion years ago, the universe was born.
0:36:50 > 0:36:51In the so-called big bang,
0:36:51 > 0:36:54everything was created simultaneously.
0:36:57 > 0:36:58See that great flash of light?
0:36:58 > 0:37:02That's all the pieces of the atoms joining together to make a gas.
0:37:02 > 0:37:05And now the gas is getting lumpy.
0:37:05 > 0:37:07It's making the giant galaxies of stars.
0:37:11 > 0:37:14The expansion of the universe that we now see
0:37:14 > 0:37:17is just a remnant of the initial violent explosion.
0:37:24 > 0:37:26The big bang means that in the past,
0:37:26 > 0:37:29the universe was much smaller than it is today.
0:37:32 > 0:37:34And it's been getting bigger ever since.
0:37:45 > 0:37:47According to the big bang theory,
0:37:47 > 0:37:52the universe has been expanding for the past 13.8 billion years.
0:37:52 > 0:37:54And for most of that time,
0:37:54 > 0:37:57you'd expect the expansion to be slowing down
0:37:57 > 0:37:59due to the combined gravitational attraction
0:37:59 > 0:38:02of all the mass in the universe
0:38:02 > 0:38:04trying to pull it back together again.
0:38:04 > 0:38:06Now, here's the clever bit,
0:38:06 > 0:38:08Cosmologists realised that by measuring
0:38:08 > 0:38:10how much the expansion was slowing,
0:38:10 > 0:38:14they could calculate how much stuff was out there.
0:38:14 > 0:38:18In a sense, it would allow them to weigh the entire universe.
0:38:21 > 0:38:25But in order to measure how the universe is expanding,
0:38:25 > 0:38:29you need a reliable way to measure distances in space.
0:38:36 > 0:38:41Something of known brightness, astronomers call a standard candle.
0:38:43 > 0:38:47The flame in this lantern produces a fixed amount of light.
0:38:47 > 0:38:51It has a specific brightness that I can measure here on the ground.
0:38:51 > 0:38:55But if I let the lantern go, it'll drift away
0:38:55 > 0:38:57and the light will appear to get dimmer and dimmer
0:38:57 > 0:38:59the further away it gets.
0:39:02 > 0:39:04Because I know how bright it really is,
0:39:04 > 0:39:08by comparing that with how bright it appears,
0:39:08 > 0:39:11I can calculate how far away it is.
0:39:34 > 0:39:36And because every lantern's the same,
0:39:36 > 0:39:39I can use the brightness to calculate the distance
0:39:39 > 0:39:42to any lantern I see in the sky.
0:39:47 > 0:39:50The astronomical equivalent of a Chinese lantern
0:39:50 > 0:39:57is a particular species of exploding star called a Type 1a supernova.
0:40:11 > 0:40:16These stars always explode when they reach the same critical mass
0:40:16 > 0:40:19and so always explode with the same brightness.
0:40:22 > 0:40:25So by measuring how bright they appear,
0:40:25 > 0:40:28we can tell how far they are from the Earth.
0:40:32 > 0:40:35As well as telling us how far away they are,
0:40:35 > 0:40:40the light reaching us from distant supernovae tells us something else.
0:40:40 > 0:40:44As it travels across the cosmos, light gets stretched
0:40:44 > 0:40:48because the space it's travelling through is expanding.
0:40:48 > 0:40:53And as its wavelength increases, the light gets redder and redder.
0:40:53 > 0:40:59And this red shift tells us how fast the universe was expanding
0:40:59 > 0:41:03when the light left its source, when the star exploded.
0:41:07 > 0:41:12But when scientists analysed light from the more distant supernovae
0:41:12 > 0:41:14they found something strange.
0:41:14 > 0:41:17It was less stretched than expected.
0:41:19 > 0:41:21It meant that, in the past,
0:41:21 > 0:41:25the universe was expanding more slowly than it is today.
0:41:25 > 0:41:30In other words, the expansion of the universe wasn't slowing down at all,
0:41:30 > 0:41:32it was speeding up.
0:41:37 > 0:41:41The only way the universe's expansion could be accelerating...
0:41:43 > 0:41:48..was if there was a mysterious new force pushing it apart.
0:41:52 > 0:41:55And just as with dark matter, physicists thought the key
0:41:55 > 0:41:58to understanding this new force
0:41:58 > 0:42:01might lie at the smallest possible scales...
0:42:03 > 0:42:08..because quantum physics appeared to provide a ready-made explanation.
0:42:11 > 0:42:16According to quantum field theory, empty space is anything but empty.
0:42:16 > 0:42:19Particles are constantly appearing and disappearing,
0:42:19 > 0:42:23created out of energy borrowed from the vacuum itself.
0:42:25 > 0:42:29The hope was that this theoretical vacuum energy
0:42:29 > 0:42:33might be the very thing that was pushing the universe apart.
0:42:34 > 0:42:37And the theory allows me to calculate the energy density
0:42:37 > 0:42:41of the vacuum, that's the amount of energy you'd expect to find
0:42:41 > 0:42:42in a given volume.
0:42:42 > 0:42:46And so if I take the energy of the vacuum
0:42:46 > 0:42:51to be a sum over J of half h-bar omega J,
0:42:51 > 0:42:53and if I take the cut-off energy
0:42:53 > 0:42:56to be of the order of 10 tera electronvolts
0:42:56 > 0:42:58which is just above the known physics
0:42:58 > 0:43:01at the Large Hadron Collider, then the formula for the vacuum...
0:43:01 > 0:43:06'All they needed to do was check the energy density the theory predicted
0:43:06 > 0:43:11'matched that needed to drive the universe's acceleration
0:43:11 > 0:43:14'and the mysterious force would be explained.'
0:43:14 > 0:43:20HE MUTTERS EQUATIONS
0:43:37 > 0:43:42So that would give me a value for the energy density
0:43:42 > 0:43:47of the vacuum of 10 to the 35 kilograms per cubic metre.
0:43:49 > 0:43:53The trouble is, the value observed by astronomers
0:43:53 > 0:43:57is 10 to the minus 27 kilograms per cubic metre.
0:43:57 > 0:44:00That means the theoretical number and the experimental number
0:44:00 > 0:44:04are out by a factor of 10 to the power 62.
0:44:04 > 0:44:07That's one followed by 62 zeros.
0:44:08 > 0:44:11To give you a sense of the scale of the error,
0:44:11 > 0:44:14there've been only 10 to the 17 seconds
0:44:14 > 0:44:19since the big bang and the diameter of the entire visible universe
0:44:19 > 0:44:22is 10 to the 27 metres...
0:44:24 > 0:44:25So it's a pretty big error.
0:44:27 > 0:44:32And that meant that whatever was actually pushing the universe apart,
0:44:32 > 0:44:35it was something completely new.
0:44:40 > 0:44:43The truth is, we know very little about what's causing
0:44:43 > 0:44:45the expansion of the universe to accelerate,
0:44:45 > 0:44:49but we do have a name for it - dark energy.
0:44:49 > 0:44:52And we know that for it to have the effect that it does,
0:44:52 > 0:44:54there must be an awful lot of it about.
0:44:57 > 0:45:01Einstein's famous equation E=mc2
0:45:01 > 0:45:05says that energy and matter are different forms of the same thing.
0:45:05 > 0:45:09And the equivalent mass of dark energy dwarfs that
0:45:09 > 0:45:11of everything else in the universe.
0:45:13 > 0:45:15And it means that, today,
0:45:15 > 0:45:19normal matter makes up just 4% of the cosmos.
0:45:19 > 0:45:2323% of it is elusive dark matter.
0:45:24 > 0:45:28And a colossal 73% of the universe
0:45:28 > 0:45:31consists of mysterious dark energy.
0:45:36 > 0:45:38Just think about it for a moment.
0:45:38 > 0:45:40100 billion galaxies,
0:45:40 > 0:45:44each one containing more than 100 billion stars,
0:45:44 > 0:45:48home in turn to billions upon billions of planets and moons.
0:45:49 > 0:45:55All of that is mere flotsam adrift on a vast and unfathomable ocean.
0:45:55 > 0:46:00Dark matter we can't see and dark energy we can barely comprehend.
0:46:05 > 0:46:09And the very nature of dark energy means the universe is getting
0:46:09 > 0:46:12more unknowable all the time.
0:46:15 > 0:46:19As space expands and distances become bigger,
0:46:19 > 0:46:24most forces get weaker, because you have the same amount of mass
0:46:24 > 0:46:27or electric charge, only now everything's further apart.
0:46:29 > 0:46:32But dark energy behaves completely differently.
0:46:32 > 0:46:37As the universe has expanded, the stronger it's become.
0:46:37 > 0:46:40The more space there is, the more dark energy there is
0:46:40 > 0:46:43and so the faster the universe expands,
0:46:43 > 0:46:47creating ever more space and ever more dark energy.
0:46:53 > 0:46:56And that has a profound consequence.
0:46:56 > 0:46:59Just as dark matter pulled the galaxies together
0:46:59 > 0:47:02to create the cosmos as we know it...
0:47:03 > 0:47:07..so dark energy will tear the universe apart.
0:47:09 > 0:47:12In the future, as space gets bigger,
0:47:12 > 0:47:15dark energy will become ever more dominant.
0:47:15 > 0:47:19And so it will ultimately shape the universe's destiny.
0:47:19 > 0:47:22And if it continues to increase as it appears to be doing today,
0:47:22 > 0:47:26then it will push the galaxies further and further apart
0:47:26 > 0:47:29until, eventually, they slip out of view,
0:47:29 > 0:47:33creating a cosmos that will become ever more dark
0:47:33 > 0:47:34and ever more desolate.
0:47:45 > 0:47:49The ultimate goal of modern cosmology is to understand
0:47:49 > 0:47:52dark energy and the fate of the universe,
0:47:52 > 0:47:56and to witness how dark matter brought everything together
0:47:56 > 0:47:58in the first place.
0:48:03 > 0:48:08And so to shed light on both the beginning and end of the universe,
0:48:08 > 0:48:13cosmologists have embarked on a quest of epic proportions -
0:48:13 > 0:48:18to map everywhere in space over the entire lifespan of the cosmos...
0:48:19 > 0:48:24..starting with the darkest period in its past,
0:48:24 > 0:48:29an era that began as the afterglow of the big bang faded away.
0:48:31 > 0:48:34We talk about the ages of the universe in the same way
0:48:34 > 0:48:38that we talk about the stages in our own lives, from its birth,
0:48:38 > 0:48:42through childhood, adolescence, adulthood and even death.
0:48:42 > 0:48:45So mapping the universe is really about
0:48:45 > 0:48:48filling in the photo album of its life.
0:48:49 > 0:48:54Here's a picture of me from 20 years ago with my children.
0:48:54 > 0:48:57I know it because I have a lot more hair there.
0:48:57 > 0:49:01And here's a picture of me in my early 20s on graduation.
0:49:01 > 0:49:03And here's one of me as a teenager.
0:49:05 > 0:49:08In the same way, by looking out into space,
0:49:08 > 0:49:10we have good images of the universe
0:49:10 > 0:49:13all the way back to its teenage years,
0:49:13 > 0:49:16when large galaxies first formed.
0:49:16 > 0:49:21But before that, we have nothing but a single image -
0:49:21 > 0:49:24a picture of the universe when it was just 400,000 years old,
0:49:24 > 0:49:29the cosmic microwave background - the afterglow of the big bang.
0:49:29 > 0:49:32It's as though, in the photo album of my life,
0:49:32 > 0:49:36I have nothing before this picture of me aged 16,
0:49:36 > 0:49:39apart from this one of me and my parents in Iraq
0:49:39 > 0:49:41when I was just a few months old.
0:49:43 > 0:49:46This gap in the childhood of the universe,
0:49:46 > 0:49:50the period between its earliest moments, through the birth
0:49:50 > 0:49:54of the first stars to the formation of large galaxies
0:49:54 > 0:49:57is a time known as the dark ages of the universe.
0:50:03 > 0:50:08The universe's dark ages lasted for around a billion years
0:50:08 > 0:50:12and they get their name because there were precious few stars
0:50:12 > 0:50:13to illuminate them.
0:50:18 > 0:50:23So to fill in those pages in the cosmic photo album, we'd need
0:50:23 > 0:50:27something capable of seeing where there was next to no light.
0:50:34 > 0:50:38During the Second World War, Bernard Lovell had developed a machine
0:50:38 > 0:50:41that could see in the dark.
0:50:41 > 0:50:43He'd worked on airborne radar
0:50:43 > 0:50:45that mapped bombers' targets on the ground.
0:50:49 > 0:50:51But his real ambition was to build
0:50:51 > 0:50:54something capable of mapping the heavens.
0:51:26 > 0:51:30The giant dish at Jodrell Bank was Bernard Lovell's baby.
0:51:30 > 0:51:34It was designed to be the world's largest fully manoeuvrable
0:51:34 > 0:51:38radio telescope, capable of scouring the entire sky
0:51:38 > 0:51:41and picking up the longest-wavelength radio signals
0:51:41 > 0:51:44coming from the deepest recesses of space.
0:52:07 > 0:52:09The Lovell Telescope has a collecting area
0:52:09 > 0:52:13of 4,560 square metres,
0:52:13 > 0:52:19made up of more than 2,400 galvanised steel plates.
0:52:24 > 0:52:28In the original designs, this bowl of the telescope
0:52:28 > 0:52:30wasn't meant to be solid like this.
0:52:30 > 0:52:34The plan was for it to be built of much lighter wire mesh.
0:52:37 > 0:52:39The dish was redesigned
0:52:39 > 0:52:44because astronomers had discovered a new way of seeing in the dark,
0:52:44 > 0:52:47something that might ultimately allow them
0:52:47 > 0:52:50to map the universe's dark ages.
0:52:52 > 0:52:55Hydrogen permeates every galaxy.
0:52:55 > 0:52:57It was produced in the big bang
0:52:57 > 0:53:01and is the basic constituent of all normal matter, including us.
0:53:01 > 0:53:03And like most normal matter,
0:53:03 > 0:53:06it wasn't thought to give off any light.
0:53:06 > 0:53:09But then astronomers discovered something remarkable.
0:53:09 > 0:53:11As it floats around in space,
0:53:11 > 0:53:16neutral hydrogen gas is constantly producing radio waves
0:53:16 > 0:53:22and, crucially, those waves are always the same wavelength - 21cm.
0:53:22 > 0:53:26And this meant that hydrogen could be used to map
0:53:26 > 0:53:28the galaxies that it fills.
0:53:30 > 0:53:36By detecting the 21cm signal, the Lovell Telescope helped reveal
0:53:36 > 0:53:39the spiral structure of the Milky Way
0:53:39 > 0:53:43and produced detailed maps of distant galaxies.
0:53:48 > 0:53:52But galaxies aren't the only place in the cosmos you find hydrogen gas.
0:53:52 > 0:53:54During the dark ages of the universe,
0:53:54 > 0:53:58there were no galaxies, but there was plenty of hydrogen.
0:53:58 > 0:54:03So by detecting the 21cm signal from these primordial gas clouds,
0:54:03 > 0:54:05you could see the universe in its infancy
0:54:05 > 0:54:08and peer into the dark ages themselves.
0:54:15 > 0:54:18And by doing so, we'll be able to watch dark matter
0:54:18 > 0:54:20pull the cosmos together...
0:54:22 > 0:54:24..and light up the heavens.
0:54:28 > 0:54:31It was during the dark ages that the hydrogen gas created
0:54:31 > 0:54:36in the big bang was compressed into stars and moulded into galaxies.
0:54:36 > 0:54:41It was in this era that the cosmos as we know it was born,
0:54:41 > 0:54:44sculpted by the gravitational pull of dark matter.
0:54:51 > 0:54:55But the machine scientists are building to map the dark ages
0:54:55 > 0:54:57will see far more.
0:54:58 > 0:55:03With an effective collecting area of more than 200 times that
0:55:03 > 0:55:07of the Lovell Telescope, the square kilometre array
0:55:07 > 0:55:10will be capable of mapping a billion galaxies,
0:55:10 > 0:55:15tracking the expansion and evolution of the entire universe
0:55:15 > 0:55:17more accurately than ever before.
0:55:22 > 0:55:25And the hope is, that by doing so,
0:55:25 > 0:55:29it will provide clues to the nature of dark energy
0:55:29 > 0:55:31and the universe's ultimate fate.
0:55:51 > 0:55:56Using hydrogen to map the cosmos might just represent the final
0:55:56 > 0:56:02chapter of humankind's exploration of the universe using light,
0:56:02 > 0:56:07a journey that began in earnest some 400 years ago.
0:56:07 > 0:56:12In December 1609, Galileo Galilei began making observations
0:56:12 > 0:56:14of the night sky.
0:56:14 > 0:56:17Before then, what was thought to be out there was essentially
0:56:17 > 0:56:19a matter of faith.
0:56:19 > 0:56:23The universe at large lay unseen and unseeable.
0:56:23 > 0:56:24But now, for the first time,
0:56:24 > 0:56:27the nature of the heavens was something knowable -
0:56:27 > 0:56:31you simply had to look up and see it.
0:56:31 > 0:56:34The light captured in Galileo's simple telescope
0:56:34 > 0:56:37began a chain of discoveries that would reveal
0:56:37 > 0:56:39the true nature of the cosmos.
0:56:48 > 0:56:49We've seen galaxies
0:56:49 > 0:56:52billions of light years' distance from the Earth.
0:56:55 > 0:56:58And as we've come to understand light's properties,
0:56:58 > 0:57:01we've discovered the stuff of which stars are made...
0:57:05 > 0:57:08..and glimpsed the beginning of the universe itself.
0:57:15 > 0:57:21But the realisation that most normal matter can't be seen
0:57:21 > 0:57:26and the discovery of dark matter and dark energy
0:57:26 > 0:57:32mean that more than 99% of the universe lies hidden in the shadows.
0:57:35 > 0:57:40And as dark energy pushes the galaxies ever further apart,
0:57:40 > 0:57:43what few lights there are will begin to go out.
0:57:45 > 0:57:48As the universe expands ever faster,
0:57:48 > 0:57:52one by one the galaxies will disappear from view.
0:57:52 > 0:57:57All that will remain visible will be the stars in our own galaxy.
0:57:57 > 0:58:00It would be almost as if we'd never invented the telescope at all.
0:58:01 > 0:58:04For the vast majority of the universe's life,
0:58:04 > 0:58:08there'll be no way of discovering all the things we have about it.
0:58:10 > 0:58:14So I don't feel disheartened that so much of the cosmos
0:58:14 > 0:58:16is hidden in the shadows.
0:58:16 > 0:58:17The real miracle is
0:58:17 > 0:58:20that when we first looked out into the depths of space
0:58:20 > 0:58:23there was any light to see at all.
0:58:33 > 0:58:36Whether you want to step into the light
0:58:36 > 0:58:38or explore the mysteries of the dark,
0:58:38 > 0:58:41let the Open University inspire you.
0:58:41 > 0:58:42Go to...
0:58:45 > 0:58:48..and follow links to The Open University.
0:58:48 > 0:58:51Subtitles by Red Bee Media Ltd