0:00:03 > 0:00:06In our night sky, you can see space -
0:00:06 > 0:00:11big, mysterious, and frankly a bit scary.
0:00:11 > 0:00:14I sometimes look up at it and ask those big questions.
0:00:14 > 0:00:17Such as, how do stars work?
0:00:17 > 0:00:19How did it all begin?
0:00:19 > 0:00:22And, what is Madonna doing in space?
0:00:22 > 0:00:25For the answers, stick with me,
0:00:25 > 0:00:30as we uncover the things you need to know about the universe.
0:00:30 > 0:00:32Right, let's get this show on the road,
0:00:32 > 0:00:37and the beginning is as good a place to start as any.
0:00:37 > 0:00:39So, how did the universe begin?
0:00:41 > 0:00:45Everyone knows the universe started with a bang -
0:00:45 > 0:00:50a bang so big, it's called the Big Bang.
0:00:50 > 0:00:54It happened everywhere in the universe at the same time,
0:00:54 > 0:00:57and was the beginning of everything we know -
0:00:57 > 0:01:00space, matter and even time itself.
0:01:02 > 0:01:05The Big Bang is the most important event in history.
0:01:05 > 0:01:08In fact, without it, there wouldn't be any history.
0:01:08 > 0:01:12Even so, the biggest brains in science don't really know
0:01:12 > 0:01:14why it happened, only that it did.
0:01:14 > 0:01:20And they know it did because there are clues out there in space.
0:01:22 > 0:01:26In 1929, astronomer Edwin Hubble discovered that
0:01:26 > 0:01:29distant galaxies are moving away from us,
0:01:29 > 0:01:33and the furthest are moving away faster than the closer ones.
0:01:35 > 0:01:41So, logically, all these galaxies came from one tiny central point -
0:01:41 > 0:01:42the Big Bang.
0:01:44 > 0:01:48Further proof was found by two young scientists in the 1960s,
0:01:48 > 0:01:52when their new radio telescope seemed to be faulty,
0:01:52 > 0:01:55due to a constant annoying hiss.
0:01:55 > 0:01:58Hmm? Grr!
0:01:58 > 0:02:01They checked the telescope and pinned the blame
0:02:01 > 0:02:03on some resident pigeons and their droppings.
0:02:03 > 0:02:05GUN COCKS
0:02:05 > 0:02:07SHOT FIRES
0:02:07 > 0:02:12They swept the dish and evicted the birds, but the hiss was still there.
0:02:12 > 0:02:18This hiss was in fact cosmic microwave background radiation,
0:02:18 > 0:02:22or CMB, which is the heat and light left over from the Big Bang.
0:02:22 > 0:02:27It's travelled over 270,000 billion, billion miles to reach us,
0:02:27 > 0:02:30and has slowly cooled on its long journey.
0:02:32 > 0:02:35In fact, we've all seen the Big Bang.
0:02:35 > 0:02:42About 1% of the static on your untuned TV is this CMB radiation.
0:02:42 > 0:02:47And scientists have used this static to calculate the age
0:02:47 > 0:02:52of the universe, which is roughly 13.7 billion years old.
0:02:52 > 0:02:55That's a seriously long time ago,
0:02:55 > 0:02:59so perhaps we may never know what caused the Big Bang.
0:02:59 > 0:03:05But we do know that the universe apparently came from nothing,
0:03:05 > 0:03:09but is now everything. So if it came from nothing,
0:03:09 > 0:03:13what is the universe made from?
0:03:13 > 0:03:17You might be surprised to learn that we don't really know.
0:03:17 > 0:03:20All the matter and energy we can see only accounts for
0:03:20 > 0:03:23about 5% of the universe's total mass,
0:03:23 > 0:03:28and a whopping 97% of all this visible stuff is made up
0:03:28 > 0:03:32of just two elements - hydrogen and helium.
0:03:34 > 0:03:37So, everything heavier here on Earth,
0:03:37 > 0:03:43like carbon, oxygen, water, baboons, jet planes and clowns,
0:03:43 > 0:03:45is very rare indeed.
0:03:45 > 0:03:48And as for the remaining 95% that's invisible,
0:03:48 > 0:03:51well, unsurprisingly, that's a bit of a mystery.
0:03:51 > 0:03:56Scientists think that a quarter of all this missing stuff is
0:03:56 > 0:04:01comprised of dark matter. It's divided into two theoretical types -
0:04:01 > 0:04:06weakly-interacting massive particles, or WIMPs for short,
0:04:06 > 0:04:11and massive compact halo objects, or MACHOs, to you and me.
0:04:11 > 0:04:15WIMPs are tiny particles of exotic matter.
0:04:15 > 0:04:18And by exotic, scientists mean
0:04:18 > 0:04:20they are different from ordinary particles.
0:04:20 > 0:04:22Not that they dance for money.
0:04:24 > 0:04:28But so far, they've proven undetectable.
0:04:29 > 0:04:33There could be WIMPs flying through you right now,
0:04:33 > 0:04:36only you can't see or feel them.
0:04:37 > 0:04:43And we can't see MACHOs, as they don't reflect or emit light.
0:04:43 > 0:04:48They could be everything from failed stars to black holes.
0:04:48 > 0:04:52Although it only exists theoretically, scientists think
0:04:52 > 0:04:56dark matter is important, as it seems to binds galaxies together.
0:04:56 > 0:04:59Otherwise they would simply fly apart -
0:04:59 > 0:05:03which might be fun to watch, but not so good for us.
0:05:03 > 0:05:05But even with dark matter,
0:05:05 > 0:05:09that still leaves about 70% of the universe simply missing.
0:05:09 > 0:05:14Scientists have called what's left dark energy,
0:05:14 > 0:05:16which is even more puzzling.
0:05:17 > 0:05:20In 1998, astronomers discovered that
0:05:20 > 0:05:24the universe's rate of expansion is actually increasing.
0:05:24 > 0:05:27Something seems to be overpowering gravity,
0:05:27 > 0:05:29which scientists thought would
0:05:29 > 0:05:32eventually slow the universe's expansion.
0:05:32 > 0:05:37They now think that this is due to dark energy.
0:05:37 > 0:05:40So, if our greatest thinkers had to take an exam
0:05:40 > 0:05:42in what the universe is made from,
0:05:42 > 0:05:44they'd have to tick the "Don't know" box.
0:05:44 > 0:05:48But it's definitely made from something, and if we look at
0:05:48 > 0:05:53the night sky, what we can actually see are stars, billions of them.
0:05:53 > 0:05:56So many, in fact, that it sort of makes your brain hurt a bit.
0:05:56 > 0:05:59So, how do stars work?
0:06:01 > 0:06:04There are hundreds of billions of stars in our galaxy alone.
0:06:04 > 0:06:09And they come in a dazzling variety of colour, size and brightness.
0:06:09 > 0:06:15All stars, including our own Sun, work in roughly the same way.
0:06:15 > 0:06:20They harness the power of nuclear reactions -
0:06:20 > 0:06:22specifically, a process we call fusion.
0:06:22 > 0:06:26Humans have also harnessed nuclear power,
0:06:26 > 0:06:29but this is fission, not fusion.
0:06:29 > 0:06:34Fission is the splitting of atoms to unleash vast amounts of energy,
0:06:34 > 0:06:36as in the original atomic bomb.
0:06:38 > 0:06:41But during fusion, the opposite occurs.
0:06:41 > 0:06:43In the deep core of a star,
0:06:43 > 0:06:48hydrogen atoms collide and fuse together, creating helium.
0:06:48 > 0:06:51These shiny new helium atoms have slightly less mass
0:06:51 > 0:06:54than the hydrogen atoms that created them.
0:06:54 > 0:06:59And this lost mass is released as gamma radiation.
0:06:59 > 0:07:04This is explained by Einstein's famous E = mc2 equation,
0:07:04 > 0:07:08which states that mass and energy are effectively
0:07:08 > 0:07:09two sides of the same coin.
0:07:09 > 0:07:12In fact, if you could measure it accurately enough,
0:07:12 > 0:07:16a hot cup of tea would weigh more than an identical cold one,
0:07:16 > 0:07:19because it has more energy.
0:07:21 > 0:07:24Mmm. Now, just try to imagine this -
0:07:24 > 0:07:28every single second, 600 million tons of hydrogen collide
0:07:28 > 0:07:32inside the Sun, and this creates new helium,
0:07:32 > 0:07:35and that releases four million tons of energy.
0:07:35 > 0:07:39Or, to put it another way, while I've been speaking,
0:07:39 > 0:07:45the Sun has lost the equivalent of 500 aircraft carriers in mass.
0:07:45 > 0:07:49Can I have another cup of tea, please?
0:07:49 > 0:07:54Fusion is the beating heart of all stars in the universe.
0:07:54 > 0:07:57And it's pretty handy for us down here on Earth,
0:07:57 > 0:08:00as it provides the visible light we depend on.
0:08:00 > 0:08:03- Wow! Cool!- Ow! My frickin' eyes!
0:08:03 > 0:08:08In fact, the light from the Sun only takes eight minutes to reach us,
0:08:08 > 0:08:10but can be up to a million years old,
0:08:10 > 0:08:14as it takes so long to journey out of the Sun's dense interior.
0:08:14 > 0:08:16Mmm.
0:08:16 > 0:08:19Makes you think about sunbathing in a whole new light, really.
0:08:19 > 0:08:23But anyway, stars are actually just like you and me.
0:08:23 > 0:08:26Apart from them being great flaming balls of fire.
0:08:26 > 0:08:30But like you and me, they are born, they live and then they die -
0:08:30 > 0:08:32often with spectacular results.
0:08:32 > 0:08:36So, what happens when a star dies?
0:08:38 > 0:08:43As the Sun grows old, it will become smaller, brighter and hotter.
0:08:47 > 0:08:50In three billion years, it will be 40% brighter,
0:08:50 > 0:08:52and so hot, it will evaporate our oceans.
0:08:55 > 0:08:58In another two billion years, its core will collapse,
0:08:58 > 0:09:01and it will expand to form a red giant -
0:09:01 > 0:09:03so massive, it will engulf the Earth.
0:09:05 > 0:09:09When this red giant Sun finally dies,
0:09:09 > 0:09:14it will throw off most of its mass in a huge nebula of gas and dust.
0:09:14 > 0:09:19And all that will remain is a dense core, called a white dwarf,
0:09:19 > 0:09:23which will slowly cool over billions of years.
0:09:25 > 0:09:29However, not all stars behave like the Sun.
0:09:30 > 0:09:35Other stars, called red dwarfs, use their fuel so economically,
0:09:35 > 0:09:38they may last for a trillion years or more.
0:09:38 > 0:09:41And at the other end of the spectrum, the biggest stars,
0:09:41 > 0:09:46hundreds of times bigger than the Sun, burn their fuel very quickly -
0:09:46 > 0:09:48they live fast and die young.
0:09:50 > 0:09:52It's these biggest stars that produce
0:09:52 > 0:09:55the strangest results when they die.
0:09:55 > 0:10:00They literally go out with a bang, exploding in violent supernovas,
0:10:00 > 0:10:03amongst the most spectacular events in the universe.
0:10:08 > 0:10:11Supernova explosions might be spectacular,
0:10:11 > 0:10:14but they could also be very dangerous for us humans.
0:10:14 > 0:10:17If one occurred within 25 light years of earth,
0:10:17 > 0:10:20it would kill all life on the planet.
0:10:20 > 0:10:22The ozone layer would be destroyed,
0:10:22 > 0:10:24bathing us in lethal doses of radiation.
0:10:24 > 0:10:28Probably best to stay indoors if this happens.
0:10:28 > 0:10:32These supernova explosions can have two outcomes.
0:10:32 > 0:10:35Firstly, the material left behind can collapse
0:10:35 > 0:10:38to form a superdense neutron star.
0:10:38 > 0:10:43A neutron star is only about the size of a city like London,
0:10:43 > 0:10:46but can weigh twice as much as our Sun.
0:10:46 > 0:10:49But when the largest stars of all explode,
0:10:49 > 0:10:52the remaining neutron core is compressed
0:10:52 > 0:10:55in a fraction of a second into a singularity.
0:10:57 > 0:11:00This is infinitely small, smaller even than an atom,
0:11:00 > 0:11:03and its gravitational pull is so massive,
0:11:03 > 0:11:06nothing can escape from it, not even light.
0:11:06 > 0:11:10This is a stellar black hole.
0:11:10 > 0:11:15They are the strangest and most destructive forces in nature -
0:11:15 > 0:11:20anything straying near a black hole will be sucked in and destroyed.
0:11:22 > 0:11:26The universe sounds like a pretty violent place,
0:11:26 > 0:11:28what with innocent stars exploding
0:11:28 > 0:11:31and being eaten by rogue black holes.
0:11:31 > 0:11:34But if stars are continually being destroyed,
0:11:34 > 0:11:39surely they're at risk of becoming an endangered species?
0:11:39 > 0:11:43So, why aren't stars extinct?
0:11:44 > 0:11:49Across the universe, stars are dying all the time, so you might think
0:11:49 > 0:11:53the night sky would slowly dim as their lights are snuffed out.
0:11:53 > 0:11:56This isn't happening - but why?
0:11:56 > 0:12:00Well, the answer is recycling.
0:12:00 > 0:12:02All the mass and energy that exists today
0:12:02 > 0:12:05was created during the Big Bang.
0:12:05 > 0:12:09And that's it, that's all there'll ever be.
0:12:09 > 0:12:13Therefore, the universe needs an efficient recycling scheme,
0:12:13 > 0:12:16and this relies on the humble atom.
0:12:17 > 0:12:21Atoms consist of a tiny central nucleus
0:12:21 > 0:12:24surrounded by a cloud of orbiting electrons.
0:12:28 > 0:12:31If an atom was the size of a football pitch,
0:12:31 > 0:12:35the nucleus would be smaller than a single blade of grass.
0:12:35 > 0:12:39The vast majority of an atom is empty space,
0:12:39 > 0:12:43which means most of everything is actually empty space.
0:12:47 > 0:12:49Atoms are also remarkably durable.
0:12:49 > 0:12:54No-one knows how long a single atom can survive, but it could be
0:12:54 > 0:12:58as long as 100 billion, trillion, trillion years.
0:13:01 > 0:13:05So long, in fact, that they can be reused almost endlessly.
0:13:07 > 0:13:12When a star dies, most of its mass is thrown out into space.
0:13:12 > 0:13:16This is when the recycling process can begin.
0:13:16 > 0:13:20Over time, a dead star's atoms condense and compact,
0:13:20 > 0:13:26until they become so hot, they ignite, forming a new star.
0:13:27 > 0:13:33You could say this is stellar reincarnation.
0:13:33 > 0:13:37Our Sun is thought to be a third-generation star,
0:13:37 > 0:13:40so every atom here on Earth has passed through
0:13:40 > 0:13:43two long-dead stars already.
0:13:43 > 0:13:45Not only that, but as atoms are
0:13:45 > 0:13:48constantly recycled here on Earth too,
0:13:48 > 0:13:52millions of your atoms once belonged to Shakespeare,
0:13:52 > 0:13:54Genghis Khan and Julius Caesar.
0:13:57 > 0:14:00It's weird to think that all of the atoms in our body might once have
0:14:00 > 0:14:06been part of ancient stars or planets, or even aliens.
0:14:06 > 0:14:10We are really just a series of cosmic hand-me-downs.
0:14:10 > 0:14:14But how did the atoms of some ancient star end up
0:14:14 > 0:14:18as part of you or me or even a geranium?
0:14:18 > 0:14:23Just how did the solar system form?
0:14:23 > 0:14:28Our solar system began life as a huge cloud of gas and dust
0:14:28 > 0:14:30called the solar nebula.
0:14:30 > 0:14:32About 4.6 billion years ago,
0:14:32 > 0:14:37this giant cloud started to coalesce under the force of gravity.
0:14:37 > 0:14:41It also began to spin itself into a flattened disk shape.
0:14:44 > 0:14:47At the centre of this spinning nebula,
0:14:47 > 0:14:5299% of all its mass compressed into a protosun.
0:14:52 > 0:14:56This was a baby star, not yet ready to ignite.
0:14:56 > 0:15:02All the remaining matter in the huge cloud around the new protosun
0:15:02 > 0:15:05slowly formed itself into rings.
0:15:05 > 0:15:10These rings would eventually become the planets we know today.
0:15:10 > 0:15:13Close to the protosun, the higher temperatures meant
0:15:13 > 0:15:17only rocky materials and metals could survive the heat.
0:15:17 > 0:15:21Therefore, the closest planets to the Sun -
0:15:21 > 0:15:24Mercury, Venus, Earth and Mars -
0:15:24 > 0:15:27are mostly composed of heavier elements, like iron.
0:15:27 > 0:15:32But in the distant, colder regions, big lumps of rock and ice managed
0:15:32 > 0:15:34to capture vast clouds of gas around them.
0:15:34 > 0:15:42These would become the gas giants - Jupiter, Saturn, Uranus and Neptune.
0:15:42 > 0:15:45Eventually, the protosun became dense enough
0:15:45 > 0:15:48to begin fusion in its core, and - drum roll, please -
0:15:48 > 0:15:52our Sun finally became a fully-fledged star.
0:15:55 > 0:15:58But for Earth, the story was really just beginning.
0:15:58 > 0:16:03When our planet was just 100 million years old, a huge object
0:16:03 > 0:16:05the size of Mars collided with us,
0:16:05 > 0:16:08throwing vast amounts of rock into space.
0:16:08 > 0:16:13This stray rock eventually reassembled itself into the Moon.
0:16:13 > 0:16:17That huge collision that formed the Moon
0:16:17 > 0:16:22also knocked the Earth off its axis by 23.5 degrees,
0:16:22 > 0:16:26and that was very handy for us, because that created the seasons.
0:16:26 > 0:16:31In its youth, Earth is thought to have been volcanic and inhospitable.
0:16:31 > 0:16:35But over time, it came to be covered in a vast ocean of water,
0:16:35 > 0:16:38perhaps carried here by icy comets and asteroids.
0:16:38 > 0:16:43However it got here, it was this water that allowed life to thrive.
0:16:43 > 0:16:46We all know our place in the solar system -
0:16:46 > 0:16:48the third rock from the Sun,
0:16:48 > 0:16:52one of eight planets orbiting our nearest star.
0:16:52 > 0:16:57But, if there is anyone or anything else out there,
0:16:57 > 0:17:00how will they be able to find us?
0:17:00 > 0:17:01Where are we?
0:17:01 > 0:17:05If there's some sort of intergalactic postal system,
0:17:05 > 0:17:10then our address might look something like this.
0:17:10 > 0:17:13Planet Earth, the solar system, local fluff,
0:17:13 > 0:17:18the Milky Way, local group, local supercluster, the universe.
0:17:21 > 0:17:23For most of human history,
0:17:23 > 0:17:28we have placed our planet slap bang at the centre of the universe.
0:17:32 > 0:17:36But since astronomer Nicolaus Copernicus realised that
0:17:36 > 0:17:38the Earth actually orbits the Sun,
0:17:38 > 0:17:42every new discovery has highlighted the fact
0:17:42 > 0:17:47that we're not particularly special at all.
0:17:47 > 0:17:51Actually, it seems as though we live in a rather unimportant cul-de-sac,
0:17:51 > 0:17:53in the grand scheme of things.
0:17:53 > 0:17:58We are just an insignificant dot in the vastness of space.
0:17:58 > 0:18:00But don't get depressed about this -
0:18:00 > 0:18:04because that assumption is, in fact, the cornerstone of modern astronomy,
0:18:04 > 0:18:08what is sometimes known as the cosmological principle.
0:18:08 > 0:18:11The cosmological principle states that,
0:18:11 > 0:18:16when viewed on a sufficiently large scale,
0:18:16 > 0:18:18the universe actually looks the same,
0:18:18 > 0:18:20in all places and in all directions.
0:18:23 > 0:18:27So when we look out into space at the other stars and galaxies,
0:18:27 > 0:18:30they might seem haphazard and irregular,
0:18:30 > 0:18:33but in fact, they are all laid out in a very symmetrical way.
0:18:39 > 0:18:41So, if we send out a galactic calling card,
0:18:41 > 0:18:45there's a distinct possibility that aliens wouldn't be able
0:18:45 > 0:18:50to find us, even with directions, due to the cosmological principle
0:18:50 > 0:18:52and the sheer size of the universe.
0:19:02 > 0:19:04We humans are obsessed with the idea
0:19:04 > 0:19:07that there could be other life in the universe.
0:19:07 > 0:19:11From books to films to people who claim to have been abducted,
0:19:11 > 0:19:15we all have our own take on what else might be out there.
0:19:18 > 0:19:22But is there any fact behind any of this fiction?
0:19:23 > 0:19:25Or are we alone?
0:19:27 > 0:19:31If there is life out there, then scientists believe
0:19:31 > 0:19:35it will probably be found in the Goldilocks zone.
0:19:35 > 0:19:38This has nothing to do with bears and porridge,
0:19:38 > 0:19:41but describes a planet that's just right,
0:19:41 > 0:19:43the perfect distance from a star
0:19:43 > 0:19:47and warm enough for water to be found on its surface.
0:19:47 > 0:19:51In our solar system, only the Earth fits the bill.
0:19:51 > 0:19:55Our nearest neighbours, Venus and Mars, are just too close
0:19:55 > 0:19:57or too far away from the Sun.
0:19:57 > 0:20:01However, this doesn't mean there couldn't be life
0:20:01 > 0:20:03somewhere else in our solar system.
0:20:03 > 0:20:07It's just that it would probably be simple bacteria,
0:20:07 > 0:20:10and unlikely to provide intelligent conversation.
0:20:10 > 0:20:12I can't say I have much confidence in your opinion!
0:20:12 > 0:20:16There's bacteria here on Earth that can live in poisonous environments,
0:20:16 > 0:20:20suggesting creatures on other worlds may evolve
0:20:20 > 0:20:22in ways we can barely imagine.
0:20:22 > 0:20:25And instead of being carbon-based, as on Earth,
0:20:25 > 0:20:28life elsewhere could have evolved
0:20:28 > 0:20:31using other elements, such as silicon.
0:20:31 > 0:20:34Such creatures could withstand much higher temperatures than us,
0:20:34 > 0:20:39so planets too hot for humans could still support life.
0:20:39 > 0:20:41In fact, the hunt for life
0:20:41 > 0:20:45beyond our solar system is already in full swing.
0:20:45 > 0:20:50NASA's Kepler Mission has identified over 1,000 potential planets
0:20:50 > 0:20:53worthy of more investigation, and of these,
0:20:53 > 0:20:5715 have been confirmed as lying in the Goldilocks zone.
0:20:57 > 0:21:03In the 1960s, a scientist named Frank Drake developed an equation
0:21:03 > 0:21:08designed to calculate the number of other civilizations
0:21:08 > 0:21:09in our own galaxy, the Milky Way.
0:21:09 > 0:21:14These factors include the rate of new stars forming in the galaxy,
0:21:14 > 0:21:17the proportion of these stars that have planets,
0:21:17 > 0:21:21the percentage of these planets that are habitable,
0:21:21 > 0:21:27and the length of time any civilization might last.
0:21:29 > 0:21:32At the moment, we can only hypothesise
0:21:32 > 0:21:34about most of these numbers,
0:21:34 > 0:21:38but conservative estimates suggest that there might be
0:21:38 > 0:21:41900 advanced civilizations in the Milky Way at any one time,
0:21:41 > 0:21:45and our galaxy is just one of billions.
0:21:45 > 0:21:49So, there's a very good chance that we're not alone.
0:21:49 > 0:21:53Unfortunately, the distances involved are so huge
0:21:53 > 0:21:57that we may never make contact with anybody out there.
0:21:57 > 0:22:00However, that doesn't mean our nearest neighbours
0:22:00 > 0:22:05haven't already discovered us, or at least our taste in pop music.
0:22:05 > 0:22:11So, just what is Madonna doing in space?
0:22:11 > 0:22:16The furthest any human has ventured from Earth is to our own Moon,
0:22:16 > 0:22:19which in space terms is barely beyond our own doorstep.
0:22:19 > 0:22:21Look at me, I'm flying!
0:22:21 > 0:22:24Oh, no, wait, maybe not.
0:22:24 > 0:22:28However, humanity has in fact travelled much further than that,
0:22:28 > 0:22:32far beyond the furthest reaches of our solar system.
0:22:32 > 0:22:37Madonna, Hitler and the Dalai Lama -
0:22:37 > 0:22:40they're all out there in deep space,
0:22:40 > 0:22:42thanks to the power of radio waves.
0:22:44 > 0:22:48Since the early 20th century, our broadcasts have
0:22:48 > 0:22:54leaked out into space, almost like a three-dimensional ripple on a pond.
0:22:56 > 0:22:59All electromagnetic waves, including radio waves,
0:22:59 > 0:23:01travel at the same speed -
0:23:01 > 0:23:08some 300 million metres per second, what we call the speed of light.
0:23:08 > 0:23:13Therefore, TV and radio signals transmitted 50 years ago have
0:23:13 > 0:23:16journeyed 50 light years into space by now,
0:23:16 > 0:23:20ample time to have reached hundreds of neighbouring stars.
0:23:22 > 0:23:26So, if there are alien civilisations out there listening in,
0:23:26 > 0:23:29the first they might know about life on Earth is
0:23:29 > 0:23:33a speech by Martin Luther King, or an episode of EastEnders,
0:23:33 > 0:23:35or even what I'm saying now.
0:23:35 > 0:23:39But it's not just us broadcasting radio waves.
0:23:39 > 0:23:44They're also being emitted by galaxies and even black holes,
0:23:44 > 0:23:46which can be a bit confusing.
0:23:46 > 0:23:50In 1967, a young Cambridge astronomy student noticed
0:23:50 > 0:23:54a strange radio signal from space
0:23:54 > 0:23:59that pulsated exactly every 1.337 seconds.
0:23:59 > 0:24:03It was so precise and regular, it didn't appear natural.
0:24:03 > 0:24:09Therefore, the object emitting the signal was dubbed LGM-1.
0:24:09 > 0:24:13LGM stood for Little Green Men.
0:24:13 > 0:24:17However, it wasn't a friendly alien, but a pulsar,
0:24:17 > 0:24:20one of the strangest natural phenomena in the universe.
0:24:20 > 0:24:25Pulsars are spinning neutron stars that emit beams of radiation -
0:24:25 > 0:24:29almost like intergalactic lighthouses.
0:24:29 > 0:24:32When you're just sitting at home,
0:24:32 > 0:24:36it's comforting to think that the universe will be around for ever.
0:24:36 > 0:24:38But don't get too comfortable,
0:24:38 > 0:24:43because observations by cosmologists suggest that it probably won't.
0:24:43 > 0:24:47So, the final question is really very obvious -
0:24:47 > 0:24:49when will the universe end?
0:24:49 > 0:24:54Scientists have developed three plausible theories
0:24:54 > 0:24:57as to what might happen at the end of it all.
0:24:57 > 0:25:01Currently, the most likely is the Big Chill.
0:25:01 > 0:25:03This is what will happen
0:25:03 > 0:25:07if the universe continues to expand forever.
0:25:07 > 0:25:11Firstly, galaxies would move away from each other.
0:25:11 > 0:25:17Then stars, and everything else, would slowly drift apart and die.
0:25:17 > 0:25:21Finally, only giant black holes will remain,
0:25:21 > 0:25:25each separated by distances 100 times greater
0:25:25 > 0:25:28than the current size of our universe.
0:25:28 > 0:25:32Eventually, even these black holes will evaporate,
0:25:32 > 0:25:38and the universe will be still, cold and effectively dead.
0:25:38 > 0:25:42But before you start to panic, you should probably know that
0:25:42 > 0:25:45we have around 100 trillion years to wait
0:25:45 > 0:25:50before even the beginning of the end, when stars start to disappear.
0:25:50 > 0:25:53And that's about 10,000 times as long
0:25:53 > 0:25:56as the universe has existed already.
0:25:56 > 0:25:58And in any case, by that time,
0:25:58 > 0:26:03we humans will probably have long since disappeared.
0:26:03 > 0:26:07Another possible end is called the Big Rip.
0:26:07 > 0:26:13This would be the spectacular and rapid destruction of the universe.
0:26:13 > 0:26:16But this will only occur if, in the future,
0:26:16 > 0:26:21the mysterious force of dark energy somehow supersedes gravity.
0:26:22 > 0:26:25First galaxies, and then literally everything,
0:26:25 > 0:26:29right down to tiny atoms, would be torn apart.
0:26:31 > 0:26:34Lastly, the Big Crunch.
0:26:36 > 0:26:39Not a breakfast cereal, but what will happen
0:26:39 > 0:26:42if the Big Bang slows down and is thrown into reverse.
0:26:44 > 0:26:48The universe would implode in an almighty crash,
0:26:48 > 0:26:50crunching down to a tiny singularity.
0:26:52 > 0:26:56It might seem morbid to think about the death of the universe,
0:26:56 > 0:27:01but some scientists think that the end might not be the end at all.
0:27:01 > 0:27:04Some believe that there might even be
0:27:04 > 0:27:08parallel universes that exist alongside our own.
0:27:08 > 0:27:12This is called multiverse theory.
0:27:12 > 0:27:16It could be that, beyond the observable horizon of our universe,
0:27:16 > 0:27:18there are other universes out there,
0:27:18 > 0:27:23each existing separately like the bubbles inside a Swiss cheese.
0:27:25 > 0:27:29Or perhaps other universes occupy a space
0:27:29 > 0:27:32that we cannot even comprehend,
0:27:32 > 0:27:38existing in extra dimensions we are, as yet, unaware of.
0:27:41 > 0:27:46The universe is magnificently, mind-blowingly weird -
0:27:46 > 0:27:50so strange, in fact, that we may never fully understand
0:27:50 > 0:27:53how or why it came to be, or what dark energy is,
0:27:53 > 0:27:57or if we're the only sentient beings in it.
0:27:57 > 0:28:01And every new discovery or theory by beard-tugging boffins in white coats
0:28:01 > 0:28:06has the potential to completely rewrite every book on the subject.
0:28:06 > 0:28:09So for now, I'm off to contemplate my part
0:28:09 > 0:28:13in the grand cosmological scheme of everything,
0:28:13 > 0:28:16knowing only one thing for certain -
0:28:16 > 0:28:21that my part - and yours, I'm afraid - is very, very small.
0:28:35 > 0:28:38Subtitles by Red Bee Media Ltd
0:28:38 > 0:28:41E-mail subtitling@bbc.co.uk