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The Invisible Universe

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Tonight, we want to report on one of the most unnerving discoveries in

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space science. Most of the universe is missing.

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Stranger still, wherever and whatever this missing stuff is,

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it controls the fate of the cosmos.

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Welcome to the invisible universe.

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We're here at the Mullard Radio Astronomy Observatory in Cambridge.

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These amazing dishes are at the forefront of one of the strangest,

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and yet most important, searches in science.

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The quest - to understand the invisible universe.

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We live in a world made of matter.

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This radio telescope is matter.

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So are the planets, the stars and interstellar dust.

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So, you might think it's easy stuff to find.

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But it turns out

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that even this ordinary matter is almost invisible.

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And that's only the start.

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As well as ordinary matter,

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there's another kind of matter we think is out there,

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but we've never actually seen.

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We call it dark matter.

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Chris visits the largest dark matter detector in the world

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to try and find it.

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And Jim Al-Khalili investigates the most puzzling mystery.

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So mysterious that almost all we know about it is a name.

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Dark energy.

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Tonight, we'll guide you through this mind-boggling

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invisible universe,

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and show you how it can control the fate of the entire cosmos.

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We start our journey with a success story -

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ordinary matter.

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That's what's called baryonic matter.

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And the thing is, that when you add up all the baryonic matter in

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the universe - all the stars, the galaxies,

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the black holes, the planets, the gas,

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the dust, everything - you find you come up very short.

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We just can't find enough of the stuff,

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given what we know about the early universe.

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But where is this missing matter?

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In the last few months, it may finally have turned up.

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To find out where it was hiding,

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Maggie met up with Amelie Saintonge.

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Amelie, how do we know that there's stuff missing in the universe?

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To figure out how much baryonic matter there is in the universe,

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we can look at the cosmic microwave background.

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And the cosmic microwave background is an image of the universe

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as it was about 400,000 years after the Big Bang.

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So, it's quite granular. What's that all about?

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The difference between the red spots and the blue spots

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is very, very small temperature fluctuations,

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across the entire sky.

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So, what do the temperature fluctuations in this picture mean?

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Well, there is a lot of information in this.

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We need to find these temperature variations,

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measure their position,

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the distance between them.

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And then we compare that with our cosmological models.

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And we can infer how much baryonic matter

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there was in this soup of

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-baryons and photons.

-So, that was the universe in the past.

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What are we observing now?

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So, now, we can go and use our telescopes to look at the galaxies

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around us. And, with that,

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we can measure their stars, the gas,

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the dust between the stars.

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And if we add all of that up,

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we come up to about 10% of the total

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that is inferred by the cosmic microwave background,

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-so there's about a 90% gap there.

-So, 90% is missing?

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-It's not visible?

-That's right.

-Aha.

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So, now we had to be a bit clever about that, and tried to go and

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find that extra missing mass.

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We assume a lot of it must be in gas

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that is located around galaxies,

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but how do we measure this thing? Big question.

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So, I have a little demo here which we can do to illustrate this.

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So, I'm going to spray some water,

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-which presumably you can't really see.

-Not really, no.

-Just vapour.

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But what about if I take this,

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and shine a light

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-from the background?

-Oh, yes, I can see it now!

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All of a sudden, we can see the mist appear.

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So, then, what are you using as your torch?

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So, we can use as a torch what we call quasars,

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supermassive black holes in very distant galaxies that are very,

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very bright. They are our cosmic torches, in some sense.

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-Yes!

-And by looking at their light,

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we can see some of the light being absorbed by the dense,

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well, the low-density gas in front of it.

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So, how much of this gas does this account for?

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Does it mop up the 90% that's missing?

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Not quite. So, if we combine all of this, all the different observation,

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-we come up to about 70%...

-OK.

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..so there was still the 30% of what we call

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the missing baryons that were not accounted for.

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So, it's been suspected for a long time that these missing baryons must

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be hiding at temperatures of about 1 million degrees.

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-That sounds hot.

-It is hot!

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It's a very difficult temperature.

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If it were slightly hotter,

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we would be able to observe it directly by X-ray light

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it would produce.

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If it were slightly colder, slightly denser,

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we could apply our technique here, with a flashlight, to be able

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to see it. A million kelvin is just in a bit of a no-man's-land,

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where we don't have easy ways of detecting it.

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But where is this hot gas?

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Scientists guessed that it lay in invisible gassy threads, called

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filaments, that occupy the space between galaxies.

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And they came up with an ingenious way of seeing them,

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using the cosmic microwave background.

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So, what some astronomers have done now is used some

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data from the Planck satellite to look for this gas

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in filaments in between galaxies.

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Now, each filament between galaxies is very diffuse.

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There is trace amounts of gas in that,

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so we are not going to be able to see it directly.

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We need to find a trick to boost that signal.

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We see light from the cosmic microwave background,

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these photons that are propagating,

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and when they hit the filaments of warm gas,

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the photons are scattered,

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they change direction slightly.

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And they lose a little bit of energy.

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So, we can pick up on these energy changes.

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This image shows the new result.

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The invisible filaments between the galaxies now rendered visible,

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using light from the beginning of the universe.

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-So, this is the filament here?

-Yes.

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So, what's our view of the universe now?

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-How does it all add up?

-With this discovery, we think that

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we have located all the baryons in the universe today,

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which is great news, it's great,

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because it confirms our models,

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and it gives us a good view of where the matter is in the universe.

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That's an amazing result.

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-Thank you so much for coming and sharing it with us.

-Pleasure.

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So, that's one part of the invisible universe we're finally able to see.

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But not all invisible matter is that easy to identify.

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Here at the MRAO in Cambridge,

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telescopes like these peer deeper and deeper into space.

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Producing ever more detailed information about

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what's out there in our universe.

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And they're uncovering new clues about a very different kind

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of invisible matter.

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One that dwarfs ordinary matter in mass,

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and seems to shape how the whole of the cosmos is held together.

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I'm talking about the mystery of dark matter.

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We think that dark matter exists because of some strange phenomena

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we've observed. Some of the first evidence came in from

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studying distant galaxies and how they rotate.

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Now, it's not a strictly accurate analogy,

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but imagine this turntable is a distant galaxy.

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As the turntable spins, the marble will fly off.

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With galaxies, a similar thing should happen.

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As they rotate, the stars within them

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should fly off into deep space.

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But they don't.

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It appears that gravity keeps them in place.

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But the problem is, there simply isn't enough mass visible

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in the galaxy to produce this much gravity.

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Something else must be providing this extra gravity

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to hold the stars in place.

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So, scientists came up with the idea of dark matter,

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a substance that has mass, so affects the gravity,

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and holds the galaxy together.

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They suspect dark matter is made of heavy subatomic particles

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affected by gravity, but little else.

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Making it totally invisible.

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So, what is dark matter,

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and how do we go about finding it?

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Chris went to visit one of the biggest dark matter laboratories

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in the world.

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This is a wonderful place to be this time of year,

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Gran Sasso, in the heart of the Italian Apennines.

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But we're not here to admire the view,

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we're heading underground in search of dark matter.

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This laboratory isn't up in the mountains because of the clear air,

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it's actually underground.

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And that's because we need to shield ourselves from cosmic rays,

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particles that are raining down on us all the time.

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And so, out here, we're being hit by them every second,

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but, once we go into the tunnel, disappear underground

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where the laboratory is, we get a million times fewer.

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That means we can see the more subtle signals

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that we're looking for, for dark matter among the cosmic particles.

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This is amazing. We're driving down

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a secret tunnel underneath a mountain, just like in

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a James Bond film. But this, this is where physics gets done.

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I'm here to meet Ranny Budnik, scientist on XENON 1,

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a detector designed to find the most elusive particles in the universe.

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This is the XENON Experiment.

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This is amazing. This place is enormous.

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-Yeah, it's really large space, spacious.

-Yeah.

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I reckon if you got somebody to design what they think a physics

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-experiment would look like...

-Yeah, this is...

-..this is pretty close.

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Here, deep under the mountain itself,

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the detector is shielded from cosmic rays and from surface radiation.

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But dark matter should pass right through.

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We have 1,400 metres of rock just to protect us from the universe...

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-OK.

-..which is barely enough.

-OK.

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The detector itself is 3.5 tonnes of the inert gas Xenon,

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held within this huge tank.

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The hope is that dark matter is made of WIMPS,

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Weakly Interacting Massive Particles,

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which will pass straight through the mountain,

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but then, just occasionally, score a rare direct hit on a Xenon nucleus.

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Which Ranny should be able to detect.

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What do we know about dark matter?

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We don't know much

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about what this particle could do,

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but we do know many things

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about what it cannot do.

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So, we know it doesn't interact strongly with light,

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or with any matter that we know.

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So, this is a problem, because you're trying to detect it,

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so how on earth do you build a dark matter detector?

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You need to look for a very rare interaction.

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If the interaction strength is very, very weak,

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that means that they do interact, but rarely.

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When you say interact,

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what should I be imagining, what's actually happening?

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What we're looking for is, basically,

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kind of a billiard ball interaction.

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They just knock something, and then our particles,

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the normal particle, is knocked,

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and then this particle gets some energy,

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-our nucleus...

-Yeah.

-..our Xenon nucleus, basically,

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is being kicked, and then the Xenon nucleus

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deposits the energy inside our detector.

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OK. So, you're looking for these direct hits.

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-Exactly.

-These rare cases where the dark matter particle

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-happens to hit directly a Xenon nucleus.

-Yes.

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And how often do we think that happens?

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So, we know that in this detector,

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in Xenon 1 tonne, we expect, at most,

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let's say, uh, in the low number of tens of events per year.

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If you see a signal, what will it tell us?

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We're going to see, if we see something,

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that would be a bunch of events,

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let's say around five,

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that are consistent with being dark matter,

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and, more importantly, very inconsistent

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with being anything normal, that we do expect to see in

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-the experiment.

-Well, that's the cheerful possibility.

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You could press the button and see nothing.

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-Exactly. Actually, what usually happens.

-Yeah.

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Or what happened all the time so far.

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So, what does that tell us?

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Each time we look at data,

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and that happened in the past, and don't find anything,

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that means we can rule out,

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we can just send them back to the drawing board

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and look for explanations

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on what could be dark matter that is not seen

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by our experiment and by other experiments.

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Well, whatever the results are,

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I hope you'll come back and tell us about them.

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-Thank you very much.

-You're welcome.

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It's wonderful to be here,

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and genuinely exciting to see these marvellous experiments in action.

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But with all this effort, they still haven't found anything.

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And that makes me wonder,

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why are astronomers so sure that dark matter exists?

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Back in the UK, I met up with cosmologist Andrew Pontzen,

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who is convinced that evidence for dark matter can be glimpsed at

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the beginning of the universe.

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In the same cosmic microwave background

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that Maggie encountered before.

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Particle physicists haven't found dark matter.

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What is it that makes astronomers so convinced that it exists?

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We've been able to take pictures of the universe when it was very young,

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using specialist telescopes

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that just look back through time, by looking to

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extraordinarily large distances.

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This is the cosmic microwave background?

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The cosmic microwave background, exactly.

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And people had predictions for what that should look like,

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long before detailed observations

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were actually technologically possible.

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And those predictions are based on a kind of competition.

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There's a competition between gravity pulling stuff together

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and pressure pushing things apart.

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That's going to give rise to kind of ripples going through

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the early universe,

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and what we're able to do, using these satellite pictures,

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is to actually measure how strong

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are the ripples as a function of scale.

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So, you can have a look at how ripply the universe is

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on, say, small scales, versus how ripply it is on large scales.

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And depending on how that balance between gravity

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and pressure plays out,

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that gives you a very distinctive set of patterns.

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So, these patterns tell you that the dark matter exists?

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The more dark matter you have, the more there's a sort of

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tendency to make big ripples,

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and the more you have of normal matter,

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the more pressure there is that's able to resist that.

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And so we get the right amount of dark matter?

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Yeah, we get the right amount of dark matter,

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based on the entire universe,

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and that's a genuine prediction coming from dark matter theory.

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So, if the detectors aren't sensitive enough to find the dark matter,

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is there a point we get to where we should be worried,

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where a non-detection would start to question

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what we know from looking at the universe?

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There's certainly a point coming up where we should start

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to worry about our simplest and, in a sense, most compelling,

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explanation of what particle is responsible for dark matter.

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These are the so-called WIMPS,

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or weakly interactive massive particles.

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And there's a very natural sort of set of expectations

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for how big and sensitive a detector you need to build

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before you'll be able to find it,

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and we are actually reaching that kind of level of sensitivity and,

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so far, of course, haven't found anything.

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So, in the next few years, at least as far as that simplest

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and most favoured explanation for what dark matter actually is,

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yeah, we should start to get a bit concerned if we don't

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hear anything pretty soon.

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Well, there's lots more to do either way. Andrew, thank you very much.

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Dark matter is, of course,

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as invisible to the amateur as it is to the professional.

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But anyone can look up and glimpse objects in the night sky

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that tell us about this elusive material.

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Pete Lawrence takes a look at a few of the objects

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that fascinate dark matter scientists.

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And he shows us how photography can help us see more.

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The idea of the invisible universe

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isn't really news to the amateur astronomer.

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We make the invisible visible every time we use our telescopes

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to look at the stars.

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Take, for example, the Andromeda Galaxy.

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It's between the constellation of Cassiopeia and Andromeda,

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just up from the star Mirach.

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So, what at first appears to be just a faint, fuzzy,

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elongated blob,

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with a telescope is revealed to be something far more complex.

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It's a spiral galaxy, made up of an estimated trillion stars

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and huge clouds of gas,

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all revolving around a supermassive black hole.

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We now believe the shape is due to dark matter

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which infuses the galaxy,

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holding the stars in position through gravity.

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Spiral galaxies like Andromeda are interesting for other reasons, too,

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and I managed to get a really quick photograph of it just now through

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a gap in the clouds, and if you look at the shot,

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you can see there's a little star-like dot

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very close to the centre of the main Andromeda galaxy.

0:19:080:19:11

Now, that's not a star, that's actually a dwarf galaxy

0:19:110:19:14

which is in orbit around the main galaxy.

0:19:140:19:16

These satellite galaxies are fascinating for astronomers,

0:19:180:19:21

because they're thought to contain proportionately more

0:19:210:19:24

dark matter than the larger galaxies.

0:19:240:19:26

They're a bit of a puzzle, too,

0:19:260:19:28

because if current theories on dark matter are correct,

0:19:280:19:31

we should be seeing more of them than we've so far found.

0:19:310:19:35

There are other dwarf galaxies out there, too,

0:19:350:19:38

which you can try and photograph.

0:19:380:19:40

At this time of year, there's a wonderful dwarf galaxy

0:19:400:19:43

visible to a camera in the constellation of Leo.

0:19:430:19:47

It's close to the bright star Regulus,

0:19:470:19:50

and is called Leo I.

0:19:500:19:52

Now, the length of exposure you need to use will depend on the quality

0:19:540:19:58

of your skies. If you use a long exposure under light-polluted skies,

0:19:580:20:02

the image will come out just pure orange.

0:20:020:20:04

So you then need to knock it back a little bit.

0:20:040:20:07

Now, the longer your exposure,

0:20:070:20:09

the more influence you're going to get from the rotation of the earth,

0:20:090:20:12

so the more star trailing you will have on a fixed platform.

0:20:120:20:16

So, then, you may need to consider going to a tracking platform,

0:20:160:20:20

like I've got here.

0:20:200:20:22

And here's one final example of how to use photography

0:20:220:20:25

to make the invisible visible.

0:20:250:20:28

Nothing to do with dark matter,

0:20:280:20:30

but it's a stunning object if you can find it.

0:20:300:20:33

It's in Orion, which rises soon after sunset

0:20:330:20:36

in the eastern sky at this time of year.

0:20:360:20:39

Orion hanging in the night sky is a wonderful sight

0:20:400:20:44

in its own right.

0:20:440:20:45

But it's when you apply long exposure photography

0:20:450:20:48

that you make the invisible visible,

0:20:480:20:50

and reveal the beautiful Barnard's Loop.

0:20:500:20:53

The loop is brightest on its eastern side,

0:20:540:20:57

and appears as a beautiful red semicircle of gas,

0:20:570:21:00

glowing due to ionisation.

0:21:000:21:02

There are many other objects you can reveal using photography

0:21:050:21:08

in the night sky. So, take a look at our website and we'll show you

0:21:080:21:12

how to find some of them. And if you do get any photos,

0:21:120:21:16

don't forget to add them to our Flickr page,

0:21:160:21:18

because we'd love to see them.

0:21:180:21:19

As we explore the invisible universe,

0:21:260:21:29

we finally come to our most problematic mystery.

0:21:290:21:32

It seems there's something else we need to explain,

0:21:330:21:36

something we know very little about,

0:21:360:21:39

but which might control the entire fate of the universe.

0:21:390:21:43

Jim Al-Khalili explains.

0:21:430:21:44

Take a moment to consider what astronomy and physics have achieved.

0:21:540:21:58

Sitting on our small rock, in an unremarkable part of an apparently

0:21:580:22:03

unimportant galaxy,

0:22:030:22:04

we've looked out and seen back to the very beginning of time.

0:22:040:22:09

We've peered into the furthest corners of the universe.

0:22:110:22:15

And we've uncovered the fundamental laws that govern the behaviour

0:22:150:22:20

of energy and matter.

0:22:200:22:21

And yet there is a problem.

0:22:230:22:26

A big puzzle that remains unsolved.

0:22:260:22:29

Let me first explain why this puzzle even exists.

0:22:320:22:35

See what happens when I throw a stone into the water.

0:22:350:22:37

The ripples spread outwards at a constant speed.

0:22:420:22:46

Now, consider matter moving outwards from the Big Bang.

0:22:460:22:49

Imagine - hypothetically, of course -

0:22:490:22:52

that we could switch off the force of gravity.

0:22:520:22:54

Then, with nothing out there to slow them down,

0:22:560:22:59

all the galaxies should move away from each other

0:22:590:23:02

at a constant speed, just like the ripples.

0:23:020:23:05

But, in reality, gravity from their combined mass

0:23:070:23:11

slows down the expansion.

0:23:110:23:13

We now know how much normal matter there is in the universe,

0:23:160:23:19

and we have a good idea how much dark matter there is out there, too,

0:23:190:23:23

so we should know how the gravity of all the stuff influences

0:23:230:23:27

the way the universe is expanding.

0:23:270:23:29

And, at this point in its evolution,

0:23:290:23:32

our calculations suggest that this expansion should be slowing down.

0:23:320:23:36

But very slowly.

0:23:360:23:38

However, astronomers have discovered that this isn't the case.

0:23:380:23:42

To get a sense of how we know this,

0:23:450:23:48

and why it's a problem,

0:23:480:23:50

I've come to the Rawlings Array at the Chilbolton Observatory.

0:23:500:23:54

In August last year, this was one of the many radio telescopes

0:23:550:23:59

around the world that observed the biggest astronomical event of 2017.

0:23:590:24:04

Deep in space, two neutron stars collided,

0:24:060:24:11

causing a stellar explosion of incredible violence.

0:24:110:24:14

This so-called kilonova

0:24:160:24:18

unleashed a massive burst of gamma rays,

0:24:180:24:22

and a powerful gravitational wave,

0:24:220:24:24

both of which were measured here on Earth.

0:24:240:24:26

Scientists have now used this event

0:24:290:24:31

to measure the expansion of the universe.

0:24:310:24:34

This is how.

0:24:360:24:37

The gravitational wave told them how much energy

0:24:370:24:41

the kilonova produced.

0:24:410:24:42

And how far away it was.

0:24:460:24:48

They also worked out from the gamma rays

0:24:490:24:52

how fast the galaxy was moving,

0:24:520:24:55

by measuring their red shift.

0:24:550:24:57

So, we now have a measure of how far away the kilonova was when

0:24:590:25:02

it exploded, and how fast it was moving.

0:25:020:25:06

So, here's the 64 million question.

0:25:060:25:08

Would this galaxy move at the speed they just measured

0:25:110:25:15

if gravity were the only force acting on it?

0:25:150:25:19

And here are the results.

0:25:190:25:20

Obviously, I'm ignoring lots of subtleties and complexities,

0:25:200:25:24

but the speed, if gravity were the only influence, would be this.

0:25:240:25:28

1,600 kilometres per second.

0:25:280:25:31

But the speed, according to the neutron star collision measurement...

0:25:310:25:35

..is this. 3,000 kilometres per second - almost double.

0:25:380:25:42

Now, these two numbers are different, very different.

0:25:420:25:45

It's more evidence for a startling conclusion.

0:25:450:25:48

The universe's expansion can't be slowing down.

0:25:500:25:53

In fact, it's speeding up.

0:25:530:25:55

So, if the universe is expanding

0:25:570:26:00

faster and faster,

0:26:000:26:02

what's making it accelerate?

0:26:020:26:04

Well, the truth is, we don't know.

0:26:080:26:10

But at least we've given it a name - dark energy,

0:26:100:26:13

a weird new force that pushes the universe faster and faster.

0:26:130:26:18

This is the ultimate invisible something in the universe,

0:26:180:26:21

because there has to be a hell of a lot of it out there, somewhere.

0:26:210:26:25

And all this matters because dark energy

0:26:300:26:32

could be the key to explaining how the universe will end.

0:26:320:26:36

Without dark energy, gravity is the most significant

0:26:360:26:39

force dictating the fate of the universe.

0:26:390:26:42

If gravity is the dominant force,

0:26:430:26:46

then it means that, one day,

0:26:460:26:48

the universe might stop expanding and start contracting.

0:26:480:26:52

Eventually, it'll collapse together,

0:26:520:26:55

in what's known as the big crunch.

0:26:550:26:58

But if dark energy turns out to dominate,

0:27:000:27:03

then the end of the universe could be much lonelier.

0:27:030:27:06

As the universe spreads out,

0:27:070:27:09

the influence of gravity becomes weaker,

0:27:090:27:11

until everything's too far apart for it to have any effect.

0:27:110:27:15

Then dark energy will be the only player in town.

0:27:150:27:18

As dark energy keeps pushing the universe apart,

0:27:240:27:28

eventually, all galaxies will move so far away

0:27:280:27:31

they'll become invisible to each other.

0:27:310:27:34

The distances between them will become so great that light

0:27:340:27:39

from one would never reach the others.

0:27:390:27:42

And the universe would disappear into darkness for ever.

0:27:420:27:46

A sobering thought.

0:27:490:27:51

So, there's a lot to play for over the next few years.

0:27:510:27:54

But don't panic, none of this is due for another 20 billion years

0:27:540:27:58

or more. And who knows, it may all turn out to be wrong, anyway.

0:27:580:28:02

That's it for this month. But do join us for the next programme.

0:28:080:28:12

Meanwhile, don't forget to check out the website with Pete's star guide,

0:28:120:28:15

our special weather forecast and all the extra material

0:28:150:28:18

that we just couldn't fit into the programme.

0:28:180:28:21

In the meantime, of course, get outside and...

0:28:210:28:24

-get looking up.

-Good night.

0:28:240:28:26

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