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We know the universe had a beginning.
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A moment 13.8 billion years ago when it sprang into life...
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..creating the vast cosmos we see today.
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Now we've discovered its origin,
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we're faced with another equally fundamental question.
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If the universe has a beginning, if it was born,
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does that then mean it'll eventually die?
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Or will it just keep on going for ever, eternal?
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You see, for us, as all-too-mortal humans, the ultimate fate
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of the universe is a question that's hard-wired into our psyche.
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Trying to answer it has driven an astonishing
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revolution in our understanding of the cosmos.
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Yet in recent years, it's also revealed a universe
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that's far stranger than we ever imagined.
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And led to one of the most shocking moments in scientific history.
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It's the latest twist in a tale stretching back over 100 years.
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In that time, key experiments and crucial discoveries...
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And there it is.
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Exactly, exactly where Hoyle predicted.
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..have brought us closer than anyone thought possible
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to finally knowing the ultimate fate of the universe.
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The sheer scale of the universe is truly staggering.
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How on earth can you predict the future of something so vast...
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..so complex...
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..so much bigger than we are?
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Since we first started grappling with this question,
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the answer has hinged on one simple idea.
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If we could chart, observe and understand how the universe has changed,
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how it has evolved to the present moment from its very
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ancient beginnings, then we should be able to extrapolate forward
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and predict how it will evolve in the future.
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Unfortunately, the slight flaw in that plan is that
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the universe operates on timescales of millions and billions of years.
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We don't.
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To understand the workings of the universe,
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we need to see beyond our limited human lifespan.
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And in this case, it turned out the sheer scale
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of the universe could be turned to our advantage.
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The universe is so vast,
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light from some of the objects we see in the night sky
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has taken millions, even billions of years to reach the Earth.
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When we look up, we're looking back in time at a record
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of the deep history of the universe.
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The problem is, we only have a snapshot, a single complex
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and confusing picture of all this history.
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It's like taking all the words in a novel, jumbling them up
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and sticking them on a single page.
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The key is to try and unpick this story, to learn how to read it,
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to recognise and understand what's going on.
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Astronomers realised that stars could help unlock that history.
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If scientists could work out how stars change,
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how they evolve in time,
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they could begin to understand the bigger story of how the universe
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was changing, the first clues to what the future might hold.
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But it would take until the middle of the 20th century
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to find the answer.
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Unlocking the secrets of the stars would take
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a moment of brilliance from this man, Fred Hoyle.
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Hoyle was a brilliant mathematician and physicist,
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one of the greatest of his day.
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He was creative, coming up with bold theories.
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Above all, he loved a problem,
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some thorny issue he could make his mark by solving.
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And in the late 1940s, he found one of the biggest.
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Hoyle wanted to know where the elements came from.
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The early universe was mostly just a sea of hydrogen and helium.
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The simplest and lightest elements.
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But we know that changed.
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Look around us now. This is no simple world we live in.
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We're surrounded by complexity, built from complex, heavy elements,
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like the oxygen I breathe and the iron in our blood.
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And of course, carbon, in the trees and in every cell in my body.
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No-one knew how to bridge the gap, how the universe
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went from that very simple beginning to all of this.
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This was the problem Hoyle seized on.
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Hoyle knew nuclear fusion must hold the answer.
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In nuclear fusion,
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lighter elements are fused together to make more complex ones.
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It was already known to happen in the heart of stars,
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where hydrogen fused together to form the more complex helium.
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Hoyle wondered how to go further, how the helium nuclei
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might fuse to make heavier elements.
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It's a remarkably simple idea. Here's our helium nucleus.
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If you could stick together two helium nuclei,
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you'd make beryllium, a heavier, more complex nucleus.
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Then, add a third helium nucleus and you get carbon.
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From there, you can carry on building up heavier and heavier elements.
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It sounds like the perfect solution.
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But there was a very good reason why the formation of carbon -
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hence all other elements - was still such a big mystery.
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The problem was, that the physics of this process just didn't work.
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Calculations showed that three helium nuclei wouldn't stick together.
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The carbon nucleus they formed was unstable and simply fell apart.
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If it broke down at carbon,
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then there was no chance of making any other heavier elements.
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It was like hitting a roadblock, every time.
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In typical bold and bullish fashion,
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Hoyle got around the problem by predicting a brand-new state of carbon.
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Hoyle took an intuitive leap.
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He decided that if three helium nuclei did come together inside a star,
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they could form carbon with a bit more energy than normal.
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In this special state, it could stay intact for just long enough to become stable.
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In that way, stars could make carbon and the roadblock was removed.
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If he was right, then Hoyle had solved the mystery.
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The elements were built in the heart of stars.
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But there was more at stake than that.
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Hoyle realised his theory could reveal how stars changed
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through their lives.
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And as the universe we see is built of stars, that would make it
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a powerful tool for predicting the future of the universe.
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Astronomers were already grouping stars based on their size,
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colour and brightness...
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..plotting them on a chart that was known as the Hertzsprung-Russell diagram.
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So here we had the diagram that they created.
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Along here is size and brightness, running from very large,
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very bright stars, all the way down to smaller, dimmer stars.
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And along this direction is colour and temperature.
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Very hot blue stars, all the way down to cooler red stars.
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Most regular-size stars fell into a long diagonal
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through the middle of the diagram,
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with a group of giant, bright stars above
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and small, dwarf stars below.
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Astronomers could see the patterns, but weren't able to unlock what they meant.
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Until Hoyle and his theory presented
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a radical new way of looking at the diagram.
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One that would reveal the life cycle of a star.
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Let's consider our own sun.
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Now, at the moment, it's sitting here in the middle of the diagram,
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happily burning hydrogen, turning it into helium.
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But if Hoyle was right, when it's run out of its hydrogen,
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it'll start fusing helium to make heavier elements.
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Now, at this point, a dramatic transformation takes place.
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Because rather than moving down the diagram in this direction,
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it expands to many times its size
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and jumps across here to live amongst the red giants.
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At this phase, it starts burning helium to make much heavier
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elements until it finally begins to produce carbon.
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Now, at that point, when it's run out of its nuclear fuel,
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it undergoes its final transformation.
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It sheds most of its outer layer and leaves behind a tiny white cinder,
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living here amongst the white dwarfs.
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All stars follow their own route around the diagram.
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Hoyle's theory provided the understanding to track each star's evolution,
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driven by the sudden ignition of a new phase of elemental formation.
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Here was the answer to the mystery of the heavy elements.
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The key to the life cycle of the stars.
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And a window onto the future of the universe.
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All thanks to Hoyle's new state of carbon.
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There was just one slight problem.
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No-one had ever seen or detected Hoyle's special form of carbon,
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not in a telltale spectra from stars, not anywhere on earth,
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not even in a laboratory experiment.
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As far as anyone could tell, it didn't exist.
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And without this special form of carbon,
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the whole theory would come crashing down.
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What happened next is a testament to Hoyle's brilliance
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and almost pig-headed self belief.
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In the 1950s, Hoyle joined the California Institute of Technology -
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Caltech - who had one of the few particle accelerators
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in existence at the time, similar to this one.
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Hoyle wanted to use the accelerator to try
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and make his high-energy carbon.
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They were not so keen.
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Here was an unknown Brit trying to take over their new machine
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in order to look for something he'd effectively made up.
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Like Hoyle, I'm a theorist.
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Experimental physics is a very different world
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and it's a different area of expertise.
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But Hoyle had the confidence, the daring, to stride into the lab
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and, as the director of the facility said,
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without a buy-or-leave, demand that they give up the research
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they were doing in favour of carrying out a complicated experiment
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to look for something that no-one even believed existed in the first place.
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I'm pretty sure I wouldn't have had the guts to do that.
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Hoyle kept at them, arguing it would be a crucial and famous discovery.
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Finally, they gave in.
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The search was on.
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Today, I'm recreating their experiment.
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The plan was to bombard a target element with a particle beam
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to see if they could create that state of carbon.
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Well, I have with me my own experimental colleagues,
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Zahne and Robin, to help me out.
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Our target will be held in the centre of this reaction chamber.
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Now, what they were looking for was a very specific signal
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that would show up in their detectors.
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If that state of carbon existed, then Hoyle predicted that it would
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show up as a spike in the energy at 7.7 million electron volts -
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the fingerprints of this special state of carbon.
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We'll be looking for the same spike in energy.
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Time to seal the chamber...
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..close the radiation doors...
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..and see for ourselves what happened.
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Right, this is the control panel.
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And they've let me in - a theorist - to get it all running.
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So the first thing I do is fire up the beam.
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Then to aim the beam at the target.
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Charged particles are now slamming into the target.
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Back in the 1950s, this was Hoyle's moment of truth.
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Now data will start coming in and the important display
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to look at is over here.
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Now, if Hoyle was right, they'd see his excited state of carbon at this
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energy here. They would expect to see a spike in energy at that point.
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And there it is.
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Exactly - exactly - where Hoyle predicted.
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Now, when this experiment was carried out some 60 years ago,
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they were flabbergasted to see that Hoyle was right.
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It's quite incredible to think that he just worked on a theoretical hunch,
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convinced his experimental colleagues to do the experiment,
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and he was right.
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He was also right about the fame.
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The director of the laboratory went on to receive
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the Nobel Prize for the discovery.
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Hoyle, however, received nothing.
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They published their findings in one of the most famous
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and heavily referenced papers in science.
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On the front cover of the paper,
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the authors put a very apt quote from Shakespeare's King Lear.
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"It is the stars, the stars above us, govern our conditions."
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It was the confirmation of this excited state of carbon that
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proved that it's inside stars that all the elements that make
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up the world around us, including ourselves, are actually forged.
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And with that discovery, we gained real insight into the life cycle of stars.
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We could begin to understand how the universe changed over time,
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both now and into the future.
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Here was the foundation for extrapolating into the future.
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And it made one clear prediction for the end of the universe.
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It was hydrogen and helium that first formed stars,
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and it was these two elements that were consumed in stars
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as they aged, creating all the heavier elements in the process.
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The logical conclusion was disturbing.
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After an almost unimaginable length of time,
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stars would use up all the hydrogen and helium in existence.
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No new stars could form,
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and existing stars would eventually run out of their fuel and die.
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The universe would go dark.
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For everything that's important to you and me, the light and life
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created by the stars, the universe would eventually come to an end.
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But there was another option.
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One that promised a very different fate...
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..and would play out long before the stars ran out of fuel.
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A fate that involved a fundamental force of the universe.
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Gravity.
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The potential for gravity to define the ultimate fate
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of the universe was first spotted by one of science's unsung heroes.
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Vesto Slipher.
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Little-known, his pioneering expert measurements
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would transform our understanding of the universe.
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In the early 1900s, astronomy was entering its golden age,
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with evermore powerful telescopes trained on the skies.
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00:18:45,680 --> 00:18:48,880
One of the biggest targets of the time was the nebulae.
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Nebulae were patches and swirls of light
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that could be seen in between the stars,
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and not much was known about these mysterious objects,
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00:19:03,320 --> 00:19:07,680
so astronomers were scrambling to find out as much about them as possible.
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00:19:07,680 --> 00:19:11,200
Slipher was interested in one particular aspect of the nebulae -
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00:19:11,200 --> 00:19:12,680
their motion.
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And for his target, he chose the most famous one of all, Andromeda.
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Slipher wanted to be the first to measure how quickly a nebula was moving.
259
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The problem was, his was not the best telescope out there.
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Not by a long chalk.
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00:19:36,120 --> 00:19:39,920
But Slipher did have one big advantage over his competitors.
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He was a superb astronomer.
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00:19:47,040 --> 00:19:51,120
This telescope is actually the same size as Slipher's.
264
00:19:51,120 --> 00:19:53,240
It has a 24-inch mirror.
265
00:19:54,400 --> 00:19:58,880
But Slipher would have loved to have got his hands on something like this.
266
00:19:58,880 --> 00:20:01,920
You see, what he needed was to get a spectrum.
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00:20:01,920 --> 00:20:05,080
Now, that involves splitting the light from the nebulae
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00:20:05,080 --> 00:20:09,120
into its different wavelengths, the different colours that it's made of.
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00:20:09,120 --> 00:20:12,880
Now, he'd have used something like this - it's a diffraction grating.
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00:20:12,880 --> 00:20:16,680
I can see it reflects this light and gives me
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00:20:16,680 --> 00:20:19,960
all the different colours of the rainbow.
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00:20:19,960 --> 00:20:24,560
What worried Slipher was that he needed to collect as much light as possible
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00:20:24,560 --> 00:20:29,720
to give him a usable spectrum, and nebulae are exceptionally faint.
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00:20:30,720 --> 00:20:35,080
He feared that getting enough light from his telescope would
275
00:20:35,080 --> 00:20:36,680
prove to be impossible.
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It may be the same size,
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00:20:43,280 --> 00:20:46,480
but this modern telescope can capture the spectrum
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00:20:46,480 --> 00:20:48,880
of Andromeda in a matter of minutes.
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00:20:52,560 --> 00:20:57,920
With his telescope, Slipher needed 14 hours to produce one spectrum.
280
00:20:57,920 --> 00:21:00,280
Two days of backbreaking efforts.
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00:21:02,440 --> 00:21:04,280
Seven hours each night,
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00:21:04,280 --> 00:21:07,960
constantly adjusting the telescope to keep it fixed on Andromeda.
283
00:21:11,920 --> 00:21:15,000
Slipher wanted to know how Andromeda was moving,
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00:21:15,000 --> 00:21:18,760
and for that he didn't just need the spectrum of light on Andromeda,
285
00:21:18,760 --> 00:21:21,000
he needed to have the absorption lines.
286
00:21:21,000 --> 00:21:25,240
Now, these are discreet gaps in the spectrum, like this.
287
00:21:25,240 --> 00:21:29,480
Now, these absorption lines should always be in the same place
288
00:21:29,480 --> 00:21:31,840
if the source isn't moving.
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00:21:31,840 --> 00:21:35,480
If they've shifted to the right, towards the red end of the spectrum,
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00:21:35,480 --> 00:21:38,680
that means that the source is moving away from us.
291
00:21:38,680 --> 00:21:42,200
If they've shifted to the left, towards the blue end of the spectrum,
292
00:21:42,200 --> 00:21:46,160
that means the source is moving towards us - a blue shift.
293
00:21:46,160 --> 00:21:52,280
Now, after two days of observing, Slipher was ready to develop his photograph.
294
00:21:52,280 --> 00:21:56,080
And he didn't get something as beautiful and clean as this.
295
00:21:58,000 --> 00:21:59,840
He got this image.
296
00:21:59,840 --> 00:22:01,640
Now this is in fact blown up.
297
00:22:01,640 --> 00:22:04,320
In fact, what he got was a much smaller image than this.
298
00:22:04,320 --> 00:22:07,960
And it's not even these lines, at the top and bottom.
299
00:22:07,960 --> 00:22:12,320
In fact, what he got was this dirty smudge in the middle.
300
00:22:12,320 --> 00:22:14,440
That was the spectrum from Andromeda.
301
00:22:15,760 --> 00:22:17,800
Now, you might think he'd failed,
302
00:22:17,800 --> 00:22:20,200
that you couldn't get anything meaningful from this.
303
00:22:20,200 --> 00:22:23,760
In fact, not only was he able to get a meaningful measurement,
304
00:22:23,760 --> 00:22:28,520
he could work out that Andromeda showed a very clear blue shift,
305
00:22:28,520 --> 00:22:30,760
that it was moving towards us.
306
00:22:30,760 --> 00:22:36,280
In fact, he worked out it was moving towards us at a speed of 300km per second,
307
00:22:36,280 --> 00:22:39,040
which actually matches modern-day estimates.
308
00:22:40,280 --> 00:22:42,560
Slipher had done it.
309
00:22:42,560 --> 00:22:45,800
The first ever measure of the speed of a nebula.
310
00:22:45,800 --> 00:22:49,720
His skill and tenacity overcoming the limits of his telescope.
311
00:22:52,720 --> 00:22:57,280
When Slipher presented his findings at an astronomy meeting in 1914,
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00:22:57,280 --> 00:22:59,840
he received a standing ovation.
313
00:22:59,840 --> 00:23:03,120
It's often easy to forget how important people like Slipher are.
314
00:23:04,120 --> 00:23:07,720
The major breakthroughs in science aren't always about
315
00:23:07,720 --> 00:23:10,520
the big idea or the beautiful theory.
316
00:23:10,520 --> 00:23:14,480
They're often simply reliant on people who are exceptionally
317
00:23:14,480 --> 00:23:18,080
skilled at observing and measuring the natural world.
318
00:23:22,320 --> 00:23:26,320
We now know that the Andromeda nebula is actually a galaxy
319
00:23:26,320 --> 00:23:28,400
like our own, the Milky Way.
320
00:23:30,240 --> 00:23:34,680
And it's Andromeda's movement that reveals how gravity can shape
321
00:23:34,680 --> 00:23:36,440
the fate of the universe.
322
00:23:42,160 --> 00:23:45,000
Since it was first born in the Big Bang,
323
00:23:45,000 --> 00:23:48,520
the universe has been expanding outwards.
324
00:23:48,520 --> 00:23:51,000
As a result, most galaxies are actually
325
00:23:51,000 --> 00:23:52,840
heading away from each other.
326
00:23:54,160 --> 00:23:56,840
When they first formed, the same would have been true
327
00:23:56,840 --> 00:23:59,200
of Andromeda and the Milky Way.
328
00:23:59,200 --> 00:24:03,720
Until gravity got to work and began to overwhelm that expansion.
329
00:24:07,320 --> 00:24:09,880
It's gravity that's dragging Andromeda
330
00:24:09,880 --> 00:24:13,280
and our own Milky Way galaxy inexorably together.
331
00:24:13,280 --> 00:24:17,880
The question is, if it can pull off this trick in our own little corner of the cosmos,
332
00:24:17,880 --> 00:24:22,000
can it do the same over the entire expanse of the universe?
333
00:24:36,000 --> 00:24:39,440
If gravity could overwhelm the expansion,
334
00:24:39,440 --> 00:24:42,320
then long before the stars are burnt out,
335
00:24:42,320 --> 00:24:48,120
our vast universe would inevitably, inescapably collapse in on itself.
336
00:24:50,200 --> 00:24:53,360
The universe would end with a big crunch.
337
00:24:57,200 --> 00:25:01,800
If gravity failed, the universe would simply continue to expand,
338
00:25:01,800 --> 00:25:05,360
far beyond even the time when the last star had died.
339
00:25:12,280 --> 00:25:15,520
Everything hinged on one factor,
340
00:25:15,520 --> 00:25:19,280
predicted by Einstein's general theory of relativity.
341
00:25:23,760 --> 00:25:25,520
Using general relativity
342
00:25:25,520 --> 00:25:29,520
revealed that there were two very different futures to the universe.
343
00:25:29,520 --> 00:25:33,040
What's more, they were able to calculate a specific figure
344
00:25:33,040 --> 00:25:36,760
that marked the boundary between these two different scenarios.
345
00:25:36,760 --> 00:25:39,760
It became known as the critical density.
346
00:25:44,520 --> 00:25:48,520
The critical density was effectively a threshold
347
00:25:48,520 --> 00:25:52,520
based on how much matter and energy - how much stuff -
348
00:25:52,520 --> 00:25:55,280
there was in the entire universe.
349
00:25:58,040 --> 00:26:01,280
If that total was above the critical density,
350
00:26:01,280 --> 00:26:05,040
then gravity would drag the entire universe back together
351
00:26:05,040 --> 00:26:06,760
into the Big Crunch.
352
00:26:10,280 --> 00:26:13,280
If the total was below the critical density,
353
00:26:13,280 --> 00:26:17,760
then the expansion of the universe will continue for ever.
354
00:26:20,080 --> 00:26:24,280
The fate of the entire universe came down to a simple question -
355
00:26:24,280 --> 00:26:26,520
what universe do we live in?
356
00:26:26,520 --> 00:26:30,000
One that is above the critical density, or one that is below?
357
00:26:35,520 --> 00:26:39,800
One way to tell was to look at the expansion of the universe.
358
00:26:40,800 --> 00:26:44,520
If the universe was above the critical density and heading for
359
00:26:44,520 --> 00:26:49,360
collapse, then the rate of expansion would already be slowing down.
360
00:26:50,360 --> 00:26:53,520
So, astronomers began working on a way to measure
361
00:26:53,520 --> 00:26:56,280
how the expansion of the universe was changing.
362
00:26:59,520 --> 00:27:03,280
They were confident until a precocious PhD student
363
00:27:03,280 --> 00:27:08,040
called Beatrice Tinsley spotted a fatal flaw in the plan.
364
00:27:11,280 --> 00:27:14,840
Tinsley, know as "little beetle" to her family and friends,
365
00:27:14,840 --> 00:27:17,040
was an extremely talented musician.
366
00:27:17,040 --> 00:27:19,040
She could have turned professional.
367
00:27:19,040 --> 00:27:22,280
But instead she decided to focus on her other great passion,
368
00:27:22,280 --> 00:27:24,040
which was astrophysics.
369
00:27:24,040 --> 00:27:26,040
Here, too, she excelled.
370
00:27:26,040 --> 00:27:30,280
But an academic career in the 1960s, if you are woman, wasn't easy,
371
00:27:30,280 --> 00:27:33,280
and her institution, the University of Texas,
372
00:27:33,280 --> 00:27:37,520
seemed determined to ignore this brilliant scientist in their midst.
373
00:27:37,520 --> 00:27:40,040
Despite that, she completed her PhD
374
00:27:40,040 --> 00:27:43,200
in less than half the time it would normally take.
375
00:27:44,200 --> 00:27:48,520
And that PhD spelled trouble for the expansion rate measurements.
376
00:27:51,040 --> 00:27:54,280
The plan was to measure how galaxies were moving
377
00:27:54,280 --> 00:27:56,520
at different distances from Earth
378
00:27:56,520 --> 00:28:00,040
and therefore at different times in the past.
379
00:28:02,520 --> 00:28:04,800
How their movement changed
380
00:28:04,800 --> 00:28:08,280
would reveal how the expansion of the universe was changing.
381
00:28:09,520 --> 00:28:13,280
Measuring the movement was relatively straightforward.
382
00:28:13,280 --> 00:28:16,520
It was measuring the distance where the problem lay.
383
00:28:18,280 --> 00:28:21,760
In our everyday world, we're surrounded by visual clues
384
00:28:21,760 --> 00:28:25,040
that give us a good sense of scale, and therefore of distance.
385
00:28:25,040 --> 00:28:28,520
But in the vastness of the universe, this is much more difficult,
386
00:28:28,520 --> 00:28:32,040
so astronomers turned to something that might seem unusual.
387
00:28:32,040 --> 00:28:33,720
Light itself.
388
00:28:37,040 --> 00:28:40,280
Light is not perhaps an obvious tape measure,
389
00:28:40,280 --> 00:28:43,120
but in this case it seemed ideal.
390
00:28:43,120 --> 00:28:45,520
Now, this relies on a very simple principle.
391
00:28:45,520 --> 00:28:50,280
How bright the light appears to me is dependant on how close I am to it
392
00:28:50,280 --> 00:28:53,520
so when I'm very close, a lot of light enters my eyes
393
00:28:53,520 --> 00:28:55,280
and it seems bright.
394
00:28:55,280 --> 00:28:59,280
But as I move away, the light has had more chance to spread out
395
00:28:59,280 --> 00:29:02,760
and less of it enters my eyes, so it appears dimmer.
396
00:29:02,760 --> 00:29:06,040
Crucially, this change in the level of brightness
397
00:29:06,040 --> 00:29:09,280
follows a very precise mathematical relationship.
398
00:29:12,040 --> 00:29:16,040
And I can use this relationship to calculate distance.
399
00:29:18,560 --> 00:29:21,040
'If I measure the difference in brightness
400
00:29:21,040 --> 00:29:23,040
'between a light next to me...'
401
00:29:23,040 --> 00:29:24,560
220.
402
00:29:25,560 --> 00:29:27,680
'..and one further away...'
403
00:29:27,680 --> 00:29:29,520
About 1.5.
404
00:29:29,520 --> 00:29:32,280
I don't know if you can see that. It's quite dark.
405
00:29:32,280 --> 00:29:35,520
'..I can work out how far away the light is.'
406
00:29:37,800 --> 00:29:41,120
And so now I have to divide these two numbers.
407
00:29:41,120 --> 00:29:45,040
Well, it's roughly 150.
408
00:29:46,040 --> 00:29:48,840
Now I have to take the square root.
409
00:29:48,840 --> 00:29:51,280
The square root of 150...
410
00:29:51,280 --> 00:29:53,280
Well, it's about 12.
411
00:29:53,280 --> 00:29:55,280
It's just over 12.
412
00:29:55,280 --> 00:29:58,600
About 12.2 metres.
413
00:29:59,600 --> 00:30:01,040
Right.
414
00:30:02,040 --> 00:30:04,960
Now to check my working.
415
00:30:07,040 --> 00:30:09,760
It's this principle that astronomers were using
416
00:30:09,760 --> 00:30:12,040
to measure the distance to galaxies.
417
00:30:15,960 --> 00:30:18,280
So, what I have here...
418
00:30:18,280 --> 00:30:20,760
is 11.5 metres.
419
00:30:20,760 --> 00:30:24,280
It's a bit less than the 12 metres I calculated, but close enough.
420
00:30:24,280 --> 00:30:26,280
I'm pretty happy with that.
421
00:30:28,280 --> 00:30:30,760
But this technique only works
422
00:30:30,760 --> 00:30:34,520
if you know how bright the distance object should be,
423
00:30:34,520 --> 00:30:38,280
so you can measure how much that brightness has changed.
424
00:30:38,280 --> 00:30:42,280
And that would turn out to be the astronomers' Achilles heel.
425
00:30:44,440 --> 00:30:47,280
They were measuring galaxies at different distances,
426
00:30:47,280 --> 00:30:50,760
so at different times during the life of the universe.
427
00:30:50,760 --> 00:30:54,040
This meant that the galaxies differed in age by millions
428
00:30:54,040 --> 00:30:55,760
or billions of years.
429
00:30:55,760 --> 00:30:58,520
You see, for the distance measurements to work,
430
00:30:58,520 --> 00:31:01,760
they had to assume that all these galaxies of different ages
431
00:31:01,760 --> 00:31:04,280
were shining with the same brightness.
432
00:31:04,280 --> 00:31:05,760
In other words,
433
00:31:05,760 --> 00:31:08,520
a galaxy's brightness doesn't change over time.
434
00:31:08,520 --> 00:31:10,280
But for Beatrice Tinsley,
435
00:31:10,280 --> 00:31:13,760
there was a fatal flaw at the heart of this assumption.
436
00:31:16,480 --> 00:31:20,280
Tinsley was fascinated by the life cycle of the stars -
437
00:31:20,280 --> 00:31:23,040
how they changed through their lives.
438
00:31:24,520 --> 00:31:28,040
Her PhD looked at what effect that would have
439
00:31:28,040 --> 00:31:30,280
on the brightness of galaxies.
440
00:31:33,040 --> 00:31:37,040
For Tinsley, it was clear that if stars have a life cycle
441
00:31:37,040 --> 00:31:40,760
during which their appearance and brightness change,
442
00:31:40,760 --> 00:31:44,520
then because galaxies are fundamentally made of stars,
443
00:31:44,520 --> 00:31:48,040
so too would their brightness change over time.
444
00:31:50,520 --> 00:31:54,520
Tinsley's findings sent shockwaves through the field.
445
00:31:54,520 --> 00:31:59,280
"A palpable sense of panic", as one astronomer of the time described it.
446
00:31:59,280 --> 00:32:02,040
And they were immediately challenged.
447
00:32:02,040 --> 00:32:04,760
You see, a huge amount of time, effort and money
448
00:32:04,760 --> 00:32:07,760
had been invested in these expansion measurements
449
00:32:07,760 --> 00:32:11,760
and yet here was this unknown young PhD student - a woman, no less -
450
00:32:11,760 --> 00:32:13,760
who was questioning it all.
451
00:32:13,760 --> 00:32:17,040
And yet there was no arguing the logic of Tinsley's work
452
00:32:17,040 --> 00:32:20,440
and, after four years, it was eventually accepted.
453
00:32:23,520 --> 00:32:26,280
With that, it was back to the drawing board.
454
00:32:29,040 --> 00:32:32,520
A new way was needed to test how close the universe was
455
00:32:32,520 --> 00:32:34,280
to the critical density
456
00:32:34,280 --> 00:32:37,960
to see if it would collapse or continue to expand.
457
00:32:44,520 --> 00:32:46,520
There was another option.
458
00:32:46,520 --> 00:32:48,760
A more direct approach.
459
00:32:52,440 --> 00:32:55,520
One obvious way to see how close the universe is
460
00:32:55,520 --> 00:32:57,280
to the critical density
461
00:32:57,280 --> 00:33:00,520
is just to count how much stuff there is out there.
462
00:33:00,520 --> 00:33:04,520
It's a simple enough idea, but rather difficult to pull off.
463
00:33:04,520 --> 00:33:08,520
After all, in something as almost unimaginably vast as the universe,
464
00:33:08,520 --> 00:33:11,520
how do you count every galaxy, every star,
465
00:33:11,520 --> 00:33:14,040
every speck of interstellar gas?
466
00:33:14,040 --> 00:33:16,040
It's almost impossible.
467
00:33:18,040 --> 00:33:22,320
So, instead, astronomers cut the universe down to size.
468
00:33:23,320 --> 00:33:26,520
They took an average count of just one small part
469
00:33:26,520 --> 00:33:29,760
and then multiplied it up from there.
470
00:33:29,760 --> 00:33:33,040
They could do this thanks to one unique characteristic
471
00:33:33,040 --> 00:33:34,760
of the universe.
472
00:33:36,200 --> 00:33:39,760
As far as we can tell, the universe is, on the largest scales,
473
00:33:39,760 --> 00:33:42,280
the same in whatever direction we look.
474
00:33:42,280 --> 00:33:45,520
So an astronomer sitting on Earth looking out into space
475
00:33:45,520 --> 00:33:49,040
will get pretty much the same view as an alien astronomer
476
00:33:49,040 --> 00:33:51,520
on a planet thousands of light years away
477
00:33:51,520 --> 00:33:54,280
looking out in a completely different direction.
478
00:33:54,280 --> 00:33:57,280
And that's why measuring how much stuff there is
479
00:33:57,280 --> 00:33:59,280
in one small part of the universe
480
00:33:59,280 --> 00:34:03,400
gives us a pretty accurate measure of how much there is overall.
481
00:34:05,040 --> 00:34:09,040
They took their averages and came up with a total amount of mass
482
00:34:09,040 --> 00:34:11,040
and energy in the universe.
483
00:34:12,760 --> 00:34:15,760
The results took everyone by surprise.
484
00:34:15,760 --> 00:34:20,040
All of them suggested the universe was well below the critical density.
485
00:34:20,040 --> 00:34:23,760
In fact, the best estimate suggested the universe had so little mass
486
00:34:23,760 --> 00:34:28,040
that its density was only a tiny fraction of the critical value.
487
00:34:29,520 --> 00:34:31,280
Obviously, if right,
488
00:34:31,280 --> 00:34:35,040
there was no way that the universe was going to collapse.
489
00:34:51,040 --> 00:34:54,200
But there was a problem with this first estimate
490
00:34:54,200 --> 00:34:58,000
of how close the universe was to the critical density.
491
00:34:58,000 --> 00:35:02,760
The results were so low, they just didn't make any sense.
492
00:35:04,280 --> 00:35:06,520
A flat white coffee, please.
493
00:35:08,040 --> 00:35:12,280
Ours is so clearly a universe of matter, mass and energy.
494
00:35:12,280 --> 00:35:14,280
They dominate our world.
495
00:35:14,280 --> 00:35:16,040
They ARE our world.
496
00:35:16,040 --> 00:35:19,040
These findings painted a picture of a universe
497
00:35:19,040 --> 00:35:23,520
so alien to our everyday experience that it is perhaps understandable
498
00:35:23,520 --> 00:35:26,480
it was such a difficult concept to embrace.
499
00:35:27,760 --> 00:35:32,280
What's more, the estimates seemed to be at odds with the universe itself.
500
00:35:34,760 --> 00:35:37,280
The scale of the mismatch was revealed
501
00:35:37,280 --> 00:35:41,040
when the universe was mapped on an unprecedented scale
502
00:35:41,040 --> 00:35:44,040
by Margaret Geller at Harvard University.
503
00:35:51,280 --> 00:35:55,280
What Geller and her team did was first take a slice of the universe
504
00:35:55,280 --> 00:36:01,040
some 500 million light-years long, 300 million light-years wide,
505
00:36:01,040 --> 00:36:04,520
but still a thin wedge of the visible universe.
506
00:36:04,520 --> 00:36:07,280
They observed as many galaxies as they could
507
00:36:07,280 --> 00:36:09,480
and plotted them against distance.
508
00:36:09,480 --> 00:36:13,040
So, every one of these dots is an individual galaxy.
509
00:36:13,040 --> 00:36:15,280
There's over a thousand of them.
510
00:36:15,280 --> 00:36:19,040
What took everyone by surprise was this pattern that they saw -
511
00:36:19,040 --> 00:36:22,520
these bubbles, or almost a honeycomb structure.
512
00:36:22,520 --> 00:36:25,520
You see, everyone had assumed that the galaxies would be
513
00:36:25,520 --> 00:36:28,280
scattered randomly throughout the universe.
514
00:36:28,280 --> 00:36:32,520
Here, for the first time, was evidence that - far from random -
515
00:36:32,520 --> 00:36:35,280
the universe actually had structure.
516
00:36:36,480 --> 00:36:40,520
And at the heart of this newly-discovered structure
517
00:36:40,520 --> 00:36:42,520
was the pull of gravity.
518
00:36:44,160 --> 00:36:47,280
Since almost the beginning of the universe,
519
00:36:47,280 --> 00:36:50,280
gravity has been drawing matter together.
520
00:36:51,280 --> 00:36:56,720
First into clouds of gas, which then clumped together to form galaxies.
521
00:36:59,520 --> 00:37:03,280
These galaxies come together to form clusters of galaxies
522
00:37:03,280 --> 00:37:05,880
and the clusters into superclusters.
523
00:37:08,240 --> 00:37:10,760
It looks like a work of art.
524
00:37:18,040 --> 00:37:22,520
These superclusters of galaxies are all joined together
525
00:37:22,520 --> 00:37:26,040
by filaments of dust and gas,
526
00:37:26,040 --> 00:37:30,000
all acting under the same irresistible pull.
527
00:37:33,680 --> 00:37:36,560
My universe has just collapsed.
528
00:37:36,560 --> 00:37:38,040
Argh!
529
00:37:41,360 --> 00:37:45,520
Here we clearly see gravity acting as an architect,
530
00:37:45,520 --> 00:37:49,760
shaping and influencing the structure of the entire universe
531
00:37:49,760 --> 00:37:52,360
on a truly cosmic scale.
532
00:37:54,880 --> 00:37:57,040
No, I think I can do better.
533
00:37:57,040 --> 00:38:00,520
'The problem was, the estimates of matter in the universe
534
00:38:00,520 --> 00:38:02,040
'were so small...'
535
00:38:02,040 --> 00:38:03,520
Open that up.
536
00:38:03,520 --> 00:38:07,040
'..they put the universe so far below the critical density,
537
00:38:07,040 --> 00:38:10,760
'that such grand structures simply could not form.'
538
00:38:10,760 --> 00:38:12,520
I don't like that.
539
00:38:12,520 --> 00:38:14,520
'According to the numbers,
540
00:38:14,520 --> 00:38:17,760
'the universe as we know it couldn't exist.'
541
00:38:17,760 --> 00:38:19,920
This is a rubbish universe.
542
00:38:28,280 --> 00:38:32,040
There had to be something missing from the counts.
543
00:38:32,040 --> 00:38:33,760
But what was it?
544
00:38:33,760 --> 00:38:37,040
And what would it mean for the critical density
545
00:38:37,040 --> 00:38:39,360
and the fate of the universe?
546
00:38:40,880 --> 00:38:44,280
One of the most colourful and controversial scientists
547
00:38:44,280 --> 00:38:47,280
of the 20th century found the first clue.
548
00:38:48,280 --> 00:38:50,640
Fritz Zwicky.
549
00:38:51,760 --> 00:38:56,040
Zwicky was an eccentric, abrasive and brilliant scientist,
550
00:38:56,040 --> 00:38:59,360
known occasionally to refer to the rest of his profession
551
00:38:59,360 --> 00:39:03,280
as "spherical bastards", which is basically anyone who's a bastard,
552
00:39:03,280 --> 00:39:05,280
whichever way you look at him.
553
00:39:05,280 --> 00:39:07,280
But even those who disliked him
554
00:39:07,280 --> 00:39:10,440
had to admit that he was capable of brilliant work.
555
00:39:14,880 --> 00:39:18,760
Zwicky was also looking at galaxy clusters
556
00:39:18,760 --> 00:39:22,520
and they would lead him to discover something extraordinary.
557
00:39:25,360 --> 00:39:29,280
This picture here is just such a galaxy cluster.
558
00:39:29,280 --> 00:39:31,760
It's called Abell 1689.
559
00:39:31,760 --> 00:39:35,400
Each one of these yellow dots is part of the cluster.
560
00:39:35,400 --> 00:39:38,280
It's quite incredible to think that each one of them
561
00:39:38,280 --> 00:39:40,360
is an entire galaxy in itself.
562
00:39:40,360 --> 00:39:44,360
It sort of gives you an impression of the sheer scale of these things.
563
00:39:45,360 --> 00:39:49,040
Zwicky was fascinated by what held the clusters together.
564
00:39:50,040 --> 00:39:53,040
The answer, of course, has to be gravity.
565
00:39:53,040 --> 00:39:56,520
Imagine these marbles are all each individual galaxies,
566
00:39:56,520 --> 00:40:00,280
moving in chaotic orbits around the centre of the cluster,
567
00:40:00,280 --> 00:40:04,440
but none of them moves fast enough to be able to break free
568
00:40:04,440 --> 00:40:06,520
and escape from the cluster.
569
00:40:07,520 --> 00:40:11,520
Because of that, Zwicky could use how fast they were travelling
570
00:40:11,520 --> 00:40:15,520
to measure the strength of gravity holding them in place.
571
00:40:15,520 --> 00:40:19,520
And the strength of gravity would tell him how much matter -
572
00:40:19,520 --> 00:40:22,520
how much stuff - there was within the cluster.
573
00:40:23,760 --> 00:40:26,760
That is where things got very strange,
574
00:40:26,760 --> 00:40:30,520
because the galaxies were moving at tremendous speeds.
575
00:40:32,640 --> 00:40:36,520
The strength of gravity needed to hold all these speeding galaxies
576
00:40:36,520 --> 00:40:40,520
within the cluster required far more mass than he could see.
577
00:40:40,520 --> 00:40:43,040
And it wasn't just a small difference.
578
00:40:43,040 --> 00:40:46,280
In fact, he needed something like a hundred times more mass
579
00:40:46,280 --> 00:40:48,040
than could be detected.
580
00:40:51,040 --> 00:40:55,760
Zwicky called this mysterious mass Dunkle Materie.
581
00:40:55,760 --> 00:40:57,520
Dark matter.
582
00:40:58,520 --> 00:41:03,280
Here was a strong candidate for the missing mass of the universe.
583
00:41:04,280 --> 00:41:09,280
But to know if it took the universe above or below the critical density,
584
00:41:09,280 --> 00:41:12,520
they had to solve one major problem.
585
00:41:12,520 --> 00:41:17,160
How to study something when there is no known way of detecting it.
586
00:41:24,920 --> 00:41:28,040
The answer would come thanks to a discovery made here
587
00:41:28,040 --> 00:41:30,280
at the Jodrell Bank Observatory.
588
00:41:30,280 --> 00:41:34,040
This giant dish is the Bernard Lovell Radio Telescope
589
00:41:34,040 --> 00:41:39,520
and, in 1973, it spotted something no-one had ever seen before.
590
00:41:45,280 --> 00:41:49,520
At the time, it was carrying out a survey of some very distant,
591
00:41:49,520 --> 00:41:51,640
very bright objects -
592
00:41:51,640 --> 00:41:53,400
quasars.
593
00:41:58,040 --> 00:42:02,280
Part way through the survey, they detected something very unusual.
594
00:42:03,280 --> 00:42:07,040
I've come here today to take another look at what they saw,
595
00:42:07,040 --> 00:42:10,520
this time using not just the telescopes here at Jodrell,
596
00:42:10,520 --> 00:42:13,760
but radio telescopes across the country.
597
00:42:22,040 --> 00:42:25,040
Right, here we are - the control room at Jodrell Bank.
598
00:42:25,040 --> 00:42:27,760
A lovely view there of the Lovell Telescope.
599
00:42:27,760 --> 00:42:30,040
Now, over here, on these screens,
600
00:42:30,040 --> 00:42:33,760
we see live data coming in from various telescopes.
601
00:42:33,760 --> 00:42:37,520
One of them, the Mark II, is a radio telescope at Jodrell Bank,
602
00:42:37,520 --> 00:42:41,440
but the rest are scattered around the country, all linked together
603
00:42:41,440 --> 00:42:45,080
through optical fibres feeding into the central computer here.
604
00:42:46,080 --> 00:42:50,280
The point is, the longer you observe an object, the better-quality image
605
00:42:50,280 --> 00:42:54,520
you get, and after 50 hours of observation, here's what they see.
606
00:42:54,520 --> 00:42:58,280
This is the same image as was seen 40 years ago,
607
00:42:58,280 --> 00:43:01,040
showing these two bright dots -
608
00:43:01,040 --> 00:43:03,040
two quasars.
609
00:43:03,040 --> 00:43:06,040
This wasn't the first time quasars had been seen
610
00:43:06,040 --> 00:43:10,040
but certainly the first time they had been spotted so close together,
611
00:43:10,040 --> 00:43:12,400
as though they were a pair.
612
00:43:14,000 --> 00:43:16,040
A pair was something new.
613
00:43:17,040 --> 00:43:20,760
They began to gather as much information about them as possible,
614
00:43:20,760 --> 00:43:23,280
including measuring their spectra -
615
00:43:23,280 --> 00:43:27,040
the unique fingerprint contained within their light.
616
00:43:30,520 --> 00:43:33,600
Here are the spectra from the two quasars.
617
00:43:33,600 --> 00:43:37,280
Now, even at first glance, I can tell they look quite similar.
618
00:43:37,280 --> 00:43:40,360
In fact, they are much more than just quite similar.
619
00:43:40,360 --> 00:43:42,200
When they first measured them,
620
00:43:42,200 --> 00:43:44,720
they saw that they were both red-shifted -
621
00:43:44,720 --> 00:43:47,520
so longer wavelengths - by exactly the same amount.
622
00:43:47,520 --> 00:43:50,240
And have a look at these emission peaks.
623
00:43:50,240 --> 00:43:53,840
They both fall at exactly the same wavelength.
624
00:43:53,840 --> 00:43:56,280
In fact, the spectra was so similar
625
00:43:56,280 --> 00:43:58,760
they thought they had made a mistake -
626
00:43:58,760 --> 00:44:01,280
that they had looked at the same object twice.
627
00:44:01,280 --> 00:44:02,760
But they hadn't.
628
00:44:02,760 --> 00:44:05,040
And that left just one possibility.
629
00:44:05,040 --> 00:44:07,760
What they thought were two separate quasars
630
00:44:07,760 --> 00:44:10,280
were in fact just one single quasar
631
00:44:10,280 --> 00:44:13,760
that had somehow been split into two images.
632
00:44:13,760 --> 00:44:16,280
A case of astronomical double vision.
633
00:44:19,440 --> 00:44:22,760
There was a theory that could explain this -
634
00:44:22,760 --> 00:44:26,600
a strange effect predicted by Albert Einstein -
635
00:44:26,600 --> 00:44:28,760
gravitational lensing.
636
00:44:33,440 --> 00:44:35,720
If you look through this lens,
637
00:44:35,720 --> 00:44:40,880
you see that everything behind it is warped into strange shapes.
638
00:44:40,880 --> 00:44:43,000
This bizarre effect is because,
639
00:44:43,000 --> 00:44:46,840
as light passes through different thicknesses of the glass,
640
00:44:46,840 --> 00:44:50,560
it bends, giving rise to a warped image.
641
00:44:50,560 --> 00:44:55,760
Now, Einstein said that matter - stuff - also warped space,
642
00:44:55,760 --> 00:44:59,760
changing the very shape of the fabric of the universe,
643
00:44:59,760 --> 00:45:03,280
and so, as light passes through regions of space
644
00:45:03,280 --> 00:45:06,360
with high concentrations of matter, it will bend,
645
00:45:06,360 --> 00:45:09,280
just like it does going through the glass of this lens,
646
00:45:09,280 --> 00:45:12,640
and so giving rise to similar visual tricks.
647
00:45:14,680 --> 00:45:16,520
How much the light is bent
648
00:45:16,520 --> 00:45:20,480
is dependent on how much the space is being warped,
649
00:45:20,480 --> 00:45:24,560
and that depends on how much mass there is.
650
00:45:24,560 --> 00:45:26,920
Between the quasar and the telescopes,
651
00:45:26,920 --> 00:45:29,760
there had to be a huge amount of mass,
652
00:45:29,760 --> 00:45:33,440
bending the light so much that the image is split,
653
00:45:33,440 --> 00:45:36,720
making the single quasar appear as two.
654
00:45:38,360 --> 00:45:41,320
Here's our culprit, or at least part of it.
655
00:45:41,320 --> 00:45:45,440
This smudge here is just one galaxy within a cluster of galaxies
656
00:45:45,440 --> 00:45:48,440
that sit between us and the distant quasar.
657
00:45:48,440 --> 00:45:50,720
So it's not just a little bit of mass,
658
00:45:50,720 --> 00:45:54,920
but hundreds of galaxies, each with billions of stars.
659
00:45:54,920 --> 00:45:58,240
Combined, they bend the light from the quasar,
660
00:45:58,240 --> 00:46:00,080
giving us the double image.
661
00:46:02,720 --> 00:46:06,880
And the double image was crucial to the study of dark matter.
662
00:46:09,160 --> 00:46:13,840
Even with all the mass and matter contained in the galaxy cluster,
663
00:46:13,840 --> 00:46:17,040
there wasn't enough to bend the light that much.
664
00:46:18,040 --> 00:46:22,040
For that, you needed Zwicky's mysterious and invisible
665
00:46:22,040 --> 00:46:23,800
dark matter.
666
00:46:23,800 --> 00:46:28,200
And carefully analysing exactly how much the light was distorted
667
00:46:28,200 --> 00:46:31,120
could reveal where that dark matter was.
668
00:46:32,520 --> 00:46:35,360
This is what you get - a map.
669
00:46:35,360 --> 00:46:39,160
In the centre is the normal matter of the galaxy cluster itself,
670
00:46:39,160 --> 00:46:43,040
but, surrounding it, stretching out much further, coloured here in red,
671
00:46:43,040 --> 00:46:44,760
is the dark matter.
672
00:46:44,760 --> 00:46:46,960
Look how far out it spreads.
673
00:46:46,960 --> 00:46:50,520
It completely dwarfs the normal matter of the galaxy cluster.
674
00:46:50,520 --> 00:46:53,520
Zwicky's mysterious and invisible matter
675
00:46:53,520 --> 00:46:56,280
revealed by a cosmic optical illusion.
676
00:46:58,640 --> 00:47:01,520
It couldn't reveal what dark matter was,
677
00:47:01,520 --> 00:47:05,760
but mapping like this, as Jodrell is still doing to this day,
678
00:47:05,760 --> 00:47:09,840
did give an idea of how much there was out there,
679
00:47:09,840 --> 00:47:13,800
and it seemed to far outweigh normal matter,
680
00:47:13,800 --> 00:47:18,360
but was it enough to take the universe over the critical density?
681
00:47:20,240 --> 00:47:24,360
Even though there appeared to be far more dark matter than normal matter,
682
00:47:24,360 --> 00:47:26,800
that still seemed to leave the universe
683
00:47:26,800 --> 00:47:29,160
way below the critical density -
684
00:47:29,160 --> 00:47:31,960
but this was still far from the end of the story.
685
00:47:31,960 --> 00:47:33,880
The discovery of dark matter
686
00:47:33,880 --> 00:47:37,960
had taken the scientific community completely by surprise.
687
00:47:37,960 --> 00:47:42,400
Trying to work out how close the universe was to the critical density
688
00:47:42,400 --> 00:47:45,480
was just throwing up more mysteries than answers.
689
00:47:50,400 --> 00:47:53,480
A shocking new discovery that initially promised
690
00:47:53,480 --> 00:47:56,120
to finally reveal the fate of the universe
691
00:47:56,120 --> 00:47:59,680
instead threw physics into crisis.
692
00:48:11,200 --> 00:48:15,960
In the 1990s, these telescopes were part of an international project
693
00:48:15,960 --> 00:48:19,440
looking to finally reveal the fate of the universe.
694
00:48:23,560 --> 00:48:27,080
They were using a new technique to once again
695
00:48:27,080 --> 00:48:31,560
look at how the expansion of the universe had changed over time.
696
00:48:40,880 --> 00:48:44,480
I've come to use this telescope - the GTC -
697
00:48:44,480 --> 00:48:48,080
to observe the object that was at the heart of those studies.
698
00:48:54,240 --> 00:48:59,040
This huge telescope - you can see the vast mirror behind it -
699
00:48:59,040 --> 00:49:02,000
is going to take a close look at a supernova,
700
00:49:02,000 --> 00:49:04,440
the explosive death of a star.
701
00:49:04,440 --> 00:49:09,040
The light reaching us from these distant epic events would be key
702
00:49:09,040 --> 00:49:12,440
to unlocking how the universe expanded in the past
703
00:49:12,440 --> 00:49:16,360
and, in turn, would reveal what would happen to it in the future.
704
00:49:21,240 --> 00:49:23,040
To measure the expansion,
705
00:49:23,040 --> 00:49:26,880
researchers were interested in a particular type of supernova.
706
00:49:39,360 --> 00:49:42,840
Our target tonight is the same class of supernovae
707
00:49:42,840 --> 00:49:45,920
that they were searching for - a type Ia.
708
00:49:45,920 --> 00:49:49,280
Now, what made type Ia supernovae so useful
709
00:49:49,280 --> 00:49:50,800
is that, when they went off,
710
00:49:50,800 --> 00:49:54,000
they created an incredibly bright spike of light.
711
00:49:54,000 --> 00:49:57,920
Briefly, the star would shine brighter than its entire galaxy.
712
00:49:57,920 --> 00:50:00,400
Not only that, but they always gave off
713
00:50:00,400 --> 00:50:03,320
almost exactly the same level of brightness.
714
00:50:03,320 --> 00:50:05,600
This meant that not only could they see them
715
00:50:05,600 --> 00:50:08,120
over vast distances and remote galaxies,
716
00:50:08,120 --> 00:50:12,080
but they could also work out exactly how far away they were.
717
00:50:12,080 --> 00:50:14,120
So, if they could find enough of them,
718
00:50:14,120 --> 00:50:16,680
they could sample conditions in the universe
719
00:50:16,680 --> 00:50:20,000
over a wide range of distances and times.
720
00:50:22,840 --> 00:50:26,640
Tonight, astronomer David Alvarez has been homing in
721
00:50:26,640 --> 00:50:29,760
on a recently discovered type Ia supernova.
722
00:50:32,360 --> 00:50:36,360
Right, David, this is very exciting. Do you have the supernova?
723
00:50:36,360 --> 00:50:39,240
This is the image of the supernova.
724
00:50:39,240 --> 00:50:41,360
- That thing there?
- That thing there.
725
00:50:41,360 --> 00:50:44,760
- Can you zoom in at all on it?
- Yeah, we can zoom in here.
726
00:50:44,760 --> 00:50:47,080
You can see the bright dot.
727
00:50:47,080 --> 00:50:49,640
And the rest of it is the galaxy?
728
00:50:49,640 --> 00:50:51,840
The rest of the light you can see there
729
00:50:51,840 --> 00:50:54,240
is the host galaxy of the supernova.
730
00:50:54,240 --> 00:50:55,560
I mean, that's incredible.
731
00:50:55,560 --> 00:50:58,640
Here's a galaxy with hundreds of billions of stars,
732
00:50:58,640 --> 00:51:01,240
but this one exploding star - this one supernova -
733
00:51:01,240 --> 00:51:05,000
is shining brighter than the whole of the rest the galaxy.
734
00:51:05,000 --> 00:51:08,600
And you know how far away this supernova is?
735
00:51:08,600 --> 00:51:10,080
You've measured the distance?
736
00:51:10,080 --> 00:51:14,240
- Yeah, the supernova is about eight billion light years away.
- Wow.
737
00:51:17,400 --> 00:51:18,800
As well as the distance,
738
00:51:18,800 --> 00:51:21,800
the spectrum of the supernova is also crucial.
739
00:51:23,640 --> 00:51:26,080
The astronomers needed the spectrum of the light
740
00:51:26,080 --> 00:51:28,360
because it gave them the redshift.
741
00:51:28,360 --> 00:51:31,960
You see, as the light travels from the distant supernova to Earth,
742
00:51:31,960 --> 00:51:34,000
the universe is expanding,
743
00:51:34,000 --> 00:51:37,280
the space the light is travelling through is stretching,
744
00:51:37,280 --> 00:51:40,160
and so the light itself is also stretching.
745
00:51:40,160 --> 00:51:42,320
Its wavelength is getting longer.
746
00:51:42,320 --> 00:51:44,120
If it leaves the supernova
747
00:51:44,120 --> 00:51:46,440
at a particular wavelength, a particular colour,
748
00:51:46,440 --> 00:51:50,040
when it arrives in our telescopes, it's at a longer wavelength -
749
00:51:50,040 --> 00:51:52,920
it's shifted towards the red end of the spectrum,
750
00:51:52,920 --> 00:51:54,560
hence a redshift.
751
00:51:54,560 --> 00:51:56,920
So knowing the redshift of the light
752
00:51:56,920 --> 00:52:00,880
tells us how much space has expanded in that time.
753
00:52:00,880 --> 00:52:05,520
In a sense, it gives us a measure of how big the universe has become.
754
00:52:07,160 --> 00:52:10,880
Because of this, measuring redshifts at greater distances -
755
00:52:10,880 --> 00:52:13,760
in effect, further back in time -
756
00:52:13,760 --> 00:52:15,680
could create a potted history
757
00:52:15,680 --> 00:52:18,920
of how the expansion of the universe was changing.
758
00:52:21,440 --> 00:52:25,000
Astronomers were convinced that gravity must have,
759
00:52:25,000 --> 00:52:28,880
at the very least, been slowing down the expansion.
760
00:52:28,880 --> 00:52:32,440
The question was - by how much?
761
00:52:32,440 --> 00:52:35,000
By plotting distance
762
00:52:35,000 --> 00:52:37,840
against the redshift's measure of expansion,
763
00:52:37,840 --> 00:52:40,240
they could finally answer that question.
764
00:52:42,240 --> 00:52:45,880
Now, if you imagine the universe has been expanding at the same rate -
765
00:52:45,880 --> 00:52:48,680
the rate that it is now - for its entire history,
766
00:52:48,680 --> 00:52:51,920
I'd get a very simple line.
767
00:52:51,920 --> 00:52:54,200
But astronomers knew this couldn't be correct
768
00:52:54,200 --> 00:52:57,920
because, of course, gravity is putting the brakes on the expansion,
769
00:52:57,920 --> 00:53:00,840
so the expansion of the universe should be slowing down
770
00:53:00,840 --> 00:53:03,120
and, if it's expanding more slowly now,
771
00:53:03,120 --> 00:53:06,160
it should've been expanding more quickly in the past.
772
00:53:06,160 --> 00:53:10,360
Space stretching more would mean a bigger redshift.
773
00:53:10,360 --> 00:53:12,360
Now, what does this mean for our supernova?
774
00:53:12,360 --> 00:53:15,880
Well, we know it was eight billion light years away.
775
00:53:16,960 --> 00:53:19,880
So we know it wouldn't fall exactly on this line,
776
00:53:19,880 --> 00:53:23,080
which corresponds to a redshift of about 0.49.
777
00:53:23,080 --> 00:53:25,840
It should sit maybe somewhere over here.
778
00:53:25,840 --> 00:53:28,800
Maybe at a redshift greater than 0.5.
779
00:53:28,800 --> 00:53:33,840
That means this line should really be curving down like that.
780
00:53:33,840 --> 00:53:36,760
But, of course, the exact shape of this line would tell them
781
00:53:36,760 --> 00:53:40,360
how much gravity is slowing down the expansion of the universe
782
00:53:40,360 --> 00:53:44,360
and that would tell them the fate of the universe.
783
00:53:44,360 --> 00:53:47,160
OK, so, David, you have the spectrum ready now.
784
00:53:47,160 --> 00:53:49,440
We have it.
785
00:53:49,440 --> 00:53:51,560
Yes, bring it up.
786
00:53:51,560 --> 00:53:53,480
And that gives you a measure of the redshift.
787
00:53:53,480 --> 00:53:55,120
So what did you measure that to be here?
788
00:53:55,120 --> 00:53:58,120
For this case, we measured 0.47.
789
00:53:58,120 --> 00:54:01,720
0.47! Well, that puts it on this side of the line.
790
00:54:01,720 --> 00:54:05,560
That means it's not a larger redshift, but a smaller redshift.
791
00:54:07,480 --> 00:54:10,280
This is fascinating because it's exactly what they saw.
792
00:54:10,280 --> 00:54:14,120
Not redshifts that were larger, but redshifts that were smaller.
793
00:54:14,120 --> 00:54:16,320
And they saw this time and time again
794
00:54:16,320 --> 00:54:18,800
and it could only have one explanation -
795
00:54:18,800 --> 00:54:22,320
smaller redshifts meant that the universe must have been expanding
796
00:54:22,320 --> 00:54:25,240
more slowly in the past than it is today.
797
00:54:25,240 --> 00:54:28,120
In other words, rather than slowing down,
798
00:54:28,120 --> 00:54:31,880
the rate of expansion of the universe is accelerating.
799
00:54:34,920 --> 00:54:37,720
As more and more supernovae were plotted,
800
00:54:37,720 --> 00:54:39,520
the picture became clearer.
801
00:54:42,480 --> 00:54:45,520
For the first few billion years after the Big Bang,
802
00:54:45,520 --> 00:54:49,320
it looked as if the expansion rates had been slowing as expected...
803
00:54:51,160 --> 00:54:53,920
..but then that changed
804
00:54:53,920 --> 00:54:57,000
and the expansion started to accelerate.
805
00:54:59,600 --> 00:55:03,000
It's hard to stress how much of a shock this was.
806
00:55:03,000 --> 00:55:06,200
Back then, everyone knew that the expansion of the universe
807
00:55:06,200 --> 00:55:07,920
had to be slowing down.
808
00:55:07,920 --> 00:55:11,200
Now, whether it would slow down enough to stop and then recollapse,
809
00:55:11,200 --> 00:55:14,280
that wasn't clear, but it had to be slowing down.
810
00:55:14,280 --> 00:55:18,480
After all, gravity had to be doing its job of putting the brakes on,
811
00:55:18,480 --> 00:55:19,760
but it wasn't.
812
00:55:19,760 --> 00:55:21,880
About six billion years ago,
813
00:55:21,880 --> 00:55:24,600
the expansion started to speed up.
814
00:55:24,600 --> 00:55:27,440
Clearly, there was some new and unexpected thing
815
00:55:27,440 --> 00:55:28,760
going on in the universe -
816
00:55:28,760 --> 00:55:30,920
something that science didn't have an answer for,
817
00:55:30,920 --> 00:55:34,120
something that was pushing the expansion of the universe
818
00:55:34,120 --> 00:55:36,160
at an accelerating rate.
819
00:55:36,160 --> 00:55:40,040
It became known, for want of another term, as dark energy.
820
00:55:44,720 --> 00:55:47,600
The best estimates suggest that dark energy
821
00:55:47,600 --> 00:55:50,360
makes up 70% of the universe.
822
00:55:52,400 --> 00:55:56,800
And that means the universe will not collapse and end in a big crunch.
823
00:55:56,800 --> 00:56:00,400
Instead, dark energy, not gravity,
824
00:56:00,400 --> 00:56:03,600
will define the ultimate fate of the universe.
825
00:56:06,320 --> 00:56:09,480
Dark energy pushes the universe apart.
826
00:56:09,480 --> 00:56:12,920
It won't carry on expanding steadily for ever.
827
00:56:12,920 --> 00:56:16,720
Instead, dark energy forces the universe to fly apart
828
00:56:16,720 --> 00:56:18,600
at an ever-increasing rate.
829
00:56:18,600 --> 00:56:20,560
Galaxies will become so far apart
830
00:56:20,560 --> 00:56:23,120
that light wouldn't be able to travel between them.
831
00:56:23,120 --> 00:56:26,880
Each one will end up as an individual island of stars
832
00:56:26,880 --> 00:56:28,240
alone in the cosmos.
833
00:56:28,240 --> 00:56:30,640
It may even become so extreme
834
00:56:30,640 --> 00:56:33,360
that galaxies themselves will be ripped apart,
835
00:56:33,360 --> 00:56:37,840
leaving individual stars all alone in the black emptiness.
836
00:56:40,880 --> 00:56:43,120
Then again, maybe not.
837
00:56:44,360 --> 00:56:47,080
After all, the effect of dark energy
838
00:56:47,080 --> 00:56:51,840
seemed to suddenly appear between six and seven billion years ago.
839
00:56:51,840 --> 00:56:54,680
Who's to say how it'll behave in the future?
840
00:56:56,440 --> 00:56:58,520
That may sound bizarre
841
00:56:58,520 --> 00:57:02,320
but, with the discovery of dark energy, all bets are off.
842
00:57:04,040 --> 00:57:07,720
It's hard to stress how little we know about dark energy.
843
00:57:07,720 --> 00:57:10,160
It has a name, but that's about it.
844
00:57:10,160 --> 00:57:11,960
We don't know what it's made of,
845
00:57:11,960 --> 00:57:14,120
why it's driving the universe apart
846
00:57:14,120 --> 00:57:17,080
and, crucially, how it'll behave in the future.
847
00:57:17,080 --> 00:57:20,640
And that leaves a big hole in our understanding of the universe
848
00:57:20,640 --> 00:57:22,360
and its ultimate fate.
849
00:57:24,440 --> 00:57:28,360
Dark energy may simply be part of the universe,
850
00:57:28,360 --> 00:57:30,400
built into the way it works...
851
00:57:33,760 --> 00:57:37,040
..or it could point to a fundamental problem
852
00:57:37,040 --> 00:57:41,480
with the most important and trusted scientific theories we have...
853
00:57:43,600 --> 00:57:46,160
..ones that are at the very heart of our understanding
854
00:57:46,160 --> 00:57:47,880
of how the world works.
855
00:57:52,320 --> 00:57:56,680
How the universe will end started as astronomy's great challenge,
856
00:57:56,680 --> 00:57:58,840
but the fate of the universe has become
857
00:57:58,840 --> 00:58:01,480
much more than just an academic question.
858
00:58:01,480 --> 00:58:05,000
Through the discovery of this strange, enigmatic energy -
859
00:58:05,000 --> 00:58:08,880
if, indeed, that's what it is - one that defies current understanding,
860
00:58:08,880 --> 00:58:12,200
it's spread to the heart of fundamental physics.
861
00:58:12,200 --> 00:58:15,160
Finding the answer to how the universe will end
862
00:58:15,160 --> 00:58:20,040
could have profound implications on how we understand our world.
863
00:58:24,800 --> 00:58:28,600
If you want to find out more about the universe and the end of time,
864
00:58:28,600 --> 00:58:32,960
go to the address below and follow the links to the Open University.
74954
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