0:00:02 > 0:00:05Tonight, we want to report on one of the most unnerving discoveries in
0:00:05 > 0:00:08space science. Most of the universe is missing.
0:00:08 > 0:00:12Stranger still, wherever and whatever this missing stuff is,
0:00:12 > 0:00:15it controls the fate of the cosmos.
0:00:15 > 0:00:17Welcome to the invisible universe.
0:00:44 > 0:00:49We're here at the Mullard Radio Astronomy Observatory in Cambridge.
0:00:49 > 0:00:53These amazing dishes are at the forefront of one of the strangest,
0:00:53 > 0:00:55and yet most important, searches in science.
0:00:55 > 0:00:58The quest - to understand the invisible universe.
0:01:00 > 0:01:03We live in a world made of matter.
0:01:03 > 0:01:06This radio telescope is matter.
0:01:06 > 0:01:09So are the planets, the stars and interstellar dust.
0:01:10 > 0:01:13So, you might think it's easy stuff to find.
0:01:13 > 0:01:15But it turns out
0:01:15 > 0:01:18that even this ordinary matter is almost invisible.
0:01:20 > 0:01:21And that's only the start.
0:01:23 > 0:01:24As well as ordinary matter,
0:01:24 > 0:01:28there's another kind of matter we think is out there,
0:01:28 > 0:01:29but we've never actually seen.
0:01:31 > 0:01:34We call it dark matter.
0:01:35 > 0:01:38Chris visits the largest dark matter detector in the world
0:01:38 > 0:01:41to try and find it.
0:01:43 > 0:01:47And Jim Al-Khalili investigates the most puzzling mystery.
0:01:47 > 0:01:51So mysterious that almost all we know about it is a name.
0:01:51 > 0:01:52Dark energy.
0:01:53 > 0:01:56Tonight, we'll guide you through this mind-boggling
0:01:56 > 0:01:57invisible universe,
0:01:57 > 0:02:02and show you how it can control the fate of the entire cosmos.
0:02:07 > 0:02:10We start our journey with a success story -
0:02:10 > 0:02:11ordinary matter.
0:02:12 > 0:02:15That's what's called baryonic matter.
0:02:15 > 0:02:18And the thing is, that when you add up all the baryonic matter in
0:02:18 > 0:02:21the universe - all the stars, the galaxies,
0:02:21 > 0:02:24the black holes, the planets, the gas,
0:02:24 > 0:02:27the dust, everything - you find you come up very short.
0:02:31 > 0:02:33We just can't find enough of the stuff,
0:02:33 > 0:02:35given what we know about the early universe.
0:02:38 > 0:02:40But where is this missing matter?
0:02:43 > 0:02:46In the last few months, it may finally have turned up.
0:02:49 > 0:02:51To find out where it was hiding,
0:02:51 > 0:02:53Maggie met up with Amelie Saintonge.
0:02:58 > 0:03:01Amelie, how do we know that there's stuff missing in the universe?
0:03:01 > 0:03:04To figure out how much baryonic matter there is in the universe,
0:03:04 > 0:03:07we can look at the cosmic microwave background.
0:03:07 > 0:03:11And the cosmic microwave background is an image of the universe
0:03:11 > 0:03:14as it was about 400,000 years after the Big Bang.
0:03:14 > 0:03:17So, it's quite granular. What's that all about?
0:03:17 > 0:03:21The difference between the red spots and the blue spots
0:03:21 > 0:03:24is very, very small temperature fluctuations,
0:03:24 > 0:03:25across the entire sky.
0:03:25 > 0:03:28So, what do the temperature fluctuations in this picture mean?
0:03:28 > 0:03:30Well, there is a lot of information in this.
0:03:30 > 0:03:33We need to find these temperature variations,
0:03:33 > 0:03:35measure their position,
0:03:35 > 0:03:37the distance between them.
0:03:37 > 0:03:40And then we compare that with our cosmological models.
0:03:40 > 0:03:44And we can infer how much baryonic matter
0:03:44 > 0:03:47there was in this soup of
0:03:47 > 0:03:50- baryons and photons.- So, that was the universe in the past.
0:03:50 > 0:03:52What are we observing now?
0:03:52 > 0:03:55So, now, we can go and use our telescopes to look at the galaxies
0:03:55 > 0:03:57around us. And, with that,
0:03:57 > 0:03:59we can measure their stars, the gas,
0:03:59 > 0:04:01the dust between the stars.
0:04:01 > 0:04:04And if we add all of that up,
0:04:04 > 0:04:07we come up to about 10% of the total
0:04:07 > 0:04:10that is inferred by the cosmic microwave background,
0:04:10 > 0:04:13- so there's about a 90% gap there. - So, 90% is missing?
0:04:13 > 0:04:15- It's not visible?- That's right. - Aha.
0:04:15 > 0:04:18So, now we had to be a bit clever about that, and tried to go and
0:04:18 > 0:04:20find that extra missing mass.
0:04:20 > 0:04:23We assume a lot of it must be in gas
0:04:23 > 0:04:26that is located around galaxies,
0:04:26 > 0:04:28but how do we measure this thing? Big question.
0:04:28 > 0:04:31So, I have a little demo here which we can do to illustrate this.
0:04:31 > 0:04:34So, I'm going to spray some water,
0:04:34 > 0:04:38- which presumably you can't really see.- Not really, no.- Just vapour.
0:04:38 > 0:04:40But what about if I take this,
0:04:40 > 0:04:42and shine a light
0:04:42 > 0:04:44- from the background?- Oh, yes, I can see it now!
0:04:44 > 0:04:48All of a sudden, we can see the mist appear.
0:04:48 > 0:04:50So, then, what are you using as your torch?
0:04:50 > 0:04:53So, we can use as a torch what we call quasars,
0:04:53 > 0:04:56supermassive black holes in very distant galaxies that are very,
0:04:56 > 0:05:00very bright. They are our cosmic torches, in some sense.
0:05:00 > 0:05:03- Yes!- And by looking at their light,
0:05:03 > 0:05:08we can see some of the light being absorbed by the dense,
0:05:08 > 0:05:10well, the low-density gas in front of it.
0:05:10 > 0:05:13So, how much of this gas does this account for?
0:05:13 > 0:05:15Does it mop up the 90% that's missing?
0:05:15 > 0:05:19Not quite. So, if we combine all of this, all the different observation,
0:05:19 > 0:05:21- we come up to about 70%...- OK.
0:05:21 > 0:05:24..so there was still the 30% of what we call
0:05:24 > 0:05:27the missing baryons that were not accounted for.
0:05:27 > 0:05:31So, it's been suspected for a long time that these missing baryons must
0:05:31 > 0:05:35be hiding at temperatures of about 1 million degrees.
0:05:35 > 0:05:37- That sounds hot.- It is hot!
0:05:37 > 0:05:39It's a very difficult temperature.
0:05:39 > 0:05:41If it were slightly hotter,
0:05:41 > 0:05:45we would be able to observe it directly by X-ray light
0:05:45 > 0:05:47it would produce.
0:05:47 > 0:05:49If it were slightly colder, slightly denser,
0:05:49 > 0:05:52we could apply our technique here, with a flashlight, to be able
0:05:52 > 0:05:56to see it. A million kelvin is just in a bit of a no-man's-land,
0:05:56 > 0:05:59where we don't have easy ways of detecting it.
0:05:59 > 0:06:02But where is this hot gas?
0:06:03 > 0:06:07Scientists guessed that it lay in invisible gassy threads, called
0:06:07 > 0:06:11filaments, that occupy the space between galaxies.
0:06:12 > 0:06:15And they came up with an ingenious way of seeing them,
0:06:15 > 0:06:18using the cosmic microwave background.
0:06:22 > 0:06:26So, what some astronomers have done now is used some
0:06:26 > 0:06:30data from the Planck satellite to look for this gas
0:06:30 > 0:06:32in filaments in between galaxies.
0:06:32 > 0:06:36Now, each filament between galaxies is very diffuse.
0:06:36 > 0:06:38There is trace amounts of gas in that,
0:06:38 > 0:06:41so we are not going to be able to see it directly.
0:06:41 > 0:06:44We need to find a trick to boost that signal.
0:06:44 > 0:06:47We see light from the cosmic microwave background,
0:06:47 > 0:06:49these photons that are propagating,
0:06:49 > 0:06:52and when they hit the filaments of warm gas,
0:06:52 > 0:06:55the photons are scattered,
0:06:55 > 0:06:56they change direction slightly.
0:06:56 > 0:06:59And they lose a little bit of energy.
0:06:59 > 0:07:01So, we can pick up on these energy changes.
0:07:02 > 0:07:05This image shows the new result.
0:07:05 > 0:07:10The invisible filaments between the galaxies now rendered visible,
0:07:10 > 0:07:13using light from the beginning of the universe.
0:07:13 > 0:07:15- So, this is the filament here?- Yes.
0:07:15 > 0:07:18So, what's our view of the universe now?
0:07:18 > 0:07:21- How does it all add up? - With this discovery, we think that
0:07:21 > 0:07:25we have located all the baryons in the universe today,
0:07:25 > 0:07:27which is great news, it's great,
0:07:27 > 0:07:29because it confirms our models,
0:07:29 > 0:07:34and it gives us a good view of where the matter is in the universe.
0:07:34 > 0:07:35That's an amazing result.
0:07:35 > 0:07:38- Thank you so much for coming and sharing it with us.- Pleasure.
0:07:39 > 0:07:44So, that's one part of the invisible universe we're finally able to see.
0:07:44 > 0:07:47But not all invisible matter is that easy to identify.
0:07:49 > 0:07:51Here at the MRAO in Cambridge,
0:07:51 > 0:07:56telescopes like these peer deeper and deeper into space.
0:07:56 > 0:07:58Producing ever more detailed information about
0:07:58 > 0:08:00what's out there in our universe.
0:08:03 > 0:08:07And they're uncovering new clues about a very different kind
0:08:07 > 0:08:08of invisible matter.
0:08:08 > 0:08:11One that dwarfs ordinary matter in mass,
0:08:11 > 0:08:15and seems to shape how the whole of the cosmos is held together.
0:08:16 > 0:08:19I'm talking about the mystery of dark matter.
0:08:22 > 0:08:26We think that dark matter exists because of some strange phenomena
0:08:26 > 0:08:28we've observed. Some of the first evidence came in from
0:08:28 > 0:08:32studying distant galaxies and how they rotate.
0:08:32 > 0:08:34Now, it's not a strictly accurate analogy,
0:08:34 > 0:08:37but imagine this turntable is a distant galaxy.
0:08:40 > 0:08:44As the turntable spins, the marble will fly off.
0:08:46 > 0:08:49With galaxies, a similar thing should happen.
0:08:49 > 0:08:52As they rotate, the stars within them
0:08:52 > 0:08:55should fly off into deep space.
0:08:55 > 0:08:56But they don't.
0:08:58 > 0:09:02It appears that gravity keeps them in place.
0:09:02 > 0:09:05But the problem is, there simply isn't enough mass visible
0:09:05 > 0:09:08in the galaxy to produce this much gravity.
0:09:10 > 0:09:14Something else must be providing this extra gravity
0:09:14 > 0:09:16to hold the stars in place.
0:09:18 > 0:09:21So, scientists came up with the idea of dark matter,
0:09:21 > 0:09:24a substance that has mass, so affects the gravity,
0:09:24 > 0:09:25and holds the galaxy together.
0:09:27 > 0:09:32They suspect dark matter is made of heavy subatomic particles
0:09:32 > 0:09:36affected by gravity, but little else.
0:09:36 > 0:09:37Making it totally invisible.
0:09:39 > 0:09:41So, what is dark matter,
0:09:41 > 0:09:44and how do we go about finding it?
0:09:45 > 0:09:48Chris went to visit one of the biggest dark matter laboratories
0:09:48 > 0:09:49in the world.
0:09:53 > 0:09:55This is a wonderful place to be this time of year,
0:09:55 > 0:09:59Gran Sasso, in the heart of the Italian Apennines.
0:09:59 > 0:10:01But we're not here to admire the view,
0:10:01 > 0:10:06we're heading underground in search of dark matter.
0:10:06 > 0:10:09This laboratory isn't up in the mountains because of the clear air,
0:10:09 > 0:10:11it's actually underground.
0:10:11 > 0:10:14And that's because we need to shield ourselves from cosmic rays,
0:10:14 > 0:10:18particles that are raining down on us all the time.
0:10:18 > 0:10:22And so, out here, we're being hit by them every second,
0:10:22 > 0:10:25but, once we go into the tunnel, disappear underground
0:10:25 > 0:10:28where the laboratory is, we get a million times fewer.
0:10:28 > 0:10:30That means we can see the more subtle signals
0:10:30 > 0:10:36that we're looking for, for dark matter among the cosmic particles.
0:10:36 > 0:10:38This is amazing. We're driving down
0:10:38 > 0:10:41a secret tunnel underneath a mountain, just like in
0:10:41 > 0:10:45a James Bond film. But this, this is where physics gets done.
0:10:49 > 0:10:53I'm here to meet Ranny Budnik, scientist on XENON 1,
0:10:53 > 0:10:58a detector designed to find the most elusive particles in the universe.
0:10:59 > 0:11:01This is the XENON Experiment.
0:11:01 > 0:11:03This is amazing. This place is enormous.
0:11:03 > 0:11:07- Yeah, it's really large space, spacious.- Yeah.
0:11:07 > 0:11:10I reckon if you got somebody to design what they think a physics
0:11:10 > 0:11:14- experiment would look like... - Yeah, this is... - ..this is pretty close.
0:11:17 > 0:11:19Here, deep under the mountain itself,
0:11:19 > 0:11:24the detector is shielded from cosmic rays and from surface radiation.
0:11:24 > 0:11:27But dark matter should pass right through.
0:11:27 > 0:11:31We have 1,400 metres of rock just to protect us from the universe...
0:11:31 > 0:11:34- OK.- ..which is barely enough.- OK.
0:11:36 > 0:11:41The detector itself is 3.5 tonnes of the inert gas Xenon,
0:11:41 > 0:11:42held within this huge tank.
0:11:44 > 0:11:48The hope is that dark matter is made of WIMPS,
0:11:48 > 0:11:50Weakly Interacting Massive Particles,
0:11:50 > 0:11:53which will pass straight through the mountain,
0:11:53 > 0:11:58but then, just occasionally, score a rare direct hit on a Xenon nucleus.
0:11:58 > 0:12:01Which Ranny should be able to detect.
0:12:04 > 0:12:06What do we know about dark matter?
0:12:06 > 0:12:07We don't know much
0:12:07 > 0:12:10about what this particle could do,
0:12:10 > 0:12:12but we do know many things
0:12:12 > 0:12:14about what it cannot do.
0:12:14 > 0:12:19So, we know it doesn't interact strongly with light,
0:12:19 > 0:12:21or with any matter that we know.
0:12:21 > 0:12:24So, this is a problem, because you're trying to detect it,
0:12:24 > 0:12:26so how on earth do you build a dark matter detector?
0:12:26 > 0:12:29You need to look for a very rare interaction.
0:12:29 > 0:12:32If the interaction strength is very, very weak,
0:12:32 > 0:12:36that means that they do interact, but rarely.
0:12:36 > 0:12:37When you say interact,
0:12:37 > 0:12:40what should I be imagining, what's actually happening?
0:12:40 > 0:12:42What we're looking for is, basically,
0:12:42 > 0:12:44kind of a billiard ball interaction.
0:12:44 > 0:12:48They just knock something, and then our particles,
0:12:48 > 0:12:51the normal particle, is knocked,
0:12:51 > 0:12:54and then this particle gets some energy,
0:12:54 > 0:12:57- our nucleus...- Yeah. - ..our Xenon nucleus, basically,
0:12:57 > 0:13:01is being kicked, and then the Xenon nucleus
0:13:01 > 0:13:03deposits the energy inside our detector.
0:13:03 > 0:13:07OK. So, you're looking for these direct hits.
0:13:07 > 0:13:10- Exactly.- These rare cases where the dark matter particle
0:13:10 > 0:13:13- happens to hit directly a Xenon nucleus.- Yes.
0:13:13 > 0:13:16And how often do we think that happens?
0:13:16 > 0:13:18So, we know that in this detector,
0:13:18 > 0:13:23in Xenon 1 tonne, we expect, at most,
0:13:23 > 0:13:28let's say, uh, in the low number of tens of events per year.
0:13:28 > 0:13:31If you see a signal, what will it tell us?
0:13:31 > 0:13:34We're going to see, if we see something,
0:13:34 > 0:13:36that would be a bunch of events,
0:13:36 > 0:13:38let's say around five,
0:13:38 > 0:13:43that are consistent with being dark matter,
0:13:43 > 0:13:46and, more importantly, very inconsistent
0:13:46 > 0:13:49with being anything normal, that we do expect to see in
0:13:49 > 0:13:52- the experiment.- Well, that's the cheerful possibility.
0:13:52 > 0:13:54You could press the button and see nothing.
0:13:54 > 0:13:58- Exactly. Actually, what usually happens.- Yeah.
0:13:58 > 0:14:01Or what happened all the time so far.
0:14:01 > 0:14:03So, what does that tell us?
0:14:03 > 0:14:05Each time we look at data,
0:14:05 > 0:14:08and that happened in the past, and don't find anything,
0:14:08 > 0:14:10that means we can rule out,
0:14:10 > 0:14:13we can just send them back to the drawing board
0:14:13 > 0:14:16and look for explanations
0:14:16 > 0:14:19on what could be dark matter that is not seen
0:14:19 > 0:14:21by our experiment and by other experiments.
0:14:21 > 0:14:22Well, whatever the results are,
0:14:22 > 0:14:24I hope you'll come back and tell us about them.
0:14:24 > 0:14:26- Thank you very much.- You're welcome.
0:14:30 > 0:14:32It's wonderful to be here,
0:14:32 > 0:14:36and genuinely exciting to see these marvellous experiments in action.
0:14:36 > 0:14:40But with all this effort, they still haven't found anything.
0:14:40 > 0:14:41And that makes me wonder,
0:14:41 > 0:14:45why are astronomers so sure that dark matter exists?
0:14:49 > 0:14:53Back in the UK, I met up with cosmologist Andrew Pontzen,
0:14:53 > 0:14:57who is convinced that evidence for dark matter can be glimpsed at
0:14:57 > 0:14:59the beginning of the universe.
0:15:00 > 0:15:02In the same cosmic microwave background
0:15:02 > 0:15:04that Maggie encountered before.
0:15:06 > 0:15:09Particle physicists haven't found dark matter.
0:15:09 > 0:15:12What is it that makes astronomers so convinced that it exists?
0:15:12 > 0:15:15We've been able to take pictures of the universe when it was very young,
0:15:15 > 0:15:17using specialist telescopes
0:15:17 > 0:15:20that just look back through time, by looking to
0:15:20 > 0:15:22extraordinarily large distances.
0:15:22 > 0:15:24This is the cosmic microwave background?
0:15:24 > 0:15:27The cosmic microwave background, exactly.
0:15:27 > 0:15:30And people had predictions for what that should look like,
0:15:30 > 0:15:33long before detailed observations
0:15:33 > 0:15:35were actually technologically possible.
0:15:35 > 0:15:39And those predictions are based on a kind of competition.
0:15:39 > 0:15:42There's a competition between gravity pulling stuff together
0:15:42 > 0:15:45and pressure pushing things apart.
0:15:45 > 0:15:48That's going to give rise to kind of ripples going through
0:15:48 > 0:15:49the early universe,
0:15:49 > 0:15:53and what we're able to do, using these satellite pictures,
0:15:53 > 0:15:56is to actually measure how strong
0:15:56 > 0:15:59are the ripples as a function of scale.
0:15:59 > 0:16:03So, you can have a look at how ripply the universe is
0:16:03 > 0:16:08on, say, small scales, versus how ripply it is on large scales.
0:16:08 > 0:16:11And depending on how that balance between gravity
0:16:11 > 0:16:13and pressure plays out,
0:16:13 > 0:16:15that gives you a very distinctive set of patterns.
0:16:15 > 0:16:19So, these patterns tell you that the dark matter exists?
0:16:19 > 0:16:22The more dark matter you have, the more there's a sort of
0:16:22 > 0:16:24tendency to make big ripples,
0:16:24 > 0:16:26and the more you have of normal matter,
0:16:26 > 0:16:29the more pressure there is that's able to resist that.
0:16:29 > 0:16:31And so we get the right amount of dark matter?
0:16:31 > 0:16:33Yeah, we get the right amount of dark matter,
0:16:33 > 0:16:35based on the entire universe,
0:16:35 > 0:16:39and that's a genuine prediction coming from dark matter theory.
0:16:39 > 0:16:42So, if the detectors aren't sensitive enough to find the dark matter,
0:16:42 > 0:16:44is there a point we get to where we should be worried,
0:16:44 > 0:16:47where a non-detection would start to question
0:16:47 > 0:16:50what we know from looking at the universe?
0:16:50 > 0:16:53There's certainly a point coming up where we should start
0:16:53 > 0:16:57to worry about our simplest and, in a sense, most compelling,
0:16:57 > 0:17:01explanation of what particle is responsible for dark matter.
0:17:01 > 0:17:03These are the so-called WIMPS,
0:17:03 > 0:17:06or weakly interactive massive particles.
0:17:06 > 0:17:11And there's a very natural sort of set of expectations
0:17:11 > 0:17:14for how big and sensitive a detector you need to build
0:17:14 > 0:17:16before you'll be able to find it,
0:17:16 > 0:17:20and we are actually reaching that kind of level of sensitivity and,
0:17:20 > 0:17:23so far, of course, haven't found anything.
0:17:23 > 0:17:27So, in the next few years, at least as far as that simplest
0:17:27 > 0:17:32and most favoured explanation for what dark matter actually is,
0:17:32 > 0:17:35yeah, we should start to get a bit concerned if we don't
0:17:35 > 0:17:36hear anything pretty soon.
0:17:36 > 0:17:40Well, there's lots more to do either way. Andrew, thank you very much.
0:17:41 > 0:17:43Dark matter is, of course,
0:17:43 > 0:17:47as invisible to the amateur as it is to the professional.
0:17:47 > 0:17:51But anyone can look up and glimpse objects in the night sky
0:17:51 > 0:17:54that tell us about this elusive material.
0:17:54 > 0:17:57Pete Lawrence takes a look at a few of the objects
0:17:57 > 0:18:01that fascinate dark matter scientists.
0:18:01 > 0:18:05And he shows us how photography can help us see more.
0:18:05 > 0:18:07The idea of the invisible universe
0:18:07 > 0:18:09isn't really news to the amateur astronomer.
0:18:09 > 0:18:13We make the invisible visible every time we use our telescopes
0:18:13 > 0:18:14to look at the stars.
0:18:16 > 0:18:19Take, for example, the Andromeda Galaxy.
0:18:19 > 0:18:23It's between the constellation of Cassiopeia and Andromeda,
0:18:23 > 0:18:26just up from the star Mirach.
0:18:26 > 0:18:30So, what at first appears to be just a faint, fuzzy,
0:18:30 > 0:18:32elongated blob,
0:18:32 > 0:18:36with a telescope is revealed to be something far more complex.
0:18:36 > 0:18:41It's a spiral galaxy, made up of an estimated trillion stars
0:18:41 > 0:18:42and huge clouds of gas,
0:18:42 > 0:18:46all revolving around a supermassive black hole.
0:18:47 > 0:18:50We now believe the shape is due to dark matter
0:18:50 > 0:18:52which infuses the galaxy,
0:18:52 > 0:18:54holding the stars in position through gravity.
0:18:56 > 0:19:00Spiral galaxies like Andromeda are interesting for other reasons, too,
0:19:00 > 0:19:04and I managed to get a really quick photograph of it just now through
0:19:04 > 0:19:06a gap in the clouds, and if you look at the shot,
0:19:06 > 0:19:08you can see there's a little star-like dot
0:19:08 > 0:19:11very close to the centre of the main Andromeda galaxy.
0:19:11 > 0:19:14Now, that's not a star, that's actually a dwarf galaxy
0:19:14 > 0:19:16which is in orbit around the main galaxy.
0:19:18 > 0:19:21These satellite galaxies are fascinating for astronomers,
0:19:21 > 0:19:24because they're thought to contain proportionately more
0:19:24 > 0:19:26dark matter than the larger galaxies.
0:19:26 > 0:19:28They're a bit of a puzzle, too,
0:19:28 > 0:19:31because if current theories on dark matter are correct,
0:19:31 > 0:19:35we should be seeing more of them than we've so far found.
0:19:35 > 0:19:38There are other dwarf galaxies out there, too,
0:19:38 > 0:19:40which you can try and photograph.
0:19:40 > 0:19:43At this time of year, there's a wonderful dwarf galaxy
0:19:43 > 0:19:47visible to a camera in the constellation of Leo.
0:19:47 > 0:19:50It's close to the bright star Regulus,
0:19:50 > 0:19:52and is called Leo I.
0:19:54 > 0:19:58Now, the length of exposure you need to use will depend on the quality
0:19:58 > 0:20:02of your skies. If you use a long exposure under light-polluted skies,
0:20:02 > 0:20:04the image will come out just pure orange.
0:20:04 > 0:20:07So you then need to knock it back a little bit.
0:20:07 > 0:20:09Now, the longer your exposure,
0:20:09 > 0:20:12the more influence you're going to get from the rotation of the earth,
0:20:12 > 0:20:16so the more star trailing you will have on a fixed platform.
0:20:16 > 0:20:20So, then, you may need to consider going to a tracking platform,
0:20:20 > 0:20:22like I've got here.
0:20:22 > 0:20:25And here's one final example of how to use photography
0:20:25 > 0:20:28to make the invisible visible.
0:20:28 > 0:20:30Nothing to do with dark matter,
0:20:30 > 0:20:33but it's a stunning object if you can find it.
0:20:33 > 0:20:36It's in Orion, which rises soon after sunset
0:20:36 > 0:20:39in the eastern sky at this time of year.
0:20:40 > 0:20:44Orion hanging in the night sky is a wonderful sight
0:20:44 > 0:20:45in its own right.
0:20:45 > 0:20:48But it's when you apply long exposure photography
0:20:48 > 0:20:50that you make the invisible visible,
0:20:50 > 0:20:53and reveal the beautiful Barnard's Loop.
0:20:54 > 0:20:57The loop is brightest on its eastern side,
0:20:57 > 0:21:00and appears as a beautiful red semicircle of gas,
0:21:00 > 0:21:02glowing due to ionisation.
0:21:05 > 0:21:08There are many other objects you can reveal using photography
0:21:08 > 0:21:12in the night sky. So, take a look at our website and we'll show you
0:21:12 > 0:21:16how to find some of them. And if you do get any photos,
0:21:16 > 0:21:18don't forget to add them to our Flickr page,
0:21:18 > 0:21:19because we'd love to see them.
0:21:26 > 0:21:29As we explore the invisible universe,
0:21:29 > 0:21:32we finally come to our most problematic mystery.
0:21:33 > 0:21:36It seems there's something else we need to explain,
0:21:36 > 0:21:39something we know very little about,
0:21:39 > 0:21:43but which might control the entire fate of the universe.
0:21:43 > 0:21:44Jim Al-Khalili explains.
0:21:54 > 0:21:58Take a moment to consider what astronomy and physics have achieved.
0:21:58 > 0:22:03Sitting on our small rock, in an unremarkable part of an apparently
0:22:03 > 0:22:04unimportant galaxy,
0:22:04 > 0:22:09we've looked out and seen back to the very beginning of time.
0:22:11 > 0:22:15We've peered into the furthest corners of the universe.
0:22:15 > 0:22:20And we've uncovered the fundamental laws that govern the behaviour
0:22:20 > 0:22:21of energy and matter.
0:22:23 > 0:22:26And yet there is a problem.
0:22:26 > 0:22:29A big puzzle that remains unsolved.
0:22:32 > 0:22:35Let me first explain why this puzzle even exists.
0:22:35 > 0:22:37See what happens when I throw a stone into the water.
0:22:42 > 0:22:46The ripples spread outwards at a constant speed.
0:22:46 > 0:22:49Now, consider matter moving outwards from the Big Bang.
0:22:49 > 0:22:52Imagine - hypothetically, of course -
0:22:52 > 0:22:54that we could switch off the force of gravity.
0:22:56 > 0:22:59Then, with nothing out there to slow them down,
0:22:59 > 0:23:02all the galaxies should move away from each other
0:23:02 > 0:23:05at a constant speed, just like the ripples.
0:23:07 > 0:23:11But, in reality, gravity from their combined mass
0:23:11 > 0:23:13slows down the expansion.
0:23:16 > 0:23:19We now know how much normal matter there is in the universe,
0:23:19 > 0:23:23and we have a good idea how much dark matter there is out there, too,
0:23:23 > 0:23:27so we should know how the gravity of all the stuff influences
0:23:27 > 0:23:29the way the universe is expanding.
0:23:29 > 0:23:32And, at this point in its evolution,
0:23:32 > 0:23:36our calculations suggest that this expansion should be slowing down.
0:23:36 > 0:23:38But very slowly.
0:23:38 > 0:23:42However, astronomers have discovered that this isn't the case.
0:23:45 > 0:23:48To get a sense of how we know this,
0:23:48 > 0:23:50and why it's a problem,
0:23:50 > 0:23:54I've come to the Rawlings Array at the Chilbolton Observatory.
0:23:55 > 0:23:59In August last year, this was one of the many radio telescopes
0:23:59 > 0:24:04around the world that observed the biggest astronomical event of 2017.
0:24:06 > 0:24:11Deep in space, two neutron stars collided,
0:24:11 > 0:24:14causing a stellar explosion of incredible violence.
0:24:16 > 0:24:18This so-called kilonova
0:24:18 > 0:24:22unleashed a massive burst of gamma rays,
0:24:22 > 0:24:24and a powerful gravitational wave,
0:24:24 > 0:24:26both of which were measured here on Earth.
0:24:29 > 0:24:31Scientists have now used this event
0:24:31 > 0:24:34to measure the expansion of the universe.
0:24:36 > 0:24:37This is how.
0:24:37 > 0:24:41The gravitational wave told them how much energy
0:24:41 > 0:24:42the kilonova produced.
0:24:46 > 0:24:48And how far away it was.
0:24:49 > 0:24:52They also worked out from the gamma rays
0:24:52 > 0:24:55how fast the galaxy was moving,
0:24:55 > 0:24:57by measuring their red shift.
0:24:59 > 0:25:02So, we now have a measure of how far away the kilonova was when
0:25:02 > 0:25:06it exploded, and how fast it was moving.
0:25:06 > 0:25:08So, here's the 64 million question.
0:25:11 > 0:25:15Would this galaxy move at the speed they just measured
0:25:15 > 0:25:19if gravity were the only force acting on it?
0:25:19 > 0:25:20And here are the results.
0:25:20 > 0:25:24Obviously, I'm ignoring lots of subtleties and complexities,
0:25:24 > 0:25:28but the speed, if gravity were the only influence, would be this.
0:25:28 > 0:25:311,600 kilometres per second.
0:25:31 > 0:25:35But the speed, according to the neutron star collision measurement...
0:25:38 > 0:25:42..is this. 3,000 kilometres per second - almost double.
0:25:42 > 0:25:45Now, these two numbers are different, very different.
0:25:45 > 0:25:48It's more evidence for a startling conclusion.
0:25:50 > 0:25:53The universe's expansion can't be slowing down.
0:25:53 > 0:25:55In fact, it's speeding up.
0:25:57 > 0:26:00So, if the universe is expanding
0:26:00 > 0:26:02faster and faster,
0:26:02 > 0:26:04what's making it accelerate?
0:26:08 > 0:26:10Well, the truth is, we don't know.
0:26:10 > 0:26:13But at least we've given it a name - dark energy,
0:26:13 > 0:26:18a weird new force that pushes the universe faster and faster.
0:26:18 > 0:26:21This is the ultimate invisible something in the universe,
0:26:21 > 0:26:25because there has to be a hell of a lot of it out there, somewhere.
0:26:30 > 0:26:32And all this matters because dark energy
0:26:32 > 0:26:36could be the key to explaining how the universe will end.
0:26:36 > 0:26:39Without dark energy, gravity is the most significant
0:26:39 > 0:26:42force dictating the fate of the universe.
0:26:43 > 0:26:46If gravity is the dominant force,
0:26:46 > 0:26:48then it means that, one day,
0:26:48 > 0:26:52the universe might stop expanding and start contracting.
0:26:52 > 0:26:55Eventually, it'll collapse together,
0:26:55 > 0:26:58in what's known as the big crunch.
0:27:00 > 0:27:03But if dark energy turns out to dominate,
0:27:03 > 0:27:06then the end of the universe could be much lonelier.
0:27:07 > 0:27:09As the universe spreads out,
0:27:09 > 0:27:11the influence of gravity becomes weaker,
0:27:11 > 0:27:15until everything's too far apart for it to have any effect.
0:27:15 > 0:27:18Then dark energy will be the only player in town.
0:27:24 > 0:27:28As dark energy keeps pushing the universe apart,
0:27:28 > 0:27:31eventually, all galaxies will move so far away
0:27:31 > 0:27:34they'll become invisible to each other.
0:27:34 > 0:27:39The distances between them will become so great that light
0:27:39 > 0:27:42from one would never reach the others.
0:27:42 > 0:27:46And the universe would disappear into darkness for ever.
0:27:49 > 0:27:51A sobering thought.
0:27:51 > 0:27:54So, there's a lot to play for over the next few years.
0:27:54 > 0:27:58But don't panic, none of this is due for another 20 billion years
0:27:58 > 0:28:02or more. And who knows, it may all turn out to be wrong, anyway.
0:28:08 > 0:28:12That's it for this month. But do join us for the next programme.
0:28:12 > 0:28:15Meanwhile, don't forget to check out the website with Pete's star guide,
0:28:15 > 0:28:18our special weather forecast and all the extra material
0:28:18 > 0:28:21that we just couldn't fit into the programme.
0:28:21 > 0:28:24In the meantime, of course, get outside and...
0:28:24 > 0:28:26- get looking up.- Good night.