0:00:07 > 0:00:13Imagine that our sun is the size of just a single grain of sand.
0:00:20 > 0:00:25Now, our sun is just one of a multitude of stars.
0:00:25 > 0:00:29It's surrounded by over 200 billion of them
0:00:29 > 0:00:33in our own Milky Way galaxy alone.
0:00:33 > 0:00:39Our sun is just a speck in the vast beach of stars.
0:00:43 > 0:00:48But the Milky Way galaxy is in itself just one
0:00:48 > 0:00:53of 100 billion galaxies, scattered throughout the cosmos.
0:01:01 > 0:01:05It's been estimated that there are more stars in the universe
0:01:05 > 0:01:11than there are grains of sand on all the beaches in all the world.
0:01:11 > 0:01:13Just think about that for a moment.
0:01:15 > 0:01:20The size and scale of the universe is awe-inspiring.
0:01:20 > 0:01:25But, as a scientist, what I find so remarkable is that the human race
0:01:25 > 0:01:30has managed to deduce so much about what it looks like.
0:01:32 > 0:01:35Let me try and put this achievement into context.
0:01:35 > 0:01:41From our vantage point, living on a minuscule speck orbiting around
0:01:41 > 0:01:47this single grain of sand, we've managed to deduce the size and shape
0:01:47 > 0:01:49of all those beaches.
0:01:49 > 0:01:55To my mind, this is one of the human race's greatest accomplishments,
0:01:55 > 0:01:59and I'd like to tell you the story of how we did it.
0:02:01 > 0:02:07This film is the astonishing story of how we gazed upwards from our
0:02:07 > 0:02:13isolated and unremarkable vantage point and began to deduce the shape,
0:02:13 > 0:02:19size and origin of everything that there is.
0:02:19 > 0:02:23It's the story of how we came to understand reality
0:02:23 > 0:02:26at the largest scale.
0:02:26 > 0:02:29It's the story of everything.
0:02:51 > 0:02:57I want you to pause for a moment and think about this one basic question.
0:02:57 > 0:03:00Here I am, sitting under the night sky.
0:03:00 > 0:03:05Above me is the atmosphere and beyond that the moon,
0:03:05 > 0:03:07and way beyond that the stars.
0:03:07 > 0:03:09But then what?
0:03:09 > 0:03:13What's the totality of everything there is?
0:03:13 > 0:03:15It's a question we've all asked at one point or another.
0:03:15 > 0:03:18I remember as a kid, growing up in Baghdad,
0:03:18 > 0:03:22during the summer, we'd take the beds up onto the roof
0:03:22 > 0:03:25and I remember lying awake at night, looking up at the stars
0:03:25 > 0:03:29and wondering whether space went on forever
0:03:29 > 0:03:32or whether the universe had an edge.
0:03:32 > 0:03:35Today, we're beginning to understand
0:03:35 > 0:03:39just how complex this question really is.
0:03:41 > 0:03:46But 500 years ago, it seemed like there was a very simple answer.
0:03:46 > 0:03:51You see, the prevailing belief was that the Earth was enclosed
0:03:51 > 0:03:58in a vast but thin shell of rotating stars that were fixed in position.
0:03:58 > 0:04:01When you look up on a starry night,
0:04:01 > 0:04:06it's not difficult to see why people believed we lived within this shell.
0:04:09 > 0:04:11But in the 16th century,
0:04:11 > 0:04:16something happened which would shatter this view of the universe.
0:04:16 > 0:04:20It was an event that would set the human race on a journey
0:04:20 > 0:04:26to uncover the true size and shape of everything.
0:04:38 > 0:04:41This is a Type Ia supernova.
0:04:41 > 0:04:44An exploding star.
0:04:44 > 0:04:48It's an event of almost unimaginable scale.
0:04:48 > 0:04:53It shines five billion times more brightly than our own sun.
0:04:58 > 0:05:03In 1572, a supernova like this would have become visible on Planet Earth.
0:05:05 > 0:05:09At the time, it was known simply as "the phenomenon".
0:05:09 > 0:05:13And to anyone who saw it, it must have been an extremely shocking
0:05:13 > 0:05:15and mysterious sight.
0:05:15 > 0:05:17This new light in the night sky
0:05:17 > 0:05:23shone more brightly than Venus and even became visible during the day.
0:05:23 > 0:05:28It's not surprising, then, that many sought a religious explanation
0:05:28 > 0:05:31for this bizarre and troubling event.
0:05:31 > 0:05:33One possible interpretation
0:05:33 > 0:05:40of the new star of 1572, which was put forward by some intellectuals,
0:05:40 > 0:05:47was that this is the star the wise men saw 1,570 years earlier.
0:05:47 > 0:05:54It's the star that shone over Bethlehem, and it's now returned.
0:05:54 > 0:05:59So something as cosmically important as the incarnation of God on Earth
0:05:59 > 0:06:03might be being proclaimed by this new star.
0:06:06 > 0:06:11The phenomenon fascinated and mystified many people across Europe.
0:06:11 > 0:06:14In England, it fired the imagination
0:06:14 > 0:06:18of the MP of the sleepy Oxfordshire town of Wallingford.
0:06:18 > 0:06:21His name was Thomas Digges.
0:06:25 > 0:06:30But just as Digges began to study this mysterious new star,
0:06:30 > 0:06:32it started to grow dimmer.
0:06:35 > 0:06:41Digges' friend, mentor and fellow astronomer, a man named John Dee,
0:06:41 > 0:06:45reasoned with him that this phenomenon could be a moving star,
0:06:45 > 0:06:48something previously thought to have been impossible.
0:06:48 > 0:06:52Perhaps it had grown brighter
0:06:52 > 0:06:57as it approached the Earth and faded as it had gone away.
0:07:01 > 0:07:06Now, although this theory was wrong, it got Digges thinking about
0:07:06 > 0:07:09the true nature of the stars that surround the Earth.
0:07:09 > 0:07:13It began to seem very unlikely that they were all arranged
0:07:13 > 0:07:15in a vast, thin shell.
0:07:15 > 0:07:20Maybe this apparent shell was just an illusion?
0:07:23 > 0:07:26It would take Thomas Digges another four years
0:07:26 > 0:07:29before he published his strange idea.
0:07:29 > 0:07:33And when he did, it was in the form of a simple diagram,
0:07:33 > 0:07:39added to a translation of the works of Nicolaus Copernicus.
0:07:39 > 0:07:42The man who'd first argued that the sun
0:07:42 > 0:07:45was at the centre of the universe.
0:07:45 > 0:07:47Have a look at this.
0:07:47 > 0:07:49On this side is Copernicus' model.
0:07:49 > 0:07:51Absolutely revolutionary.
0:07:51 > 0:07:55He has the sun at the centre with the Earth in orbit around it,
0:07:55 > 0:07:57along with the other planets.
0:07:57 > 0:08:00And in the outermost shell is that of the fixed stars -
0:08:00 > 0:08:03the stellarum fixarum.
0:08:03 > 0:08:08On this side is Digges' diagram, included in the English translation.
0:08:08 > 0:08:11Exactly the same, but he's taken Copernicus' stars
0:08:11 > 0:08:14out of their fixed shell
0:08:14 > 0:08:16and scattered them out into endless space.
0:08:19 > 0:08:24Digges' diagram was describing a radical new picture of the cosmos.
0:08:27 > 0:08:35One where the stars in the night sky now existed in an infinite space.
0:08:40 > 0:08:46Digges shows it, unlike Copernicus, as being infinite.
0:08:46 > 0:08:51This is a sphere, he says, of the stars fixed infinitely up.
0:08:51 > 0:08:57And that is a moment when perhaps Europeans start to think
0:08:57 > 0:09:04of the world as unbounded, as infinite, as a world without end.
0:09:08 > 0:09:12Digges' new picture of the universe was revolutionary.
0:09:12 > 0:09:18Previously, we'd been contained within a small shell of stars.
0:09:18 > 0:09:23Now we were suspended within an infinite static universe.
0:09:27 > 0:09:32But this picture of everything produced a strange paradox.
0:09:32 > 0:09:38If this infinite universe contained an infinite number of stars,
0:09:38 > 0:09:42then why was it dark at night?
0:09:45 > 0:09:48In the traditional old-fashioned view of the universe,
0:09:48 > 0:09:52the universe was infinite and static.
0:09:52 > 0:09:54It was very soon recognised that
0:09:54 > 0:09:58a static infinite universe was ridiculous.
0:09:58 > 0:10:02And that is because, in such a universe,
0:10:02 > 0:10:06there would be an infinite number of stars and every line
0:10:06 > 0:10:11of sight from us would intercept one of these stars.
0:10:11 > 0:10:15The universe - static infinite universe - could not be dark.
0:10:15 > 0:10:18It should be glowing as bright as the sun.
0:10:18 > 0:10:20And we know that's not our universe.
0:10:20 > 0:10:24In our universe, the night sky is dark.
0:10:32 > 0:10:35Although Thomas Digges first raised this question,
0:10:35 > 0:10:39the problem came to be known as Olbers' paradox.
0:10:39 > 0:10:42As simple as the question sounds,
0:10:42 > 0:10:48it would take until the 20th century to find a truly satisfactory answer
0:10:48 > 0:10:52for why the night sky is not as bright as the day.
0:10:58 > 0:11:02Solving Olbers' paradox would require many great scientists,
0:11:02 > 0:11:05who weren't afraid to think differently.
0:11:05 > 0:11:06Radically differently.
0:11:06 > 0:11:11You see, solving the paradox is all about understanding the shape, size
0:11:11 > 0:11:14and origin of everything there is.
0:11:14 > 0:11:18Without this understanding, the puzzle would be impossible to solve.
0:11:18 > 0:11:19You see, stuck here on Earth,
0:11:19 > 0:11:22we don't have access to interstellar travel.
0:11:22 > 0:11:26So we have to allow our minds to make that intellectual leap.
0:11:32 > 0:11:34By simply looking up,
0:11:34 > 0:11:40Digges and his contemporaries had begun a scientific journey
0:11:40 > 0:11:43to understand what everything might actually look like.
0:11:45 > 0:11:49But, for 200 years after Thomas Digges' insight,
0:11:49 > 0:11:52little progress was made in understanding the most
0:11:52 > 0:11:54distant reaches of the cosmos.
0:11:55 > 0:12:01At the end of the 18th century, however, all that would change.
0:12:02 > 0:12:07Until the end of the 1700s, everything that lies outside
0:12:07 > 0:12:13the solar system is, for astronomers, pretty uninteresting.
0:12:13 > 0:12:18Astronomy until then was the science of our system -
0:12:18 > 0:12:22of the Earth and the planets, satellites and comets.
0:12:22 > 0:12:26The stars were a kind of glorified and rather interesting backdrop.
0:12:26 > 0:12:30This changes around 1800.
0:12:42 > 0:12:48This small and unremarkable house in Bath was once home to the astronomer
0:12:48 > 0:12:52William Herschel and his sister, and devoted assistant, Caroline.
0:12:52 > 0:12:57Together, they would develop and build a new generation of telescopes
0:12:57 > 0:13:00that would allow them to see further out into space
0:13:00 > 0:13:03than any human had ever done before.
0:13:05 > 0:13:09William Herschel was born in Hanover, but moved to England
0:13:09 > 0:13:14in 1761 to pursue a career as a musician and composer.
0:13:14 > 0:13:18But he soon developed a passion for astronomy
0:13:18 > 0:13:22and began building telescopes in his spare time.
0:13:23 > 0:13:28Herschel soon perfected a technique for producing telescopes
0:13:28 > 0:13:31borrowed from Sir Isaac Newton.
0:13:31 > 0:13:34The telescopes used metal mirrors
0:13:34 > 0:13:37that were capable of capturing much more starlight
0:13:37 > 0:13:42than the glass lenses that were popular among other astronomers.
0:13:47 > 0:13:49This tiny room at the back of Herschel's house
0:13:49 > 0:13:51used to be his workshop.
0:13:51 > 0:13:55It was here that he'd smelt various metals together in the furnace
0:13:55 > 0:13:59to make the reflecting mirrors for his telescopes.
0:13:59 > 0:14:02And he would experiment with different metals,
0:14:02 > 0:14:07different combinations, to get them as reflective as possible.
0:14:07 > 0:14:09Then, with his sister Caroline to help him,
0:14:09 > 0:14:12he'd spend literally hours on end
0:14:12 > 0:14:18polishing the surface of the mirrors to achieve the precision required.
0:14:18 > 0:14:20And you have to remember, this was quite
0:14:20 > 0:14:22a dangerous, confined environment.
0:14:22 > 0:14:27The floor still bears the scars of the molten metal
0:14:27 > 0:14:29that they'd spilt, cracking the paving stones.
0:14:33 > 0:14:37With his powerful telescopes, Herschel and his sister Caroline
0:14:37 > 0:14:43would scour the heavens, night after night, cataloguing the stars.
0:14:44 > 0:14:48The universe they were seeing was revealing itself
0:14:48 > 0:14:55to be one of dynamic complexity, a universe of natural, organic motion,
0:14:55 > 0:14:58a place of endless wonder.
0:15:00 > 0:15:05Herschel's revolutionary telescope design made him famous.
0:15:05 > 0:15:09With it, he'd discover a new planet, Uranus,
0:15:09 > 0:15:14a discovery that would earn him the job of the King's astronomer.
0:15:14 > 0:15:18This new role gave him the time and resources to start a much
0:15:18 > 0:15:24grander task, to try and map all the stars in the universe, in an attempt
0:15:24 > 0:15:27to draw a picture of everything.
0:15:31 > 0:15:36In 1785, Herschel published this remarkable image.
0:15:36 > 0:15:40It shows an approximation of the Milky Way,
0:15:40 > 0:15:44with our sun residing at the centre.
0:15:44 > 0:15:49Herschel had seen that we are part of a vast disc of stars,
0:15:49 > 0:15:54a huge galaxy of suns that seemed to have a clear boundary.
0:15:59 > 0:16:02It appeared as though Herschel's craftsmanship
0:16:02 > 0:16:06had actually allowed him to see to the edge of everything.
0:16:12 > 0:16:16But soon a nagging problem began to emerge.
0:16:16 > 0:16:19Dotted around the sky, Herschel and others
0:16:19 > 0:16:23had been observing strange, cloud-like objects,
0:16:23 > 0:16:25known as nebulae.
0:16:29 > 0:16:33Some of these nebulae seemed to have distinctive form
0:16:33 > 0:16:35and complex structure.
0:16:37 > 0:16:40Some astronomers began to suggest a radical idea.
0:16:41 > 0:16:46Perhaps the Milky Way was not everything that there was.
0:16:46 > 0:16:51Perhaps some of these nebulae were in fact themselves gigantic
0:16:51 > 0:16:57galaxies of stars, just like ours, that actually existed in deep space.
0:16:59 > 0:17:03Unfortunately, there was no way to answer this question satisfactorily.
0:17:05 > 0:17:07The problem was that,
0:17:07 > 0:17:10for all Herschel's great technological achievements,
0:17:10 > 0:17:14and for all of those long, cold nights that he spent with Caroline
0:17:14 > 0:17:19outside, gazing painstakingly at the heavens, there was one problem
0:17:19 > 0:17:20they couldn't solve.
0:17:20 > 0:17:26They had no way of accurately measuring distances in outer space.
0:17:28 > 0:17:31It would not be until after Herschel's death
0:17:31 > 0:17:33that a cunning method was developed
0:17:33 > 0:17:36to measure the distances to objects deep into space.
0:17:38 > 0:17:41The technique was known as stellar parallax.
0:17:44 > 0:17:48If you look at an object like your finger from two vantage points,
0:17:48 > 0:17:51it will shift in your frame of reference.
0:17:51 > 0:17:56By observing how much it shifts, you can calculate
0:17:56 > 0:17:58how far away it is.
0:17:58 > 0:18:02My finger is moving a lot more between each frame
0:18:02 > 0:18:05than the building that is behind it.
0:18:09 > 0:18:13Now, an astronomer called Friedrich Bessel worked out that if you took
0:18:13 > 0:18:18images of stars when the Earth was at either side of its orbit around
0:18:18 > 0:18:24the sun, it would be possible to actually see the stars shifting.
0:18:24 > 0:18:26By observing how much they shifted,
0:18:26 > 0:18:29you could then work out their distance from us.
0:18:34 > 0:18:37Bessel calculated that the relatively close star,
0:18:37 > 0:18:4461 Cygni, must be some 100 trillion kilometres away.
0:18:44 > 0:18:46But amazing though this technique was,
0:18:46 > 0:18:49it was still very severely limited.
0:18:51 > 0:18:56The diameter of the Earth's orbit is 300 million kilometres.
0:18:56 > 0:19:00This means the parallax method can only measure objects
0:19:00 > 0:19:04out to about 300 trillion kilometres,
0:19:04 > 0:19:07only a tiny fraction of the size of the Milky Way.
0:19:13 > 0:19:16It soon became clear that there was plenty in the heavens
0:19:16 > 0:19:19that was practically impossible to measure,
0:19:19 > 0:19:22particularly those mysterious nebulae.
0:19:25 > 0:19:29They would remain an enigma until the beginning of the 20th century,
0:19:29 > 0:19:33when they ignited a great debate.
0:19:33 > 0:19:39One group of astronomers agrees that there is only one galaxy, ours,
0:19:39 > 0:19:41the Milky Way and everything else we see,
0:19:41 > 0:19:49the globular clusters, the nebulae, are all somehow inside that galaxy.
0:19:49 > 0:19:54Then there are other astronomers who argue no, many of these nebulae are
0:19:54 > 0:20:00themselves giant island universes, unimaginably far away from us.
0:20:00 > 0:20:02There was evidence on both sides.
0:20:06 > 0:20:09This mystery remained a source of bitter debate
0:20:09 > 0:20:13until the beginning of the 1920s.
0:20:14 > 0:20:18The woman who would help solve the problem is one of the great
0:20:18 > 0:20:21unsung heroes of science.
0:20:21 > 0:20:24She worked at the Harvard College Observatory
0:20:24 > 0:20:27and her name was Henrietta Leavitt.
0:20:28 > 0:20:32Leavitt's job was to count and catalogue the stars
0:20:32 > 0:20:36producing images from observatories around the world.
0:20:36 > 0:20:39She was a brilliant scientist who loved her work.
0:20:42 > 0:20:45This is one of the photographic plates of space
0:20:45 > 0:20:47that Leavitt worked with.
0:20:47 > 0:20:50You can see her bright marks highlighting
0:20:50 > 0:20:53tiny details within the image.
0:20:53 > 0:20:55With meticulous care,
0:20:55 > 0:20:59hundreds of subtle features of stars have been noted.
0:21:05 > 0:21:10It was this ability that would help her come up with an ingenious idea,
0:21:10 > 0:21:15one that would help unravel the true size of the universe.
0:21:15 > 0:21:20The idea rested on finding an objective way of defining
0:21:20 > 0:21:22the true brightness of a star.
0:21:26 > 0:21:32Leavitt became fascinated by a type of star known as a Cepheid variable,
0:21:32 > 0:21:35which pulses in the night sky.
0:21:35 > 0:21:36Her breakthrough was discovering
0:21:36 > 0:21:42that their brightness was precisely related to the speed they blinked.
0:21:42 > 0:21:44Let me explain.
0:21:46 > 0:21:50These two stars are blinking at the same rate, which means they should
0:21:50 > 0:21:53be exactly the same brightness.
0:21:53 > 0:21:58If one star appears dimmer, you can then calculate how much further away
0:21:58 > 0:22:01it is than the brighter one.
0:22:01 > 0:22:05Leavitt's method meant that she knew
0:22:05 > 0:22:08the true brightness of the Cepheid variables.
0:22:08 > 0:22:12She had found a method to measure the distance to stars that lay
0:22:12 > 0:22:15far beyond the reaches of parallax.
0:22:17 > 0:22:19But without access to a telescope,
0:22:19 > 0:22:22she could go no further with her work.
0:22:22 > 0:22:24She was forbidden from working
0:22:24 > 0:22:28in the supremely male-dominated world of the observatory.
0:22:29 > 0:22:32But her discovery now gave astronomers a tool
0:22:32 > 0:22:36to measure the distances to the mysterious nebulae.
0:22:38 > 0:22:42The idea that our Milky Way might contain everything that existed
0:22:42 > 0:22:45was about to crumble.
0:22:45 > 0:22:50The scale of the universe is really only understood
0:22:50 > 0:22:52amazingly recently.
0:22:52 > 0:22:56In the 1920s, it was absolutely plausible
0:22:56 > 0:23:00that the universe consists of one galaxy,
0:23:00 > 0:23:06and some of the best astronomers in the world, in the US for example,
0:23:06 > 0:23:10seriously held that view, and had good evidence that it was true.
0:23:10 > 0:23:11And they were wrong.
0:23:13 > 0:23:17The evidence to finally settle the great debate would be found
0:23:17 > 0:23:20thanks to the powerful new Hooker telescope
0:23:20 > 0:23:23being built at the Mount Wilson Observatory
0:23:23 > 0:23:26just outside Los Angeles.
0:23:26 > 0:23:29Using this incredible piece of technology,
0:23:29 > 0:23:33and Henrietta Leavitt's ingenious method for calculating distance,
0:23:33 > 0:23:37a young astronomer would make a discovery that would change
0:23:37 > 0:23:42our view of the universe and for ever immortalise his name.
0:23:42 > 0:23:46The astronomer was called Edwin Hubble.
0:23:46 > 0:23:51Hubble was a very different kind of scientist to Leavitt.
0:23:51 > 0:23:56He was a larger-than-life character, extrovert, with a huge ego.
0:23:56 > 0:24:00But he was still a hugely-talented and visionary scientist.
0:24:00 > 0:24:03He was born and grew up in America but spent some time in England,
0:24:03 > 0:24:06and this seems to have had a lasting impression
0:24:06 > 0:24:09because he would be heard walking around the observatory
0:24:09 > 0:24:11shouting things like, "By Jove!"
0:24:11 > 0:24:15and "What-ho!" in a completely over-the-top British accent.
0:24:17 > 0:24:20The talented, passionate and eccentric Hubble
0:24:20 > 0:24:24rapidly gained a name for himself in the world of astronomy.
0:24:24 > 0:24:26But it wouldn't be until 1923,
0:24:26 > 0:24:29that he would discover something in what was then known
0:24:29 > 0:24:31as the Andromeda Nebula
0:24:31 > 0:24:34that would reveal the true scale of our universe.
0:24:35 > 0:24:38I've come to the University College London Observatory
0:24:38 > 0:24:40to meet astronomer Dr Steve Fossey,
0:24:40 > 0:24:46to see for myself just what Hubble's revelation was.
0:24:46 > 0:24:48We're going to key in the co-ordinates
0:24:48 > 0:24:50of Andromeda to the console here.
0:24:50 > 0:24:53So zero hours 43 minutes...
0:24:53 > 0:24:57'For Hubble and his assistant, Milton Humason,
0:24:57 > 0:25:01'studying Andromeda was a long and painstaking process.
0:25:01 > 0:25:07'But today, we can quickly locate and photograph it in great detail.'
0:25:07 > 0:25:11- This is an image that we took a couple of weeks ago.- Right.
0:25:11 > 0:25:14If I zoom in, you'll see just there
0:25:14 > 0:25:18is the Hubble Cepheid, the first Cepheid that he found
0:25:18 > 0:25:22that unlocked the whole problem.
0:25:22 > 0:25:27Because presumably that is when he could use Leavitt's method
0:25:27 > 0:25:28of working out how far away it is.
0:25:28 > 0:25:32Exactly. Once he had seen this and identified it as a variable,
0:25:32 > 0:25:36he then had the key to determining just how bright that object was.
0:25:36 > 0:25:39And worked out that it couldn't have been in our own galaxy.
0:25:39 > 0:25:41It had to be millions of light years away.
0:25:41 > 0:25:44Absolutely, that is exactly it. You see the nuclear region,
0:25:44 > 0:25:48but as we adjust the contrast here, I can stretch the contrast
0:25:48 > 0:25:52- just to bring out some of the detail in the galaxy.- Oh, wow!
0:25:52 > 0:25:55Spiral arms. You see the dust lanes in silhouette
0:25:55 > 0:26:00against the billions of stars that are within Andromeda.
0:26:02 > 0:26:06By finding one of the variable stars in Andromeda,
0:26:06 > 0:26:10and measuring exactly how long it took to pulse,
0:26:10 > 0:26:13Hubble was able to use Leavitt's work
0:26:13 > 0:26:16to calculate exactly how far away it was.
0:26:19 > 0:26:21This is the photographic plate
0:26:21 > 0:26:26where Hubble marked his new Cepheid variable star.
0:26:26 > 0:26:31Using it, he calculated that Andromeda was many, many times
0:26:31 > 0:26:35more distant than the furthest reaches of the Milky Way.
0:26:38 > 0:26:42Andromeda was indeed an island universe,
0:26:42 > 0:26:46a vast galaxy of stars.
0:26:48 > 0:26:54We now know that Andromeda is over 2.5 million light years away.
0:26:54 > 0:26:59This means that the light that reaches us from Andromeda today,
0:26:59 > 0:27:04left on its journey before modern humans had evolved.
0:27:04 > 0:27:07- That's our neighbour. - That's our neighbour,
0:27:07 > 0:27:09our nearest large, galactic neighbour.
0:27:09 > 0:27:13I have to remember that what I am looking at here is the real thing.
0:27:13 > 0:27:18These are photons that have travelled millions of years
0:27:18 > 0:27:21- to reach my eye.- Exactly.
0:27:21 > 0:27:23These are photons directly from Andromeda
0:27:23 > 0:27:25that are arriving in my eye.
0:27:33 > 0:27:36Today, we have the power to see Andromeda
0:27:36 > 0:27:39as Hubble had only dreamed of.
0:27:45 > 0:27:47We now estimate that Andromeda
0:27:47 > 0:27:51contains over a trillion stars.
0:27:53 > 0:27:57And it is just one of a vast multitude of galaxies
0:27:57 > 0:28:01scattered throughout our universe.
0:28:23 > 0:28:30In 1923, the universe had been the size of the Milky Way.
0:28:30 > 0:28:34By 1924, the space that surrounds us
0:28:34 > 0:28:38had been revealed to be billions of times bigger
0:28:38 > 0:28:43and home to almost unimaginable cosmic complexity.
0:28:46 > 0:28:50Hubble had shown that there are a multitude of galaxies outside
0:28:50 > 0:28:56of our own and had pushed back the boundaries of the universe.
0:28:56 > 0:28:58But he had not seen an edge of space.
0:28:58 > 0:29:02He had not seen everything.
0:29:02 > 0:29:03There was still no clue
0:29:03 > 0:29:08as to how big our universe was, or even what shape it might be.
0:29:11 > 0:29:15To understand the strange truth about everything would require
0:29:15 > 0:29:18more than just observations.
0:29:18 > 0:29:21It would require mathematics -
0:29:21 > 0:29:25a powerful new type of mathematics that would be able to describe
0:29:25 > 0:29:29the bizarre properties of space itself.
0:29:30 > 0:29:33When you're trying to understand the universe, it's easy to think,
0:29:33 > 0:29:36what you do is you make lots and lots of observations, see what's there,
0:29:36 > 0:29:39and you fit it all together into your grand picture.
0:29:39 > 0:29:42But the problem is, unless you have some sort of idea
0:29:42 > 0:29:46what the picture should be, you don't know what observations to make,
0:29:46 > 0:29:47you don't know what's significant.
0:29:47 > 0:29:50And throughout the history of science,
0:29:50 > 0:29:53every so often someone has to come up with a new mathematical idea.
0:29:55 > 0:30:00The new mathematical ideas about space were so weird,
0:30:00 > 0:30:05so far removed from common sense, that it would take over 2,000 years
0:30:05 > 0:30:09and the genius of Albert Einstein to formulate them.
0:30:11 > 0:30:15But when they were ready, these strange new types
0:30:15 > 0:30:17of mathematics would lead to a revolution
0:30:17 > 0:30:21in our understanding of the space that surrounds us.
0:30:25 > 0:30:31OK. So what is space?
0:30:35 > 0:30:40We think we know the answer. I can talk about this room being spacious.
0:30:40 > 0:30:42There's a lot of space in here.
0:30:42 > 0:30:46Or a confined space. There's not enough volume, not enough space.
0:30:46 > 0:30:50But does space only exist when there's stuff in it?
0:30:50 > 0:30:54Does space only have a meaning when it's enclosed by walls?
0:31:00 > 0:31:05Think of the distance between two objects.
0:31:05 > 0:31:11Does that gap still exist if you take the objects away?
0:31:12 > 0:31:16What meaning can we give to distance
0:31:16 > 0:31:19if it doesn't have a start and end point?
0:31:21 > 0:31:23Ultimately, the question is this -
0:31:23 > 0:31:27does space in itself have form?
0:31:27 > 0:31:30Does it have structure or shape?
0:31:32 > 0:31:35Or is it just the place where things happen?
0:31:37 > 0:31:41The properties of space were first described by the mathematician
0:31:41 > 0:31:48Euclid over 2,000 years ago, in his legendary text, The Elements.
0:31:48 > 0:31:52In it, he laid down a set of simple, logical rules about space,
0:31:52 > 0:31:57in what today, we call Euclidian geometry.
0:31:57 > 0:32:01Euclidian geometry is the geometry we see around us every day.
0:32:01 > 0:32:04If you're sitting in a room and it's the usual rectangular room,
0:32:04 > 0:32:08what you see is lots of straight lines, right-angles, you see
0:32:08 > 0:32:12parallel lines, the window, the two sides of the window are parallel.
0:32:12 > 0:32:16If you extended them, they'd stay exactly the same distance apart,
0:32:16 > 0:32:18they would never meet.
0:32:18 > 0:32:21And the other thing you would see if you look a little closer
0:32:21 > 0:32:25is that any triangle you draw, the angles in the triangle
0:32:25 > 0:32:27always add up to 180 degrees.
0:32:27 > 0:32:30That's characteristic of Euclidian geometry.
0:32:30 > 0:32:33And people used to think that this was how geometry was,
0:32:33 > 0:32:35that nothing else was possible.
0:32:44 > 0:32:48For Euclid himself, and for almost all mathematicians
0:32:48 > 0:32:50for the next 2,000 years,
0:32:50 > 0:32:54these rules weren't just true mathematically,
0:32:54 > 0:32:59they were also true statements about physical reality itself.
0:32:59 > 0:33:02So they thought that two parallel lines
0:33:02 > 0:33:04would remain parallel for ever.
0:33:04 > 0:33:08That a triangle in real space would always have
0:33:08 > 0:33:11angles adding up to 180 degrees.
0:33:11 > 0:33:13But weird as though this might sound,
0:33:13 > 0:33:15it's not actually always true.
0:33:17 > 0:33:22Almost 250 years ago, in a small town in northern Germany,
0:33:22 > 0:33:28a mathematician was born who had the ability and originality to start
0:33:28 > 0:33:30to unravel Euclid's geometry
0:33:30 > 0:33:33and begin to change our ideas about space.
0:33:33 > 0:33:38His name was Carl Friedrich Gauss.
0:33:38 > 0:33:43Gauss tackled many great problems in his career, but from a young age,
0:33:43 > 0:33:48he began to speculate that the rules of Euclid may not be as absolute
0:33:48 > 0:33:50as everyone had assumed.
0:33:52 > 0:33:57Specifically, Gauss began to see that in curved spaces,
0:33:57 > 0:34:03other types of geometry could exist, with different rules to Euclid's.
0:34:04 > 0:34:09For example, on the surface of a sphere, the angles of a triangle
0:34:09 > 0:34:12can add up to more than 180 degrees.
0:34:17 > 0:34:23Many others would refine and develop Gauss's ideas.
0:34:23 > 0:34:27But one of his greatest achievements would be to give us a cunning method
0:34:27 > 0:34:30of accurately measuring curvature.
0:34:30 > 0:34:35It would become known simply as the Remarkable Theorem.
0:34:38 > 0:34:41Let me explain with this globe.
0:34:41 > 0:34:43We can see that it's three-dimensional,
0:34:43 > 0:34:45because we can stand back and look at it.
0:34:45 > 0:34:49But what if you were an ant, stuck on the surface?
0:34:49 > 0:34:52How would it know that that surface is curved?
0:34:52 > 0:34:56So, imagine you're the ant, and you start off at the North Pole.
0:34:56 > 0:35:04And facing south, you move down towards the equator.
0:35:04 > 0:35:09At the equator, you still face south, and you shuffle sideways,
0:35:09 > 0:35:11along the equator.
0:35:12 > 0:35:17Then, you reach a certain point, and then you start walking backwards
0:35:17 > 0:35:21so you're still facing the same direction, and head back
0:35:21 > 0:35:22to the North Pole.
0:35:24 > 0:35:26Now, look what's happened here.
0:35:26 > 0:35:30You've been pointing south all the way round, and yet when you arrive
0:35:30 > 0:35:35back at your starting point, you're facing in a different direction.
0:35:35 > 0:35:39Understanding this gives us a way of calculating the curvature
0:35:39 > 0:35:42of a surface without ever leaving it.
0:35:44 > 0:35:47'This was an amazing insight.'
0:35:47 > 0:35:52But it only applies to curved surfaces, which are two-dimensional.
0:35:52 > 0:35:57It would take a brilliant student of Gauss's, Bernhard Riemann,
0:35:57 > 0:36:01to develop these ideas in a way that could be applied
0:36:01 > 0:36:05to the three-dimensional space that surrounds us.
0:36:05 > 0:36:08It would be a daring, outlandish,
0:36:08 > 0:36:12and to non-mathematicians, absurd-sounding concept.
0:36:13 > 0:36:19Aged just 26, Riemann encapsulated his strange new ideas about geometry
0:36:19 > 0:36:24in a lecture that was to become legendary among mathematicians.
0:36:24 > 0:36:31In June 1854, Riemann delivered his lectures to an enraptured audience.
0:36:31 > 0:36:37In them, he detailed how he'd taken Gauss's ideas on curved surfaces,
0:36:37 > 0:36:41and generalised them, so that they applied not only to curved
0:36:41 > 0:36:46two-dimensional surfaces, but the curvature of space in any dimension.
0:36:57 > 0:37:00OK, so I'm sure this all sounds rather complicated.
0:37:00 > 0:37:05What exactly do we mean by curved space in any dimension?
0:37:05 > 0:37:07So let me try and explain.
0:37:07 > 0:37:12Here's the thing, Gauss talked about curved two-dimensional surfaces.
0:37:12 > 0:37:16Well, here we have a sheet of paper, and it's two-dimensional.
0:37:16 > 0:37:21So if I curve it, we can visualise and see this curvature.
0:37:21 > 0:37:24But only because it's embedded in three dimensions.
0:37:24 > 0:37:27Now, what if we curved three dimensions?
0:37:27 > 0:37:30Presumably, we'd need a fourth dimension.
0:37:31 > 0:37:36But how do you get to this four-dimensional space?
0:37:36 > 0:37:41It's impossible to step outside of our three-dimensional world.
0:37:41 > 0:37:43Wherever you travel in the universe,
0:37:43 > 0:37:48no matter how far you go, you're always stuck in three dimensions.
0:37:48 > 0:37:52The genius of Riemann was to show that you didn't
0:37:52 > 0:37:57need to stand in a fourth dimension to tell if space was curved.
0:37:57 > 0:38:01You could actually do it from the inside.
0:38:01 > 0:38:07But for Riemann, this would always remain a purely mathematical idea.
0:38:07 > 0:38:12It would take Albert Einstein to tie these mathematical ideas together,
0:38:12 > 0:38:17and apply bendy, curved, non-Euclidian geometries
0:38:17 > 0:38:21to the real space that surrounds us.
0:38:21 > 0:38:24I think the most important point about the whole story
0:38:24 > 0:38:26of non-Euclidian geometry is it shows
0:38:26 > 0:38:29how mathematics and the real world relate.
0:38:29 > 0:38:34And it starts out with mathematicians pottering around, asking,
0:38:34 > 0:38:37"Could there be a geometry different from Euclid's?"
0:38:37 > 0:38:40and if anyone said, "Why are you studying that?"
0:38:40 > 0:38:43They'd say, "Haven't got a clue." "What's it useful for?"
0:38:43 > 0:38:45"No idea. It's just interesting."
0:38:45 > 0:38:50But they pottered around and they found a surprising answer -
0:38:50 > 0:38:52that different geometries were possible.
0:38:52 > 0:38:57And even at that point, nobody had any real applications for this idea.
0:38:57 > 0:39:02And then when the moment is ripe, Einstein comes along and says,
0:39:02 > 0:39:05"That's what I need, that's real physics."
0:39:05 > 0:39:08And suddenly this piece of esoteric mathematics
0:39:08 > 0:39:12becomes vital to the scientific enterprise.
0:39:13 > 0:39:19Einstein would reveal that we live not in the flat world of Euclid,
0:39:19 > 0:39:24but in the strange, curved worlds of Gauss and Riemann.
0:39:24 > 0:39:28In the space of a few, short years,
0:39:28 > 0:39:31Einstein went from wrestling with some of the most difficult
0:39:31 > 0:39:37and abstract mathematical ideas to dinner dates with Charlie Chaplin.
0:39:37 > 0:39:41And it was all thanks to the pinnacle of his life's work -
0:39:41 > 0:39:45the general theory of relativity.
0:39:45 > 0:39:50In the general theory of relativity, Einstein took the mathematics
0:39:50 > 0:39:51of Gauss and Riemann
0:39:51 > 0:39:56and used it to paint a revolutionary picture of the physical world.
0:39:56 > 0:40:01He showed that just as Gauss had suspected, the geometry of the space
0:40:01 > 0:40:07around us isn't always of the regular, flat, Euclidian kind.
0:40:16 > 0:40:19'But if space is bent, and warped all around us,
0:40:19 > 0:40:24'surely we must be able to observe that this is the case?
0:40:24 > 0:40:28'Well, we do - just not in the way you might expect.
0:40:28 > 0:40:31'This was Einstein's major insight.
0:40:31 > 0:40:34'He showed that it was the ability for space to bend and warp,
0:40:34 > 0:40:37'for it to be flexible, and change its geometry,
0:40:37 > 0:40:41'that gives rise to the force we call gravity.'
0:40:45 > 0:40:47Right.
0:40:47 > 0:40:49Now, since Newton's time,
0:40:49 > 0:40:53gravity was thought to be a force that pulls all objects together.
0:40:53 > 0:40:55So if I drop this apple,
0:40:55 > 0:40:59it's as though there's an invisible rubber band that's pulling it
0:40:59 > 0:41:00down towards the earth.
0:41:00 > 0:41:04But Einstein's general theory of relativity gives us a completely
0:41:04 > 0:41:07different picture, and a totally new perspective.
0:41:09 > 0:41:13So although gravity appears to be a force,
0:41:13 > 0:41:17it's nothing more than the curvature of space itself.
0:41:17 > 0:41:21When an object falls, it's not being pulled by gravity at all,
0:41:21 > 0:41:25it's just following the simplest path through bent space.
0:41:28 > 0:41:33But the equations of general relativity didn't end there.
0:41:33 > 0:41:35They revealed that it was the presence of mass
0:41:35 > 0:41:39that caused space to curve and distort.
0:41:39 > 0:41:44The reason we have gravity on Earth is because the Earth is actually
0:41:44 > 0:41:49bending the space around it.
0:41:49 > 0:41:53In Einsteinian theory of the universe,
0:41:53 > 0:41:59space becomes a dynamic entity that reacts to its contents.
0:41:59 > 0:42:04Space knows about the presence of gravitating bodies, and responds
0:42:04 > 0:42:10to the presence by changing its geometry in really interesting ways.
0:42:10 > 0:42:15So what was in the 16th, 17th, 18th, 19th century, a very boring,
0:42:15 > 0:42:18still object, suddenly in Einsteinian theory,
0:42:18 > 0:42:22it becomes a dynamic, almost alive body.
0:42:26 > 0:42:30Einstein's theory revealed that space itself,
0:42:30 > 0:42:34the entire universe, everything,
0:42:34 > 0:42:41wasn't just unimaginably large, it also had a shape, and structure.
0:42:43 > 0:42:45It was malleable.
0:42:45 > 0:42:49Everything could be bent and warped.
0:43:00 > 0:43:04Gauss, Riemann and Einstein, had between them come up with
0:43:04 > 0:43:10a description of how the space and time we exist in can be warped.
0:43:10 > 0:43:15They showed that space and time are not the fixed, unchanging
0:43:15 > 0:43:18stage on which the actions of the universe are played out.
0:43:18 > 0:43:22They are actually part of the performance.
0:43:27 > 0:43:32It was soon realised that because the general theory of relativity
0:43:32 > 0:43:36applied to everything, it gave physicists a way of being
0:43:36 > 0:43:41able to step outside the universe, and imagine how it might be behaving
0:43:41 > 0:43:44in its entirety.
0:43:44 > 0:43:47And when they did this, they saw something
0:43:47 > 0:43:49that was extremely disturbing.
0:43:53 > 0:43:57The equations were giving a description of the universe
0:43:57 > 0:44:00that seemed ridiculous.
0:44:00 > 0:44:04They were describing something that was actually expanding.
0:44:09 > 0:44:13It seemed preposterous that the entire universe
0:44:13 > 0:44:18could be some sort of moving, organic, expanding entity.
0:44:20 > 0:44:23It was such a strange prediction
0:44:23 > 0:44:26that even Einstein refused to believe it.
0:44:33 > 0:44:38Einstein had overturned common sense notions of space and time held by
0:44:38 > 0:44:41humans over thousands of years.
0:44:41 > 0:44:43But he still couldn't accept
0:44:43 > 0:44:47that the whole universe might be dynamic and changing.
0:44:47 > 0:44:51In fact, he was so convinced that it was static, the he was prepared
0:44:51 > 0:44:56to modify his original equations by adding an extra term
0:44:56 > 0:45:02called the cosmological constant that would stabilise the universe.
0:45:02 > 0:45:05But Einstein was trying to fix something that wasn't broken.
0:45:07 > 0:45:12It's at this point that our story returns to Edwin Hubble.
0:45:12 > 0:45:17Armed with the Hooker telescope, Hubble would reveal the truth that
0:45:17 > 0:45:20Einstein had refused to believe.
0:45:21 > 0:45:26After discovering that our galaxy was just one of many, Hubble began
0:45:26 > 0:45:31to study the ways in which these other galaxies were moving.
0:45:36 > 0:45:41Hubble knew that, as a light source approaches us, the light wave would
0:45:41 > 0:45:45become compressed and appear blue.
0:45:46 > 0:45:51If an object was receding, the light waves would become stretched out
0:45:51 > 0:45:53and appear red.
0:45:58 > 0:46:00What he saw was astounding.
0:46:00 > 0:46:04All distant galaxies were being red shifted.
0:46:04 > 0:46:07They were all moving away from us.
0:46:07 > 0:46:11Not only that, but the further away a galaxy was,
0:46:11 > 0:46:14the faster it was moving away.
0:46:20 > 0:46:25Hubble's observations and Einstein's general theory of relativity
0:46:25 > 0:46:27were in agreement.
0:46:27 > 0:46:32But, and this is the crucial point here, it's not that the galaxies
0:46:32 > 0:46:36are flying away from each other through space.
0:46:36 > 0:46:39But rather that the fabric of space itself
0:46:39 > 0:46:43in between the galaxies is expanding.
0:46:43 > 0:46:48So the universe in its entirety is getting bigger.
0:46:48 > 0:46:52This is what Hubble and Einstein's work revealed.
0:47:00 > 0:47:05Einstein soon visited Hubble to see the data for himself.
0:47:05 > 0:47:10He would go on to admit that changing his equations had been
0:47:10 > 0:47:13his biggest scientific blunder.
0:47:13 > 0:47:17So, why was space expanding in this way?
0:47:20 > 0:47:24Both Hubble and Einstein soon came to agree.
0:47:25 > 0:47:28If the fabric of space was expanding
0:47:28 > 0:47:32it meant, previously, the universe was smaller.
0:47:34 > 0:47:37Rewind the clock far enough back...
0:47:38 > 0:47:41..and it appeared as if there was a point
0:47:41 > 0:47:46when our entire universe began.
0:48:00 > 0:48:04The data were pointing towards a moment of creation.
0:48:13 > 0:48:18But many scientists were not convinced by this apparent Big Bang.
0:48:18 > 0:48:22It seemed like a leap too far.
0:48:22 > 0:48:24But there was one piece of evidence
0:48:24 > 0:48:28that had the power to convince everyone.
0:48:36 > 0:48:41It seemed that if the Big Bang had happened, then some time after
0:48:41 > 0:48:42the instance of creation,
0:48:42 > 0:48:47a flash of light should have been emitted throughout the universe.
0:48:47 > 0:48:52Every part of the cosmos should now be filled with this light.
0:48:55 > 0:48:58And it turned out it was.
0:48:58 > 0:49:02It just happened to be in a rather unusual form.
0:49:04 > 0:49:09As unlikely as it sounds, the relic of the Big Bang fireball
0:49:09 > 0:49:12was actually visible on television.
0:49:16 > 0:49:19Let me explain how this is possible.
0:49:19 > 0:49:21Imagine this balloon is our universe.
0:49:24 > 0:49:27Here it is just a few hundred thousand years old.
0:49:27 > 0:49:30At this point, something very strange happens,
0:49:30 > 0:49:32because the universe suddenly becomes transparent
0:49:32 > 0:49:35to visible light as atoms form.
0:49:35 > 0:49:40It's as though a fog has lifted and light is suddenly able
0:49:40 > 0:49:42to travel freely through the universe.
0:49:47 > 0:49:52At every point in space, photons began to travel
0:49:52 > 0:49:58unimpeded and the entire universe is filled with a blinding light.
0:49:58 > 0:50:02But this light, released in the hot turmoil of the early universe,
0:50:02 > 0:50:05didn't stay bright for ever.
0:50:05 > 0:50:09As space expanded, it stretched through the spectrum
0:50:09 > 0:50:13from visible light down into microwaves.
0:50:17 > 0:50:22And it's these microwaves that get picked up by television aerials.
0:50:22 > 0:50:24Incredibly,
0:50:24 > 0:50:29almost one per cent of this static is the afterglow of creation itself.
0:50:29 > 0:50:32It's the stretched out remnants
0:50:32 > 0:50:35of the very earliest light in the universe.
0:50:43 > 0:50:48Today, with satellites, it's become possible to make an incredibly
0:50:48 > 0:50:54precise map of the universe at the moment it became light.
0:51:00 > 0:51:05This is the fossilised light of the first dawn.
0:51:08 > 0:51:12Convincing evidence that the universe had a beginning.
0:51:16 > 0:51:22Using the microwave radiation, cosmologists could even date it.
0:51:22 > 0:51:28Our entire universe is 13.7 billion years old.
0:51:32 > 0:51:36This beginning of everything would be the final piece of information
0:51:36 > 0:51:41needed to answer the question Thomas Digges had first posed
0:51:41 > 0:51:44over 400 years ago.
0:51:44 > 0:51:48It would finally give us a satisfactory explanation
0:51:48 > 0:51:51for why it gets dark at night.
0:51:53 > 0:51:58OK, so here it is, here's where I hope this all makes sense.
0:51:58 > 0:52:02The further away a star is, the longer it would take
0:52:02 > 0:52:05for its light to reach the Earth.
0:52:05 > 0:52:08So, if the universe has been around forever,
0:52:08 > 0:52:13then all the light that's out there will have had time to reach us
0:52:13 > 0:52:18and the night sky would be ablaze with starlight. But it's not.
0:52:20 > 0:52:23And here's why.
0:52:23 > 0:52:25Imagine when the universe was much younger
0:52:25 > 0:52:29and smaller than it is today.
0:52:29 > 0:52:33A beam of light on the other side of the universe begins a journey
0:52:33 > 0:52:36towards our vantage point.
0:52:36 > 0:52:39But, as space expands,
0:52:39 > 0:52:46the distance the light has to cross keeps getting bigger and bigger.
0:52:46 > 0:52:51Fast forward to today, and this light still hasn't reached us.
0:52:51 > 0:52:54So, no matter how hard we look into the sky,
0:52:54 > 0:52:58we simply won't be able to see it.
0:52:58 > 0:53:02We can only see the stars whose light has had time to reach us
0:53:02 > 0:53:07in the 13.7 billion years since the Big Bang.
0:53:07 > 0:53:11This region is known as the observable universe.
0:53:11 > 0:53:17And there are not enough stars here to light up the night sky.
0:53:17 > 0:53:22So, we only ever see the stars and galaxies whose light
0:53:22 > 0:53:29has had a chance to reach us, and that's why it gets dark at night.
0:53:40 > 0:53:43The simplest fact that we take for granted,
0:53:43 > 0:53:45that the sky at night is dark,
0:53:45 > 0:53:48is in fact incredibly profound.
0:53:48 > 0:53:54It took 200 years of theorising, of thinking, it took the development
0:53:54 > 0:53:57of general relativity, before we could understand
0:53:57 > 0:54:00why the sky at night is dark.
0:54:11 > 0:54:16By reasoning and observing and imagining, we've found
0:54:16 > 0:54:22ever better ways to project outside of the confines of our small rock
0:54:22 > 0:54:24tumbling through space.
0:54:24 > 0:54:30We've become ever more skilled at creating pictures
0:54:30 > 0:54:31of everything.
0:54:37 > 0:54:42This is a computer simulation of the universe in its infancy.
0:54:42 > 0:54:47Using it, we can see how the force of gravity has shaped the universe
0:54:47 > 0:54:50over billions of years.
0:54:50 > 0:54:53The brightest white and yellow regions in this image
0:54:53 > 0:54:58show where galaxies and clusters of galaxies form.
0:54:58 > 0:55:01You can see how, as the universe evolves,
0:55:01 > 0:55:06a strange and hidden structure begins to emerge.
0:55:13 > 0:55:16This is the cosmic web.
0:55:16 > 0:55:20It's our best picture yet of what everything might look like
0:55:20 > 0:55:23at the largest scales.
0:55:26 > 0:55:31It shows massive clusters of galaxies linked together
0:55:31 > 0:55:37in vast filaments, each one containing trillions of stars.
0:55:44 > 0:55:48Its scale is sometimes difficult to appreciate.
0:55:48 > 0:55:52But it would take light almost 10 billion years
0:55:52 > 0:55:56to cross the distance in this image.
0:56:05 > 0:56:11But this incredible picture of everything is destined to change.
0:56:11 > 0:56:15We are starting to understand that, in the distant future, the universe
0:56:15 > 0:56:20will become a terrifyingly bleak and desolate place.
0:56:23 > 0:56:27In 1998, a team of astronomers published a paper
0:56:27 > 0:56:31in which they looked at supernova explosions in distant galaxies.
0:56:31 > 0:56:34They were hoping to measure very accurately
0:56:34 > 0:56:37how fast the universe was expanding.
0:56:37 > 0:56:41Now, they expected to find that the rate of expansion was slowing down,
0:56:41 > 0:56:46just because of the pull of gravity of all the matter in the universe.
0:56:46 > 0:56:49But they were in for a big surprise.
0:56:49 > 0:56:53The universe was getting bigger, faster.
0:56:56 > 0:57:00The rate of expansion was accelerating.
0:57:00 > 0:57:05There seemed to be some mysterious force pushing everything apart.
0:57:05 > 0:57:08We still don't understand its origin,
0:57:08 > 0:57:11but it's been dubbed dark energy.
0:57:17 > 0:57:22There's one fascinating yet disturbing consequence of this.
0:57:22 > 0:57:27If the expansion of the universe continues to accelerate
0:57:27 > 0:57:32then our visible universe will begin to empty.
0:57:32 > 0:57:35Let me explain. Imagine that I'm in a distant
0:57:35 > 0:57:37galaxy that you can see from Earth.
0:57:37 > 0:57:43As the space between us stretches, there will come a time in the future
0:57:43 > 0:57:48when it is expanding so rapidly that light can't outrun it,
0:57:48 > 0:57:52and the galaxy will disappear from view.
0:57:54 > 0:57:56What this means
0:57:56 > 0:58:02is that, far into the future, some 100 billion years from now,
0:58:02 > 0:58:06if intelligent life forms still exist in our galaxy,
0:58:06 > 0:58:12they'll look out into space and see only the stars in our own Milky Way.
0:58:12 > 0:58:16All the other galaxies will have disappeared.
0:58:16 > 0:58:23And they will be alone in a vast, dark, empty expanse.
0:58:32 > 0:58:37I have here a box. What would happen if I were to remove everything
0:58:37 > 0:58:39I possibly could from inside it?
0:58:39 > 0:58:44What then exists inside the space in the box?
0:58:44 > 0:58:46Is it really nothing?
0:58:59 > 0:59:01Subtitles by Red Bee Media Ltd
0:59:01 > 0:59:03E-mail subtitling@bbc.co.uk