0:00:06 > 0:00:09We know the universe had a beginning.
0:00:11 > 0:00:16A moment 13.8 billion years ago when it sprang into life...
0:00:19 > 0:00:22..creating the vast cosmos we see today.
0:00:23 > 0:00:25Now we've discovered its origin,
0:00:25 > 0:00:28we're faced with another equally fundamental question.
0:00:29 > 0:00:32If the universe has a beginning, if it was born,
0:00:32 > 0:00:35does that then mean it'll eventually die?
0:00:35 > 0:00:38Or will it just keep on going for ever, eternal?
0:00:38 > 0:00:42You see, for us, as all-too-mortal humans, the ultimate fate
0:00:42 > 0:00:46of the universe is a question that's hard-wired into our psyche.
0:00:46 > 0:00:49Trying to answer it has driven an astonishing
0:00:49 > 0:00:52revolution in our understanding of the cosmos.
0:00:53 > 0:00:57Yet in recent years, it's also revealed a universe
0:00:57 > 0:01:00that's far stranger than we ever imagined.
0:01:02 > 0:01:06And led to one of the most shocking moments in scientific history.
0:01:09 > 0:01:14It's the latest twist in a tale stretching back over 100 years.
0:01:17 > 0:01:21In that time, key experiments and crucial discoveries...
0:01:21 > 0:01:23And there it is.
0:01:23 > 0:01:26Exactly, exactly where Hoyle predicted.
0:01:26 > 0:01:29..have brought us closer than anyone thought possible
0:01:29 > 0:01:33to finally knowing the ultimate fate of the universe.
0:01:46 > 0:01:50The sheer scale of the universe is truly staggering.
0:01:54 > 0:01:58How on earth can you predict the future of something so vast...
0:02:01 > 0:02:02..so complex...
0:02:05 > 0:02:07..so much bigger than we are?
0:02:11 > 0:02:14Since we first started grappling with this question,
0:02:14 > 0:02:17the answer has hinged on one simple idea.
0:02:20 > 0:02:25If we could chart, observe and understand how the universe has changed,
0:02:25 > 0:02:28how it has evolved to the present moment from its very
0:02:28 > 0:02:32ancient beginnings, then we should be able to extrapolate forward
0:02:32 > 0:02:35and predict how it will evolve in the future.
0:02:35 > 0:02:39Unfortunately, the slight flaw in that plan is that
0:02:39 > 0:02:44the universe operates on timescales of millions and billions of years.
0:02:44 > 0:02:45We don't.
0:02:48 > 0:02:51To understand the workings of the universe,
0:02:51 > 0:02:54we need to see beyond our limited human lifespan.
0:02:58 > 0:03:01And in this case, it turned out the sheer scale
0:03:01 > 0:03:04of the universe could be turned to our advantage.
0:03:24 > 0:03:26The universe is so vast,
0:03:26 > 0:03:30light from some of the objects we see in the night sky
0:03:30 > 0:03:34has taken millions, even billions of years to reach the Earth.
0:03:38 > 0:03:42When we look up, we're looking back in time at a record
0:03:42 > 0:03:44of the deep history of the universe.
0:03:48 > 0:03:53The problem is, we only have a snapshot, a single complex
0:03:53 > 0:03:55and confusing picture of all this history.
0:03:55 > 0:03:58It's like taking all the words in a novel, jumbling them up
0:03:58 > 0:04:01and sticking them on a single page.
0:04:01 > 0:04:05The key is to try and unpick this story, to learn how to read it,
0:04:05 > 0:04:07to recognise and understand what's going on.
0:04:09 > 0:04:14Astronomers realised that stars could help unlock that history.
0:04:17 > 0:04:20If scientists could work out how stars change,
0:04:20 > 0:04:22how they evolve in time,
0:04:22 > 0:04:25they could begin to understand the bigger story of how the universe
0:04:25 > 0:04:29was changing, the first clues to what the future might hold.
0:04:32 > 0:04:36But it would take until the middle of the 20th century
0:04:36 > 0:04:37to find the answer.
0:04:39 > 0:04:42Unlocking the secrets of the stars would take
0:04:42 > 0:04:46a moment of brilliance from this man, Fred Hoyle.
0:04:48 > 0:04:51Hoyle was a brilliant mathematician and physicist,
0:04:51 > 0:04:53one of the greatest of his day.
0:04:53 > 0:04:56He was creative, coming up with bold theories.
0:04:56 > 0:04:58Above all, he loved a problem,
0:04:58 > 0:05:01some thorny issue he could make his mark by solving.
0:05:01 > 0:05:05And in the late 1940s, he found one of the biggest.
0:05:09 > 0:05:12Hoyle wanted to know where the elements came from.
0:05:15 > 0:05:20The early universe was mostly just a sea of hydrogen and helium.
0:05:20 > 0:05:22The simplest and lightest elements.
0:05:25 > 0:05:28But we know that changed.
0:05:34 > 0:05:38Look around us now. This is no simple world we live in.
0:05:38 > 0:05:44We're surrounded by complexity, built from complex, heavy elements,
0:05:44 > 0:05:47like the oxygen I breathe and the iron in our blood.
0:05:47 > 0:05:51And of course, carbon, in the trees and in every cell in my body.
0:05:52 > 0:05:55No-one knew how to bridge the gap, how the universe
0:05:55 > 0:05:59went from that very simple beginning to all of this.
0:05:59 > 0:06:01This was the problem Hoyle seized on.
0:06:06 > 0:06:10Hoyle knew nuclear fusion must hold the answer.
0:06:10 > 0:06:12In nuclear fusion,
0:06:12 > 0:06:16lighter elements are fused together to make more complex ones.
0:06:22 > 0:06:25It was already known to happen in the heart of stars,
0:06:25 > 0:06:29where hydrogen fused together to form the more complex helium.
0:06:32 > 0:06:36Hoyle wondered how to go further, how the helium nuclei
0:06:36 > 0:06:39might fuse to make heavier elements.
0:06:42 > 0:06:45It's a remarkably simple idea. Here's our helium nucleus.
0:06:47 > 0:06:50If you could stick together two helium nuclei,
0:06:50 > 0:06:53you'd make beryllium, a heavier, more complex nucleus.
0:06:53 > 0:06:58Then, add a third helium nucleus and you get carbon.
0:06:58 > 0:07:02From there, you can carry on building up heavier and heavier elements.
0:07:02 > 0:07:04It sounds like the perfect solution.
0:07:04 > 0:07:08But there was a very good reason why the formation of carbon -
0:07:08 > 0:07:11hence all other elements - was still such a big mystery.
0:07:12 > 0:07:16The problem was, that the physics of this process just didn't work.
0:07:18 > 0:07:22Calculations showed that three helium nuclei wouldn't stick together.
0:07:22 > 0:07:27The carbon nucleus they formed was unstable and simply fell apart.
0:07:28 > 0:07:30If it broke down at carbon,
0:07:30 > 0:07:33then there was no chance of making any other heavier elements.
0:07:33 > 0:07:36It was like hitting a roadblock, every time.
0:07:42 > 0:07:45In typical bold and bullish fashion,
0:07:45 > 0:07:49Hoyle got around the problem by predicting a brand-new state of carbon.
0:08:01 > 0:08:03Hoyle took an intuitive leap.
0:08:03 > 0:08:08He decided that if three helium nuclei did come together inside a star,
0:08:08 > 0:08:11they could form carbon with a bit more energy than normal.
0:08:11 > 0:08:17In this special state, it could stay intact for just long enough to become stable.
0:08:17 > 0:08:21In that way, stars could make carbon and the roadblock was removed.
0:08:24 > 0:08:29If he was right, then Hoyle had solved the mystery.
0:08:29 > 0:08:31The elements were built in the heart of stars.
0:08:34 > 0:08:36But there was more at stake than that.
0:08:43 > 0:08:47Hoyle realised his theory could reveal how stars changed
0:08:47 > 0:08:48through their lives.
0:08:53 > 0:08:57And as the universe we see is built of stars, that would make it
0:08:57 > 0:09:01a powerful tool for predicting the future of the universe.
0:09:08 > 0:09:12Astronomers were already grouping stars based on their size,
0:09:12 > 0:09:13colour and brightness...
0:09:16 > 0:09:19..plotting them on a chart that was known as the Hertzsprung-Russell diagram.
0:09:25 > 0:09:27So here we had the diagram that they created.
0:09:27 > 0:09:32Along here is size and brightness, running from very large,
0:09:32 > 0:09:36very bright stars, all the way down to smaller, dimmer stars.
0:09:36 > 0:09:39And along this direction is colour and temperature.
0:09:39 > 0:09:44Very hot blue stars, all the way down to cooler red stars.
0:09:44 > 0:09:48Most regular-size stars fell into a long diagonal
0:09:48 > 0:09:51through the middle of the diagram,
0:09:51 > 0:09:54with a group of giant, bright stars above
0:09:54 > 0:09:56and small, dwarf stars below.
0:09:57 > 0:10:03Astronomers could see the patterns, but weren't able to unlock what they meant.
0:10:05 > 0:10:08Until Hoyle and his theory presented
0:10:08 > 0:10:11a radical new way of looking at the diagram.
0:10:11 > 0:10:14One that would reveal the life cycle of a star.
0:10:16 > 0:10:18Let's consider our own sun.
0:10:18 > 0:10:21Now, at the moment, it's sitting here in the middle of the diagram,
0:10:21 > 0:10:24happily burning hydrogen, turning it into helium.
0:10:24 > 0:10:29But if Hoyle was right, when it's run out of its hydrogen,
0:10:29 > 0:10:32it'll start fusing helium to make heavier elements.
0:10:32 > 0:10:35Now, at this point, a dramatic transformation takes place.
0:10:35 > 0:10:38Because rather than moving down the diagram in this direction,
0:10:38 > 0:10:41it expands to many times its size
0:10:41 > 0:10:46and jumps across here to live amongst the red giants.
0:10:46 > 0:10:49At this phase, it starts burning helium to make much heavier
0:10:49 > 0:10:52elements until it finally begins to produce carbon.
0:10:54 > 0:10:57Now, at that point, when it's run out of its nuclear fuel,
0:10:57 > 0:10:59it undergoes its final transformation.
0:10:59 > 0:11:05It sheds most of its outer layer and leaves behind a tiny white cinder,
0:11:05 > 0:11:08living here amongst the white dwarfs.
0:11:10 > 0:11:14All stars follow their own route around the diagram.
0:11:14 > 0:11:19Hoyle's theory provided the understanding to track each star's evolution,
0:11:19 > 0:11:24driven by the sudden ignition of a new phase of elemental formation.
0:11:28 > 0:11:33Here was the answer to the mystery of the heavy elements.
0:11:33 > 0:11:36The key to the life cycle of the stars.
0:11:36 > 0:11:40And a window onto the future of the universe.
0:11:40 > 0:11:43All thanks to Hoyle's new state of carbon.
0:11:44 > 0:11:47There was just one slight problem.
0:11:47 > 0:11:51No-one had ever seen or detected Hoyle's special form of carbon,
0:11:51 > 0:11:55not in a telltale spectra from stars, not anywhere on earth,
0:11:55 > 0:11:57not even in a laboratory experiment.
0:11:57 > 0:12:00As far as anyone could tell, it didn't exist.
0:12:00 > 0:12:02And without this special form of carbon,
0:12:02 > 0:12:05the whole theory would come crashing down.
0:12:07 > 0:12:11What happened next is a testament to Hoyle's brilliance
0:12:11 > 0:12:14and almost pig-headed self belief.
0:12:23 > 0:12:27In the 1950s, Hoyle joined the California Institute of Technology -
0:12:27 > 0:12:31Caltech - who had one of the few particle accelerators
0:12:31 > 0:12:34in existence at the time, similar to this one.
0:12:35 > 0:12:38Hoyle wanted to use the accelerator to try
0:12:38 > 0:12:41and make his high-energy carbon.
0:12:41 > 0:12:42They were not so keen.
0:12:46 > 0:12:50Here was an unknown Brit trying to take over their new machine
0:12:50 > 0:12:53in order to look for something he'd effectively made up.
0:13:07 > 0:13:10Like Hoyle, I'm a theorist.
0:13:10 > 0:13:12Experimental physics is a very different world
0:13:12 > 0:13:16and it's a different area of expertise.
0:13:16 > 0:13:21But Hoyle had the confidence, the daring, to stride into the lab
0:13:21 > 0:13:23and, as the director of the facility said,
0:13:23 > 0:13:28without a buy-or-leave, demand that they give up the research
0:13:28 > 0:13:31they were doing in favour of carrying out a complicated experiment
0:13:31 > 0:13:35to look for something that no-one even believed existed in the first place.
0:13:35 > 0:13:39I'm pretty sure I wouldn't have had the guts to do that.
0:13:39 > 0:13:44Hoyle kept at them, arguing it would be a crucial and famous discovery.
0:13:46 > 0:13:47Finally, they gave in.
0:13:47 > 0:13:49The search was on.
0:13:51 > 0:13:53Today, I'm recreating their experiment.
0:13:55 > 0:14:00The plan was to bombard a target element with a particle beam
0:14:00 > 0:14:02to see if they could create that state of carbon.
0:14:02 > 0:14:05Well, I have with me my own experimental colleagues,
0:14:05 > 0:14:07Zahne and Robin, to help me out.
0:14:08 > 0:14:12Our target will be held in the centre of this reaction chamber.
0:14:14 > 0:14:17Now, what they were looking for was a very specific signal
0:14:17 > 0:14:19that would show up in their detectors.
0:14:19 > 0:14:23If that state of carbon existed, then Hoyle predicted that it would
0:14:23 > 0:14:28show up as a spike in the energy at 7.7 million electron volts -
0:14:28 > 0:14:30the fingerprints of this special state of carbon.
0:14:32 > 0:14:35We'll be looking for the same spike in energy.
0:14:36 > 0:14:38Time to seal the chamber...
0:14:40 > 0:14:42..close the radiation doors...
0:14:43 > 0:14:46..and see for ourselves what happened.
0:14:48 > 0:14:50Right, this is the control panel.
0:14:50 > 0:14:54And they've let me in - a theorist - to get it all running.
0:14:54 > 0:14:56So the first thing I do is fire up the beam.
0:14:59 > 0:15:01Then to aim the beam at the target.
0:15:03 > 0:15:07Charged particles are now slamming into the target.
0:15:07 > 0:15:10Back in the 1950s, this was Hoyle's moment of truth.
0:15:12 > 0:15:16Now data will start coming in and the important display
0:15:16 > 0:15:17to look at is over here.
0:15:19 > 0:15:23Now, if Hoyle was right, they'd see his excited state of carbon at this
0:15:23 > 0:15:28energy here. They would expect to see a spike in energy at that point.
0:15:30 > 0:15:32And there it is.
0:15:33 > 0:15:36Exactly - exactly - where Hoyle predicted.
0:15:36 > 0:15:40Now, when this experiment was carried out some 60 years ago,
0:15:40 > 0:15:44they were flabbergasted to see that Hoyle was right.
0:15:44 > 0:15:48It's quite incredible to think that he just worked on a theoretical hunch,
0:15:48 > 0:15:51convinced his experimental colleagues to do the experiment,
0:15:51 > 0:15:52and he was right.
0:15:55 > 0:15:58He was also right about the fame.
0:15:58 > 0:16:01The director of the laboratory went on to receive
0:16:01 > 0:16:03the Nobel Prize for the discovery.
0:16:04 > 0:16:07Hoyle, however, received nothing.
0:16:10 > 0:16:12They published their findings in one of the most famous
0:16:12 > 0:16:16and heavily referenced papers in science.
0:16:16 > 0:16:17On the front cover of the paper,
0:16:17 > 0:16:22the authors put a very apt quote from Shakespeare's King Lear.
0:16:22 > 0:16:25"It is the stars, the stars above us, govern our conditions."
0:16:27 > 0:16:30It was the confirmation of this excited state of carbon that
0:16:30 > 0:16:34proved that it's inside stars that all the elements that make
0:16:34 > 0:16:38up the world around us, including ourselves, are actually forged.
0:16:38 > 0:16:43And with that discovery, we gained real insight into the life cycle of stars.
0:16:43 > 0:16:47We could begin to understand how the universe changed over time,
0:16:47 > 0:16:49both now and into the future.
0:16:53 > 0:16:57Here was the foundation for extrapolating into the future.
0:16:59 > 0:17:03And it made one clear prediction for the end of the universe.
0:17:06 > 0:17:10It was hydrogen and helium that first formed stars,
0:17:10 > 0:17:13and it was these two elements that were consumed in stars
0:17:13 > 0:17:17as they aged, creating all the heavier elements in the process.
0:17:17 > 0:17:20The logical conclusion was disturbing.
0:17:20 > 0:17:22After an almost unimaginable length of time,
0:17:22 > 0:17:27stars would use up all the hydrogen and helium in existence.
0:17:27 > 0:17:29No new stars could form,
0:17:29 > 0:17:33and existing stars would eventually run out of their fuel and die.
0:17:34 > 0:17:36The universe would go dark.
0:17:38 > 0:17:44For everything that's important to you and me, the light and life
0:17:44 > 0:17:50created by the stars, the universe would eventually come to an end.
0:17:55 > 0:17:57But there was another option.
0:17:57 > 0:17:59One that promised a very different fate...
0:18:00 > 0:18:04..and would play out long before the stars ran out of fuel.
0:18:05 > 0:18:09A fate that involved a fundamental force of the universe.
0:18:11 > 0:18:13Gravity.
0:18:16 > 0:18:20The potential for gravity to define the ultimate fate
0:18:20 > 0:18:25of the universe was first spotted by one of science's unsung heroes.
0:18:25 > 0:18:27Vesto Slipher.
0:18:27 > 0:18:30Little-known, his pioneering expert measurements
0:18:30 > 0:18:33would transform our understanding of the universe.
0:18:35 > 0:18:40In the early 1900s, astronomy was entering its golden age,
0:18:40 > 0:18:43with evermore powerful telescopes trained on the skies.
0:18:45 > 0:18:48One of the biggest targets of the time was the nebulae.
0:18:54 > 0:18:57Nebulae were patches and swirls of light
0:18:57 > 0:18:59that could be seen in between the stars,
0:18:59 > 0:19:03and not much was known about these mysterious objects,
0:19:03 > 0:19:07so astronomers were scrambling to find out as much about them as possible.
0:19:07 > 0:19:11Slipher was interested in one particular aspect of the nebulae -
0:19:11 > 0:19:12their motion.
0:19:12 > 0:19:17And for his target, he chose the most famous one of all, Andromeda.
0:19:22 > 0:19:27Slipher wanted to be the first to measure how quickly a nebula was moving.
0:19:29 > 0:19:33The problem was, his was not the best telescope out there.
0:19:33 > 0:19:34Not by a long chalk.
0:19:36 > 0:19:39But Slipher did have one big advantage over his competitors.
0:19:42 > 0:19:44He was a superb astronomer.
0:19:47 > 0:19:51This telescope is actually the same size as Slipher's.
0:19:51 > 0:19:53It has a 24-inch mirror.
0:19:54 > 0:19:58But Slipher would have loved to have got his hands on something like this.
0:19:58 > 0:20:01You see, what he needed was to get a spectrum.
0:20:01 > 0:20:05Now, that involves splitting the light from the nebulae
0:20:05 > 0:20:09into its different wavelengths, the different colours that it's made of.
0:20:09 > 0:20:12Now, he'd have used something like this - it's a diffraction grating.
0:20:12 > 0:20:16I can see it reflects this light and gives me
0:20:16 > 0:20:19all the different colours of the rainbow.
0:20:19 > 0:20:24What worried Slipher was that he needed to collect as much light as possible
0:20:24 > 0:20:29to give him a usable spectrum, and nebulae are exceptionally faint.
0:20:30 > 0:20:35He feared that getting enough light from his telescope would
0:20:35 > 0:20:36prove to be impossible.
0:20:40 > 0:20:43It may be the same size,
0:20:43 > 0:20:46but this modern telescope can capture the spectrum
0:20:46 > 0:20:48of Andromeda in a matter of minutes.
0:20:52 > 0:20:57With his telescope, Slipher needed 14 hours to produce one spectrum.
0:20:57 > 0:21:00Two days of backbreaking efforts.
0:21:02 > 0:21:04Seven hours each night,
0:21:04 > 0:21:07constantly adjusting the telescope to keep it fixed on Andromeda.
0:21:11 > 0:21:15Slipher wanted to know how Andromeda was moving,
0:21:15 > 0:21:18and for that he didn't just need the spectrum of light on Andromeda,
0:21:18 > 0:21:21he needed to have the absorption lines.
0:21:21 > 0:21:25Now, these are discreet gaps in the spectrum, like this.
0:21:25 > 0:21:29Now, these absorption lines should always be in the same place
0:21:29 > 0:21:31if the source isn't moving.
0:21:31 > 0:21:35If they've shifted to the right, towards the red end of the spectrum,
0:21:35 > 0:21:38that means that the source is moving away from us.
0:21:38 > 0:21:42If they've shifted to the left, towards the blue end of the spectrum,
0:21:42 > 0:21:46that means the source is moving towards us - a blue shift.
0:21:46 > 0:21:52Now, after two days of observing, Slipher was ready to develop his photograph.
0:21:52 > 0:21:56And he didn't get something as beautiful and clean as this.
0:21:58 > 0:21:59He got this image.
0:21:59 > 0:22:01Now this is in fact blown up.
0:22:01 > 0:22:04In fact, what he got was a much smaller image than this.
0:22:04 > 0:22:07And it's not even these lines, at the top and bottom.
0:22:07 > 0:22:12In fact, what he got was this dirty smudge in the middle.
0:22:12 > 0:22:14That was the spectrum from Andromeda.
0:22:15 > 0:22:17Now, you might think he'd failed,
0:22:17 > 0:22:20that you couldn't get anything meaningful from this.
0:22:20 > 0:22:23In fact, not only was he able to get a meaningful measurement,
0:22:23 > 0:22:28he could work out that Andromeda showed a very clear blue shift,
0:22:28 > 0:22:30that it was moving towards us.
0:22:30 > 0:22:36In fact, he worked out it was moving towards us at a speed of 300km per second,
0:22:36 > 0:22:39which actually matches modern-day estimates.
0:22:40 > 0:22:42Slipher had done it.
0:22:42 > 0:22:45The first ever measure of the speed of a nebula.
0:22:45 > 0:22:49His skill and tenacity overcoming the limits of his telescope.
0:22:52 > 0:22:57When Slipher presented his findings at an astronomy meeting in 1914,
0:22:57 > 0:22:59he received a standing ovation.
0:22:59 > 0:23:03It's often easy to forget how important people like Slipher are.
0:23:04 > 0:23:07The major breakthroughs in science aren't always about
0:23:07 > 0:23:10the big idea or the beautiful theory.
0:23:10 > 0:23:14They're often simply reliant on people who are exceptionally
0:23:14 > 0:23:18skilled at observing and measuring the natural world.
0:23:22 > 0:23:26We now know that the Andromeda nebula is actually a galaxy
0:23:26 > 0:23:28like our own, the Milky Way.
0:23:30 > 0:23:34And it's Andromeda's movement that reveals how gravity can shape
0:23:34 > 0:23:36the fate of the universe.
0:23:42 > 0:23:45Since it was first born in the Big Bang,
0:23:45 > 0:23:48the universe has been expanding outwards.
0:23:48 > 0:23:51As a result, most galaxies are actually
0:23:51 > 0:23:52heading away from each other.
0:23:54 > 0:23:56When they first formed, the same would have been true
0:23:56 > 0:23:59of Andromeda and the Milky Way.
0:23:59 > 0:24:03Until gravity got to work and began to overwhelm that expansion.
0:24:07 > 0:24:09It's gravity that's dragging Andromeda
0:24:09 > 0:24:13and our own Milky Way galaxy inexorably together.
0:24:13 > 0:24:17The question is, if it can pull off this trick in our own little corner of the cosmos,
0:24:17 > 0:24:22can it do the same over the entire expanse of the universe?
0:24:36 > 0:24:39If gravity could overwhelm the expansion,
0:24:39 > 0:24:42then long before the stars are burnt out,
0:24:42 > 0:24:48our vast universe would inevitably, inescapably collapse in on itself.
0:24:50 > 0:24:53The universe would end with a big crunch.
0:24:57 > 0:25:01If gravity failed, the universe would simply continue to expand,
0:25:01 > 0:25:05far beyond even the time when the last star had died.
0:25:12 > 0:25:15Everything hinged on one factor,
0:25:15 > 0:25:19predicted by Einstein's general theory of relativity.
0:25:23 > 0:25:25Using general relativity
0:25:25 > 0:25:29revealed that there were two very different futures to the universe.
0:25:29 > 0:25:33What's more, they were able to calculate a specific figure
0:25:33 > 0:25:36that marked the boundary between these two different scenarios.
0:25:36 > 0:25:39It became known as the critical density.
0:25:44 > 0:25:48The critical density was effectively a threshold
0:25:48 > 0:25:52based on how much matter and energy - how much stuff -
0:25:52 > 0:25:55there was in the entire universe.
0:25:58 > 0:26:01If that total was above the critical density,
0:26:01 > 0:26:05then gravity would drag the entire universe back together
0:26:05 > 0:26:06into the Big Crunch.
0:26:10 > 0:26:13If the total was below the critical density,
0:26:13 > 0:26:17then the expansion of the universe will continue for ever.
0:26:20 > 0:26:24The fate of the entire universe came down to a simple question -
0:26:24 > 0:26:26what universe do we live in?
0:26:26 > 0:26:30One that is above the critical density, or one that is below?
0:26:35 > 0:26:39One way to tell was to look at the expansion of the universe.
0:26:40 > 0:26:44If the universe was above the critical density and heading for
0:26:44 > 0:26:49collapse, then the rate of expansion would already be slowing down.
0:26:50 > 0:26:53So, astronomers began working on a way to measure
0:26:53 > 0:26:56how the expansion of the universe was changing.
0:26:59 > 0:27:03They were confident until a precocious PhD student
0:27:03 > 0:27:08called Beatrice Tinsley spotted a fatal flaw in the plan.
0:27:11 > 0:27:14Tinsley, know as "little beetle" to her family and friends,
0:27:14 > 0:27:17was an extremely talented musician.
0:27:17 > 0:27:19She could have turned professional.
0:27:19 > 0:27:22But instead she decided to focus on her other great passion,
0:27:22 > 0:27:24which was astrophysics.
0:27:24 > 0:27:26Here, too, she excelled.
0:27:26 > 0:27:30But an academic career in the 1960s, if you are woman, wasn't easy,
0:27:30 > 0:27:33and her institution, the University of Texas,
0:27:33 > 0:27:37seemed determined to ignore this brilliant scientist in their midst.
0:27:37 > 0:27:40Despite that, she completed her PhD
0:27:40 > 0:27:43in less than half the time it would normally take.
0:27:44 > 0:27:48And that PhD spelled trouble for the expansion rate measurements.
0:27:51 > 0:27:54The plan was to measure how galaxies were moving
0:27:54 > 0:27:56at different distances from Earth
0:27:56 > 0:28:00and therefore at different times in the past.
0:28:02 > 0:28:04How their movement changed
0:28:04 > 0:28:08would reveal how the expansion of the universe was changing.
0:28:09 > 0:28:13Measuring the movement was relatively straightforward.
0:28:13 > 0:28:16It was measuring the distance where the problem lay.
0:28:18 > 0:28:21In our everyday world, we're surrounded by visual clues
0:28:21 > 0:28:25that give us a good sense of scale, and therefore of distance.
0:28:25 > 0:28:28But in the vastness of the universe, this is much more difficult,
0:28:28 > 0:28:32so astronomers turned to something that might seem unusual.
0:28:32 > 0:28:33Light itself.
0:28:37 > 0:28:40Light is not perhaps an obvious tape measure,
0:28:40 > 0:28:43but in this case it seemed ideal.
0:28:43 > 0:28:45Now, this relies on a very simple principle.
0:28:45 > 0:28:50How bright the light appears to me is dependant on how close I am to it
0:28:50 > 0:28:53so when I'm very close, a lot of light enters my eyes
0:28:53 > 0:28:55and it seems bright.
0:28:55 > 0:28:59But as I move away, the light has had more chance to spread out
0:28:59 > 0:29:02and less of it enters my eyes, so it appears dimmer.
0:29:02 > 0:29:06Crucially, this change in the level of brightness
0:29:06 > 0:29:09follows a very precise mathematical relationship.
0:29:12 > 0:29:16And I can use this relationship to calculate distance.
0:29:18 > 0:29:21'If I measure the difference in brightness
0:29:21 > 0:29:23'between a light next to me...'
0:29:23 > 0:29:24220.
0:29:25 > 0:29:27'..and one further away...'
0:29:27 > 0:29:29About 1.5.
0:29:29 > 0:29:32I don't know if you can see that. It's quite dark.
0:29:32 > 0:29:35'..I can work out how far away the light is.'
0:29:37 > 0:29:41And so now I have to divide these two numbers.
0:29:41 > 0:29:45Well, it's roughly 150.
0:29:46 > 0:29:48Now I have to take the square root.
0:29:48 > 0:29:51The square root of 150...
0:29:51 > 0:29:53Well, it's about 12.
0:29:53 > 0:29:55It's just over 12.
0:29:55 > 0:29:58About 12.2 metres.
0:29:59 > 0:30:01Right.
0:30:02 > 0:30:04Now to check my working.
0:30:07 > 0:30:09It's this principle that astronomers were using
0:30:09 > 0:30:12to measure the distance to galaxies.
0:30:15 > 0:30:18So, what I have here...
0:30:18 > 0:30:20is 11.5 metres.
0:30:20 > 0:30:24It's a bit less than the 12 metres I calculated, but close enough.
0:30:24 > 0:30:26I'm pretty happy with that.
0:30:28 > 0:30:30But this technique only works
0:30:30 > 0:30:34if you know how bright the distance object should be,
0:30:34 > 0:30:38so you can measure how much that brightness has changed.
0:30:38 > 0:30:42And that would turn out to be the astronomers' Achilles heel.
0:30:44 > 0:30:47They were measuring galaxies at different distances,
0:30:47 > 0:30:50so at different times during the life of the universe.
0:30:50 > 0:30:54This meant that the galaxies differed in age by millions
0:30:54 > 0:30:55or billions of years.
0:30:55 > 0:30:58You see, for the distance measurements to work,
0:30:58 > 0:31:01they had to assume that all these galaxies of different ages
0:31:01 > 0:31:04were shining with the same brightness.
0:31:04 > 0:31:05In other words,
0:31:05 > 0:31:08a galaxy's brightness doesn't change over time.
0:31:08 > 0:31:10But for Beatrice Tinsley,
0:31:10 > 0:31:13there was a fatal flaw at the heart of this assumption.
0:31:16 > 0:31:20Tinsley was fascinated by the life cycle of the stars -
0:31:20 > 0:31:23how they changed through their lives.
0:31:24 > 0:31:28Her PhD looked at what effect that would have
0:31:28 > 0:31:30on the brightness of galaxies.
0:31:33 > 0:31:37For Tinsley, it was clear that if stars have a life cycle
0:31:37 > 0:31:40during which their appearance and brightness change,
0:31:40 > 0:31:44then because galaxies are fundamentally made of stars,
0:31:44 > 0:31:48so too would their brightness change over time.
0:31:50 > 0:31:54Tinsley's findings sent shockwaves through the field.
0:31:54 > 0:31:59"A palpable sense of panic", as one astronomer of the time described it.
0:31:59 > 0:32:02And they were immediately challenged.
0:32:02 > 0:32:04You see, a huge amount of time, effort and money
0:32:04 > 0:32:07had been invested in these expansion measurements
0:32:07 > 0:32:11and yet here was this unknown young PhD student - a woman, no less -
0:32:11 > 0:32:13who was questioning it all.
0:32:13 > 0:32:17And yet there was no arguing the logic of Tinsley's work
0:32:17 > 0:32:20and, after four years, it was eventually accepted.
0:32:23 > 0:32:26With that, it was back to the drawing board.
0:32:29 > 0:32:32A new way was needed to test how close the universe was
0:32:32 > 0:32:34to the critical density
0:32:34 > 0:32:37to see if it would collapse or continue to expand.
0:32:44 > 0:32:46There was another option.
0:32:46 > 0:32:48A more direct approach.
0:32:52 > 0:32:55One obvious way to see how close the universe is
0:32:55 > 0:32:57to the critical density
0:32:57 > 0:33:00is just to count how much stuff there is out there.
0:33:00 > 0:33:04It's a simple enough idea, but rather difficult to pull off.
0:33:04 > 0:33:08After all, in something as almost unimaginably vast as the universe,
0:33:08 > 0:33:11how do you count every galaxy, every star,
0:33:11 > 0:33:14every speck of interstellar gas?
0:33:14 > 0:33:16It's almost impossible.
0:33:18 > 0:33:22So, instead, astronomers cut the universe down to size.
0:33:23 > 0:33:26They took an average count of just one small part
0:33:26 > 0:33:29and then multiplied it up from there.
0:33:29 > 0:33:33They could do this thanks to one unique characteristic
0:33:33 > 0:33:34of the universe.
0:33:36 > 0:33:39As far as we can tell, the universe is, on the largest scales,
0:33:39 > 0:33:42the same in whatever direction we look.
0:33:42 > 0:33:45So an astronomer sitting on Earth looking out into space
0:33:45 > 0:33:49will get pretty much the same view as an alien astronomer
0:33:49 > 0:33:51on a planet thousands of light years away
0:33:51 > 0:33:54looking out in a completely different direction.
0:33:54 > 0:33:57And that's why measuring how much stuff there is
0:33:57 > 0:33:59in one small part of the universe
0:33:59 > 0:34:03gives us a pretty accurate measure of how much there is overall.
0:34:05 > 0:34:09They took their averages and came up with a total amount of mass
0:34:09 > 0:34:11and energy in the universe.
0:34:12 > 0:34:15The results took everyone by surprise.
0:34:15 > 0:34:20All of them suggested the universe was well below the critical density.
0:34:20 > 0:34:23In fact, the best estimate suggested the universe had so little mass
0:34:23 > 0:34:28that its density was only a tiny fraction of the critical value.
0:34:29 > 0:34:31Obviously, if right,
0:34:31 > 0:34:35there was no way that the universe was going to collapse.
0:34:51 > 0:34:54But there was a problem with this first estimate
0:34:54 > 0:34:58of how close the universe was to the critical density.
0:34:58 > 0:35:02The results were so low, they just didn't make any sense.
0:35:04 > 0:35:06A flat white coffee, please.
0:35:08 > 0:35:12Ours is so clearly a universe of matter, mass and energy.
0:35:12 > 0:35:14They dominate our world.
0:35:14 > 0:35:16They ARE our world.
0:35:16 > 0:35:19These findings painted a picture of a universe
0:35:19 > 0:35:23so alien to our everyday experience that it is perhaps understandable
0:35:23 > 0:35:26it was such a difficult concept to embrace.
0:35:27 > 0:35:32What's more, the estimates seemed to be at odds with the universe itself.
0:35:34 > 0:35:37The scale of the mismatch was revealed
0:35:37 > 0:35:41when the universe was mapped on an unprecedented scale
0:35:41 > 0:35:44by Margaret Geller at Harvard University.
0:35:51 > 0:35:55What Geller and her team did was first take a slice of the universe
0:35:55 > 0:36:01some 500 million light-years long, 300 million light-years wide,
0:36:01 > 0:36:04but still a thin wedge of the visible universe.
0:36:04 > 0:36:07They observed as many galaxies as they could
0:36:07 > 0:36:09and plotted them against distance.
0:36:09 > 0:36:13So, every one of these dots is an individual galaxy.
0:36:13 > 0:36:15There's over a thousand of them.
0:36:15 > 0:36:19What took everyone by surprise was this pattern that they saw -
0:36:19 > 0:36:22these bubbles, or almost a honeycomb structure.
0:36:22 > 0:36:25You see, everyone had assumed that the galaxies would be
0:36:25 > 0:36:28scattered randomly throughout the universe.
0:36:28 > 0:36:32Here, for the first time, was evidence that - far from random -
0:36:32 > 0:36:35the universe actually had structure.
0:36:36 > 0:36:40And at the heart of this newly-discovered structure
0:36:40 > 0:36:42was the pull of gravity.
0:36:44 > 0:36:47Since almost the beginning of the universe,
0:36:47 > 0:36:50gravity has been drawing matter together.
0:36:51 > 0:36:56First into clouds of gas, which then clumped together to form galaxies.
0:36:59 > 0:37:03These galaxies come together to form clusters of galaxies
0:37:03 > 0:37:05and the clusters into superclusters.
0:37:08 > 0:37:10It looks like a work of art.
0:37:18 > 0:37:22These superclusters of galaxies are all joined together
0:37:22 > 0:37:26by filaments of dust and gas,
0:37:26 > 0:37:30all acting under the same irresistible pull.
0:37:33 > 0:37:36My universe has just collapsed.
0:37:36 > 0:37:38Argh!
0:37:41 > 0:37:45Here we clearly see gravity acting as an architect,
0:37:45 > 0:37:49shaping and influencing the structure of the entire universe
0:37:49 > 0:37:52on a truly cosmic scale.
0:37:54 > 0:37:57No, I think I can do better.
0:37:57 > 0:38:00'The problem was, the estimates of matter in the universe
0:38:00 > 0:38:02'were so small...'
0:38:02 > 0:38:03Open that up.
0:38:03 > 0:38:07'..they put the universe so far below the critical density,
0:38:07 > 0:38:10'that such grand structures simply could not form.'
0:38:10 > 0:38:12I don't like that.
0:38:12 > 0:38:14'According to the numbers,
0:38:14 > 0:38:17'the universe as we know it couldn't exist.'
0:38:17 > 0:38:19This is a rubbish universe.
0:38:28 > 0:38:32There had to be something missing from the counts.
0:38:32 > 0:38:33But what was it?
0:38:33 > 0:38:37And what would it mean for the critical density
0:38:37 > 0:38:39and the fate of the universe?
0:38:40 > 0:38:44One of the most colourful and controversial scientists
0:38:44 > 0:38:47of the 20th century found the first clue.
0:38:48 > 0:38:50Fritz Zwicky.
0:38:51 > 0:38:56Zwicky was an eccentric, abrasive and brilliant scientist,
0:38:56 > 0:38:59known occasionally to refer to the rest of his profession
0:38:59 > 0:39:03as "spherical bastards", which is basically anyone who's a bastard,
0:39:03 > 0:39:05whichever way you look at him.
0:39:05 > 0:39:07But even those who disliked him
0:39:07 > 0:39:10had to admit that he was capable of brilliant work.
0:39:14 > 0:39:18Zwicky was also looking at galaxy clusters
0:39:18 > 0:39:22and they would lead him to discover something extraordinary.
0:39:25 > 0:39:29This picture here is just such a galaxy cluster.
0:39:29 > 0:39:31It's called Abell 1689.
0:39:31 > 0:39:35Each one of these yellow dots is part of the cluster.
0:39:35 > 0:39:38It's quite incredible to think that each one of them
0:39:38 > 0:39:40is an entire galaxy in itself.
0:39:40 > 0:39:44It sort of gives you an impression of the sheer scale of these things.
0:39:45 > 0:39:49Zwicky was fascinated by what held the clusters together.
0:39:50 > 0:39:53The answer, of course, has to be gravity.
0:39:53 > 0:39:56Imagine these marbles are all each individual galaxies,
0:39:56 > 0:40:00moving in chaotic orbits around the centre of the cluster,
0:40:00 > 0:40:04but none of them moves fast enough to be able to break free
0:40:04 > 0:40:06and escape from the cluster.
0:40:07 > 0:40:11Because of that, Zwicky could use how fast they were travelling
0:40:11 > 0:40:15to measure the strength of gravity holding them in place.
0:40:15 > 0:40:19And the strength of gravity would tell him how much matter -
0:40:19 > 0:40:22how much stuff - there was within the cluster.
0:40:23 > 0:40:26That is where things got very strange,
0:40:26 > 0:40:30because the galaxies were moving at tremendous speeds.
0:40:32 > 0:40:36The strength of gravity needed to hold all these speeding galaxies
0:40:36 > 0:40:40within the cluster required far more mass than he could see.
0:40:40 > 0:40:43And it wasn't just a small difference.
0:40:43 > 0:40:46In fact, he needed something like a hundred times more mass
0:40:46 > 0:40:48than could be detected.
0:40:51 > 0:40:55Zwicky called this mysterious mass Dunkle Materie.
0:40:55 > 0:40:57Dark matter.
0:40:58 > 0:41:03Here was a strong candidate for the missing mass of the universe.
0:41:04 > 0:41:09But to know if it took the universe above or below the critical density,
0:41:09 > 0:41:12they had to solve one major problem.
0:41:12 > 0:41:17How to study something when there is no known way of detecting it.
0:41:24 > 0:41:28The answer would come thanks to a discovery made here
0:41:28 > 0:41:30at the Jodrell Bank Observatory.
0:41:30 > 0:41:34This giant dish is the Bernard Lovell Radio Telescope
0:41:34 > 0:41:39and, in 1973, it spotted something no-one had ever seen before.
0:41:45 > 0:41:49At the time, it was carrying out a survey of some very distant,
0:41:49 > 0:41:51very bright objects -
0:41:51 > 0:41:53quasars.
0:41:58 > 0:42:02Part way through the survey, they detected something very unusual.
0:42:03 > 0:42:07I've come here today to take another look at what they saw,
0:42:07 > 0:42:10this time using not just the telescopes here at Jodrell,
0:42:10 > 0:42:13but radio telescopes across the country.
0:42:22 > 0:42:25Right, here we are - the control room at Jodrell Bank.
0:42:25 > 0:42:27A lovely view there of the Lovell Telescope.
0:42:27 > 0:42:30Now, over here, on these screens,
0:42:30 > 0:42:33we see live data coming in from various telescopes.
0:42:33 > 0:42:37One of them, the Mark II, is a radio telescope at Jodrell Bank,
0:42:37 > 0:42:41but the rest are scattered around the country, all linked together
0:42:41 > 0:42:45through optical fibres feeding into the central computer here.
0:42:46 > 0:42:50The point is, the longer you observe an object, the better-quality image
0:42:50 > 0:42:54you get, and after 50 hours of observation, here's what they see.
0:42:54 > 0:42:58This is the same image as was seen 40 years ago,
0:42:58 > 0:43:01showing these two bright dots -
0:43:01 > 0:43:03two quasars.
0:43:03 > 0:43:06This wasn't the first time quasars had been seen
0:43:06 > 0:43:10but certainly the first time they had been spotted so close together,
0:43:10 > 0:43:12as though they were a pair.
0:43:14 > 0:43:16A pair was something new.
0:43:17 > 0:43:20They began to gather as much information about them as possible,
0:43:20 > 0:43:23including measuring their spectra -
0:43:23 > 0:43:27the unique fingerprint contained within their light.
0:43:30 > 0:43:33Here are the spectra from the two quasars.
0:43:33 > 0:43:37Now, even at first glance, I can tell they look quite similar.
0:43:37 > 0:43:40In fact, they are much more than just quite similar.
0:43:40 > 0:43:42When they first measured them,
0:43:42 > 0:43:44they saw that they were both red-shifted -
0:43:44 > 0:43:47so longer wavelengths - by exactly the same amount.
0:43:47 > 0:43:50And have a look at these emission peaks.
0:43:50 > 0:43:53They both fall at exactly the same wavelength.
0:43:53 > 0:43:56In fact, the spectra was so similar
0:43:56 > 0:43:58they thought they had made a mistake -
0:43:58 > 0:44:01that they had looked at the same object twice.
0:44:01 > 0:44:02But they hadn't.
0:44:02 > 0:44:05And that left just one possibility.
0:44:05 > 0:44:07What they thought were two separate quasars
0:44:07 > 0:44:10were in fact just one single quasar
0:44:10 > 0:44:13that had somehow been split into two images.
0:44:13 > 0:44:16A case of astronomical double vision.
0:44:19 > 0:44:22There was a theory that could explain this -
0:44:22 > 0:44:26a strange effect predicted by Albert Einstein -
0:44:26 > 0:44:28gravitational lensing.
0:44:33 > 0:44:35If you look through this lens,
0:44:35 > 0:44:40you see that everything behind it is warped into strange shapes.
0:44:40 > 0:44:43This bizarre effect is because,
0:44:43 > 0:44:46as light passes through different thicknesses of the glass,
0:44:46 > 0:44:50it bends, giving rise to a warped image.
0:44:50 > 0:44:55Now, Einstein said that matter - stuff - also warped space,
0:44:55 > 0:44:59changing the very shape of the fabric of the universe,
0:44:59 > 0:45:03and so, as light passes through regions of space
0:45:03 > 0:45:06with high concentrations of matter, it will bend,
0:45:06 > 0:45:09just like it does going through the glass of this lens,
0:45:09 > 0:45:12and so giving rise to similar visual tricks.
0:45:14 > 0:45:16How much the light is bent
0:45:16 > 0:45:20is dependent on how much the space is being warped,
0:45:20 > 0:45:24and that depends on how much mass there is.
0:45:24 > 0:45:26Between the quasar and the telescopes,
0:45:26 > 0:45:29there had to be a huge amount of mass,
0:45:29 > 0:45:33bending the light so much that the image is split,
0:45:33 > 0:45:36making the single quasar appear as two.
0:45:38 > 0:45:41Here's our culprit, or at least part of it.
0:45:41 > 0:45:45This smudge here is just one galaxy within a cluster of galaxies
0:45:45 > 0:45:48that sit between us and the distant quasar.
0:45:48 > 0:45:50So it's not just a little bit of mass,
0:45:50 > 0:45:54but hundreds of galaxies, each with billions of stars.
0:45:54 > 0:45:58Combined, they bend the light from the quasar,
0:45:58 > 0:46:00giving us the double image.
0:46:02 > 0:46:06And the double image was crucial to the study of dark matter.
0:46:09 > 0:46:13Even with all the mass and matter contained in the galaxy cluster,
0:46:13 > 0:46:17there wasn't enough to bend the light that much.
0:46:18 > 0:46:22For that, you needed Zwicky's mysterious and invisible
0:46:22 > 0:46:23dark matter.
0:46:23 > 0:46:28And carefully analysing exactly how much the light was distorted
0:46:28 > 0:46:31could reveal where that dark matter was.
0:46:32 > 0:46:35This is what you get - a map.
0:46:35 > 0:46:39In the centre is the normal matter of the galaxy cluster itself,
0:46:39 > 0:46:43but, surrounding it, stretching out much further, coloured here in red,
0:46:43 > 0:46:44is the dark matter.
0:46:44 > 0:46:46Look how far out it spreads.
0:46:46 > 0:46:50It completely dwarfs the normal matter of the galaxy cluster.
0:46:50 > 0:46:53Zwicky's mysterious and invisible matter
0:46:53 > 0:46:56revealed by a cosmic optical illusion.
0:46:58 > 0:47:01It couldn't reveal what dark matter was,
0:47:01 > 0:47:05but mapping like this, as Jodrell is still doing to this day,
0:47:05 > 0:47:09did give an idea of how much there was out there,
0:47:09 > 0:47:13and it seemed to far outweigh normal matter,
0:47:13 > 0:47:18but was it enough to take the universe over the critical density?
0:47:20 > 0:47:24Even though there appeared to be far more dark matter than normal matter,
0:47:24 > 0:47:26that still seemed to leave the universe
0:47:26 > 0:47:29way below the critical density -
0:47:29 > 0:47:31but this was still far from the end of the story.
0:47:31 > 0:47:33The discovery of dark matter
0:47:33 > 0:47:37had taken the scientific community completely by surprise.
0:47:37 > 0:47:42Trying to work out how close the universe was to the critical density
0:47:42 > 0:47:45was just throwing up more mysteries than answers.
0:47:50 > 0:47:53A shocking new discovery that initially promised
0:47:53 > 0:47:56to finally reveal the fate of the universe
0:47:56 > 0:47:59instead threw physics into crisis.
0:48:11 > 0:48:15In the 1990s, these telescopes were part of an international project
0:48:15 > 0:48:19looking to finally reveal the fate of the universe.
0:48:23 > 0:48:27They were using a new technique to once again
0:48:27 > 0:48:31look at how the expansion of the universe had changed over time.
0:48:40 > 0:48:44I've come to use this telescope - the GTC -
0:48:44 > 0:48:48to observe the object that was at the heart of those studies.
0:48:54 > 0:48:59This huge telescope - you can see the vast mirror behind it -
0:48:59 > 0:49:02is going to take a close look at a supernova,
0:49:02 > 0:49:04the explosive death of a star.
0:49:04 > 0:49:09The light reaching us from these distant epic events would be key
0:49:09 > 0:49:12to unlocking how the universe expanded in the past
0:49:12 > 0:49:16and, in turn, would reveal what would happen to it in the future.
0:49:21 > 0:49:23To measure the expansion,
0:49:23 > 0:49:26researchers were interested in a particular type of supernova.
0:49:39 > 0:49:42Our target tonight is the same class of supernovae
0:49:42 > 0:49:45that they were searching for - a type Ia.
0:49:45 > 0:49:49Now, what made type Ia supernovae so useful
0:49:49 > 0:49:50is that, when they went off,
0:49:50 > 0:49:54they created an incredibly bright spike of light.
0:49:54 > 0:49:57Briefly, the star would shine brighter than its entire galaxy.
0:49:57 > 0:50:00Not only that, but they always gave off
0:50:00 > 0:50:03almost exactly the same level of brightness.
0:50:03 > 0:50:05This meant that not only could they see them
0:50:05 > 0:50:08over vast distances and remote galaxies,
0:50:08 > 0:50:12but they could also work out exactly how far away they were.
0:50:12 > 0:50:14So, if they could find enough of them,
0:50:14 > 0:50:16they could sample conditions in the universe
0:50:16 > 0:50:20over a wide range of distances and times.
0:50:22 > 0:50:26Tonight, astronomer David Alvarez has been homing in
0:50:26 > 0:50:29on a recently discovered type Ia supernova.
0:50:32 > 0:50:36Right, David, this is very exciting. Do you have the supernova?
0:50:36 > 0:50:39This is the image of the supernova.
0:50:39 > 0:50:41- That thing there?- That thing there.
0:50:41 > 0:50:44- Can you zoom in at all on it? - Yeah, we can zoom in here.
0:50:44 > 0:50:47You can see the bright dot.
0:50:47 > 0:50:49And the rest of it is the galaxy?
0:50:49 > 0:50:51The rest of the light you can see there
0:50:51 > 0:50:54is the host galaxy of the supernova.
0:50:54 > 0:50:55I mean, that's incredible.
0:50:55 > 0:50:58Here's a galaxy with hundreds of billions of stars,
0:50:58 > 0:51:01but this one exploding star - this one supernova -
0:51:01 > 0:51:05is shining brighter than the whole of the rest the galaxy.
0:51:05 > 0:51:08And you know how far away this supernova is?
0:51:08 > 0:51:10You've measured the distance?
0:51:10 > 0:51:14- Yeah, the supernova is about eight billion light years away.- Wow.
0:51:17 > 0:51:18As well as the distance,
0:51:18 > 0:51:21the spectrum of the supernova is also crucial.
0:51:23 > 0:51:26The astronomers needed the spectrum of the light
0:51:26 > 0:51:28because it gave them the redshift.
0:51:28 > 0:51:31You see, as the light travels from the distant supernova to Earth,
0:51:31 > 0:51:34the universe is expanding,
0:51:34 > 0:51:37the space the light is travelling through is stretching,
0:51:37 > 0:51:40and so the light itself is also stretching.
0:51:40 > 0:51:42Its wavelength is getting longer.
0:51:42 > 0:51:44If it leaves the supernova
0:51:44 > 0:51:46at a particular wavelength, a particular colour,
0:51:46 > 0:51:50when it arrives in our telescopes, it's at a longer wavelength -
0:51:50 > 0:51:52it's shifted towards the red end of the spectrum,
0:51:52 > 0:51:54hence a redshift.
0:51:54 > 0:51:56So knowing the redshift of the light
0:51:56 > 0:52:00tells us how much space has expanded in that time.
0:52:00 > 0:52:05In a sense, it gives us a measure of how big the universe has become.
0:52:07 > 0:52:10Because of this, measuring redshifts at greater distances -
0:52:10 > 0:52:13in effect, further back in time -
0:52:13 > 0:52:15could create a potted history
0:52:15 > 0:52:18of how the expansion of the universe was changing.
0:52:21 > 0:52:25Astronomers were convinced that gravity must have,
0:52:25 > 0:52:28at the very least, been slowing down the expansion.
0:52:28 > 0:52:32The question was - by how much?
0:52:32 > 0:52:35By plotting distance
0:52:35 > 0:52:37against the redshift's measure of expansion,
0:52:37 > 0:52:40they could finally answer that question.
0:52:42 > 0:52:45Now, if you imagine the universe has been expanding at the same rate -
0:52:45 > 0:52:48the rate that it is now - for its entire history,
0:52:48 > 0:52:51I'd get a very simple line.
0:52:51 > 0:52:54But astronomers knew this couldn't be correct
0:52:54 > 0:52:57because, of course, gravity is putting the brakes on the expansion,
0:52:57 > 0:53:00so the expansion of the universe should be slowing down
0:53:00 > 0:53:03and, if it's expanding more slowly now,
0:53:03 > 0:53:06it should've been expanding more quickly in the past.
0:53:06 > 0:53:10Space stretching more would mean a bigger redshift.
0:53:10 > 0:53:12Now, what does this mean for our supernova?
0:53:12 > 0:53:15Well, we know it was eight billion light years away.
0:53:16 > 0:53:19So we know it wouldn't fall exactly on this line,
0:53:19 > 0:53:23which corresponds to a redshift of about 0.49.
0:53:23 > 0:53:25It should sit maybe somewhere over here.
0:53:25 > 0:53:28Maybe at a redshift greater than 0.5.
0:53:28 > 0:53:33That means this line should really be curving down like that.
0:53:33 > 0:53:36But, of course, the exact shape of this line would tell them
0:53:36 > 0:53:40how much gravity is slowing down the expansion of the universe
0:53:40 > 0:53:44and that would tell them the fate of the universe.
0:53:44 > 0:53:47OK, so, David, you have the spectrum ready now.
0:53:47 > 0:53:49We have it.
0:53:49 > 0:53:51Yes, bring it up.
0:53:51 > 0:53:53And that gives you a measure of the redshift.
0:53:53 > 0:53:55So what did you measure that to be here?
0:53:55 > 0:53:58For this case, we measured 0.47.
0:53:58 > 0:54:010.47! Well, that puts it on this side of the line.
0:54:01 > 0:54:05That means it's not a larger redshift, but a smaller redshift.
0:54:07 > 0:54:10This is fascinating because it's exactly what they saw.
0:54:10 > 0:54:14Not redshifts that were larger, but redshifts that were smaller.
0:54:14 > 0:54:16And they saw this time and time again
0:54:16 > 0:54:18and it could only have one explanation -
0:54:18 > 0:54:22smaller redshifts meant that the universe must have been expanding
0:54:22 > 0:54:25more slowly in the past than it is today.
0:54:25 > 0:54:28In other words, rather than slowing down,
0:54:28 > 0:54:31the rate of expansion of the universe is accelerating.
0:54:34 > 0:54:37As more and more supernovae were plotted,
0:54:37 > 0:54:39the picture became clearer.
0:54:42 > 0:54:45For the first few billion years after the Big Bang,
0:54:45 > 0:54:49it looked as if the expansion rates had been slowing as expected...
0:54:51 > 0:54:53..but then that changed
0:54:53 > 0:54:57and the expansion started to accelerate.
0:54:59 > 0:55:03It's hard to stress how much of a shock this was.
0:55:03 > 0:55:06Back then, everyone knew that the expansion of the universe
0:55:06 > 0:55:07had to be slowing down.
0:55:07 > 0:55:11Now, whether it would slow down enough to stop and then recollapse,
0:55:11 > 0:55:14that wasn't clear, but it had to be slowing down.
0:55:14 > 0:55:18After all, gravity had to be doing its job of putting the brakes on,
0:55:18 > 0:55:19but it wasn't.
0:55:19 > 0:55:21About six billion years ago,
0:55:21 > 0:55:24the expansion started to speed up.
0:55:24 > 0:55:27Clearly, there was some new and unexpected thing
0:55:27 > 0:55:28going on in the universe -
0:55:28 > 0:55:30something that science didn't have an answer for,
0:55:30 > 0:55:34something that was pushing the expansion of the universe
0:55:34 > 0:55:36at an accelerating rate.
0:55:36 > 0:55:40It became known, for want of another term, as dark energy.
0:55:44 > 0:55:47The best estimates suggest that dark energy
0:55:47 > 0:55:50makes up 70% of the universe.
0:55:52 > 0:55:56And that means the universe will not collapse and end in a big crunch.
0:55:56 > 0:56:00Instead, dark energy, not gravity,
0:56:00 > 0:56:03will define the ultimate fate of the universe.
0:56:06 > 0:56:09Dark energy pushes the universe apart.
0:56:09 > 0:56:12It won't carry on expanding steadily for ever.
0:56:12 > 0:56:16Instead, dark energy forces the universe to fly apart
0:56:16 > 0:56:18at an ever-increasing rate.
0:56:18 > 0:56:20Galaxies will become so far apart
0:56:20 > 0:56:23that light wouldn't be able to travel between them.
0:56:23 > 0:56:26Each one will end up as an individual island of stars
0:56:26 > 0:56:28alone in the cosmos.
0:56:28 > 0:56:30It may even become so extreme
0:56:30 > 0:56:33that galaxies themselves will be ripped apart,
0:56:33 > 0:56:37leaving individual stars all alone in the black emptiness.
0:56:40 > 0:56:43Then again, maybe not.
0:56:44 > 0:56:47After all, the effect of dark energy
0:56:47 > 0:56:51seemed to suddenly appear between six and seven billion years ago.
0:56:51 > 0:56:54Who's to say how it'll behave in the future?
0:56:56 > 0:56:58That may sound bizarre
0:56:58 > 0:57:02but, with the discovery of dark energy, all bets are off.
0:57:04 > 0:57:07It's hard to stress how little we know about dark energy.
0:57:07 > 0:57:10It has a name, but that's about it.
0:57:10 > 0:57:11We don't know what it's made of,
0:57:11 > 0:57:14why it's driving the universe apart
0:57:14 > 0:57:17and, crucially, how it'll behave in the future.
0:57:17 > 0:57:20And that leaves a big hole in our understanding of the universe
0:57:20 > 0:57:22and its ultimate fate.
0:57:24 > 0:57:28Dark energy may simply be part of the universe,
0:57:28 > 0:57:30built into the way it works...
0:57:33 > 0:57:37..or it could point to a fundamental problem
0:57:37 > 0:57:41with the most important and trusted scientific theories we have...
0:57:43 > 0:57:46..ones that are at the very heart of our understanding
0:57:46 > 0:57:47of how the world works.
0:57:52 > 0:57:56How the universe will end started as astronomy's great challenge,
0:57:56 > 0:57:58but the fate of the universe has become
0:57:58 > 0:58:01much more than just an academic question.
0:58:01 > 0:58:05Through the discovery of this strange, enigmatic energy -
0:58:05 > 0:58:08if, indeed, that's what it is - one that defies current understanding,
0:58:08 > 0:58:12it's spread to the heart of fundamental physics.
0:58:12 > 0:58:15Finding the answer to how the universe will end
0:58:15 > 0:58:20could have profound implications on how we understand our world.
0:58:24 > 0:58:28If you want to find out more about the universe and the end of time,
0:58:28 > 0:58:32go to the address below and follow the links to the Open University.