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Discover the new Facebook emoticons!Transcript.subtitles-style-s0 td:first-child {background-color: #FFFFFF}.subtitles-style-s2 td:first-child {background-color: #FFFF00}.subtitles-style-s1 td:first-child {background-color: #00FFFF}.subtitles-style-s3 td:first-child {background-color: #00FF00}LineFromToTonight, we want to report on one of the most unnerving discoveries in0:00:020:00:05space science. Most of the universe is missing.0:00:050:00:08Stranger still, wherever and whatever this missing stuff is,0:00:080:00:12it controls the fate of the cosmos.0:00:120:00:15Welcome to the invisible universe.0:00:150:00:17We're here at the Mullard Radio Astronomy Observatory in Cambridge.0:00:440:00:49These amazing dishes are at the forefront of one of the strangest,0:00:490:00:53and yet most important, searches in science.0:00:530:00:55The quest - to understand the invisible universe.0:00:550:00:58We live in a world made of matter.0:01:000:01:03This radio telescope is matter.0:01:030:01:06So are the planets, the stars and interstellar dust.0:01:060:01:09So, you might think it's easy stuff to find.0:01:100:01:13But it turns out0:01:130:01:15that even this ordinary matter is almost invisible.0:01:150:01:18And that's only the start.0:01:200:01:21As well as ordinary matter,0:01:230:01:24there's another kind of matter we think is out there,0:01:240:01:28but we've never actually seen.0:01:280:01:29We call it dark matter.0:01:310:01:34Chris visits the largest dark matter detector in the world0:01:350:01:38to try and find it.0:01:380:01:41And Jim Al-Khalili investigates the most puzzling mystery.0:01:430:01:47So mysterious that almost all we know about it is a name.0:01:470:01:51Dark energy.0:01:510:01:52Tonight, we'll guide you through this mind-boggling0:01:530:01:56invisible universe,0:01:560:01:57and show you how it can control the fate of the entire cosmos.0:01:570:02:02We start our journey with a success story -0:02:070:02:10ordinary matter.0:02:100:02:11That's what's called baryonic matter.0:02:120:02:15And the thing is, that when you add up all the baryonic matter in0:02:150:02:18the universe - all the stars, the galaxies,0:02:180:02:21the black holes, the planets, the gas,0:02:210:02:24the dust, everything - you find you come up very short.0:02:240:02:27We just can't find enough of the stuff,0:02:310:02:33given what we know about the early universe.0:02:330:02:35But where is this missing matter?0:02:380:02:40In the last few months, it may finally have turned up.0:02:430:02:46To find out where it was hiding,0:02:490:02:51Maggie met up with Amelie Saintonge.0:02:510:02:53Amelie, how do we know that there's stuff missing in the universe?0:02:580:03:01To figure out how much baryonic matter there is in the universe,0:03:010:03:04we can look at the cosmic microwave background.0:03:040:03:07And the cosmic microwave background is an image of the universe0:03:070:03:11as it was about 400,000 years after the Big Bang.0:03:110:03:14So, it's quite granular. What's that all about?0:03:140:03:17The difference between the red spots and the blue spots0:03:170:03:21is very, very small temperature fluctuations,0:03:210:03:24across the entire sky.0:03:240:03:25So, what do the temperature fluctuations in this picture mean?0:03:250:03:28Well, there is a lot of information in this.0:03:280:03:30We need to find these temperature variations,0:03:300:03:33measure their position,0:03:330:03:35the distance between them.0:03:350:03:37And then we compare that with our cosmological models.0:03:370:03:40And we can infer how much baryonic matter0:03:400:03:44there was in this soup of0:03:440:03:47-baryons and photons.-So, that was the universe in the past.0:03:470:03:50What are we observing now?0:03:500:03:52So, now, we can go and use our telescopes to look at the galaxies0:03:520:03:55around us. And, with that,0:03:550:03:57we can measure their stars, the gas,0:03:570:03:59the dust between the stars.0:03:590:04:01And if we add all of that up,0:04:010:04:04we come up to about 10% of the total0:04:040:04:07that is inferred by the cosmic microwave background,0:04:070:04:10-so there's about a 90% gap there. -So, 90% is missing?0:04:100:04:13-It's not visible?-That's right. -Aha.0:04:130:04:15So, now we had to be a bit clever about that, and tried to go and0:04:150:04:18find that extra missing mass.0:04:180:04:20We assume a lot of it must be in gas0:04:200:04:23that is located around galaxies,0:04:230:04:26but how do we measure this thing? Big question.0:04:260:04:28So, I have a little demo here which we can do to illustrate this.0:04:280:04:31So, I'm going to spray some water,0:04:310:04:34-which presumably you can't really see.-Not really, no.-Just vapour.0:04:340:04:38But what about if I take this,0:04:380:04:40and shine a light0:04:400:04:42-from the background?-Oh, yes, I can see it now!0:04:420:04:44All of a sudden, we can see the mist appear.0:04:440:04:48So, then, what are you using as your torch?0:04:480:04:50So, we can use as a torch what we call quasars,0:04:500:04:53supermassive black holes in very distant galaxies that are very,0:04:530:04:56very bright. They are our cosmic torches, in some sense.0:04:560:05:00-Yes!-And by looking at their light,0:05:000:05:03we can see some of the light being absorbed by the dense,0:05:030:05:08well, the low-density gas in front of it.0:05:080:05:10So, how much of this gas does this account for?0:05:100:05:13Does it mop up the 90% that's missing?0:05:130:05:15Not quite. So, if we combine all of this, all the different observation,0:05:150:05:19-we come up to about 70%...-OK.0:05:190:05:21..so there was still the 30% of what we call0:05:210:05:24the missing baryons that were not accounted for.0:05:240:05:27So, it's been suspected for a long time that these missing baryons must0:05:270:05:31be hiding at temperatures of about 1 million degrees.0:05:310:05:35-That sounds hot.-It is hot!0:05:350:05:37It's a very difficult temperature.0:05:370:05:39If it were slightly hotter,0:05:390:05:41we would be able to observe it directly by X-ray light0:05:410:05:45it would produce.0:05:450:05:47If it were slightly colder, slightly denser,0:05:470:05:49we could apply our technique here, with a flashlight, to be able0:05:490:05:52to see it. A million kelvin is just in a bit of a no-man's-land,0:05:520:05:56where we don't have easy ways of detecting it.0:05:560:05:59But where is this hot gas?0:05:590:06:02Scientists guessed that it lay in invisible gassy threads, called0:06:030:06:07filaments, that occupy the space between galaxies.0:06:070:06:11And they came up with an ingenious way of seeing them,0:06:120:06:15using the cosmic microwave background.0:06:150:06:18So, what some astronomers have done now is used some0:06:220:06:26data from the Planck satellite to look for this gas0:06:260:06:30in filaments in between galaxies.0:06:300:06:32Now, each filament between galaxies is very diffuse.0:06:320:06:36There is trace amounts of gas in that,0:06:360:06:38so we are not going to be able to see it directly.0:06:380:06:41We need to find a trick to boost that signal.0:06:410:06:44We see light from the cosmic microwave background,0:06:440:06:47these photons that are propagating,0:06:470:06:49and when they hit the filaments of warm gas,0:06:490:06:52the photons are scattered,0:06:520:06:55they change direction slightly.0:06:550:06:56And they lose a little bit of energy.0:06:560:06:59So, we can pick up on these energy changes.0:06:590:07:01This image shows the new result.0:07:020:07:05The invisible filaments between the galaxies now rendered visible,0:07:050:07:10using light from the beginning of the universe.0:07:100:07:13-So, this is the filament here?-Yes.0:07:130:07:15So, what's our view of the universe now?0:07:150:07:18-How does it all add up? -With this discovery, we think that0:07:180:07:21we have located all the baryons in the universe today,0:07:210:07:25which is great news, it's great,0:07:250:07:27because it confirms our models,0:07:270:07:29and it gives us a good view of where the matter is in the universe.0:07:290:07:34That's an amazing result.0:07:340:07:35-Thank you so much for coming and sharing it with us.-Pleasure.0:07:350:07:38So, that's one part of the invisible universe we're finally able to see.0:07:390:07:44But not all invisible matter is that easy to identify.0:07:440:07:47Here at the MRAO in Cambridge,0:07:490:07:51telescopes like these peer deeper and deeper into space.0:07:510:07:56Producing ever more detailed information about0:07:560:07:58what's out there in our universe.0:07:580:08:00And they're uncovering new clues about a very different kind0:08:030:08:07of invisible matter.0:08:070:08:08One that dwarfs ordinary matter in mass,0:08:080:08:11and seems to shape how the whole of the cosmos is held together.0:08:110:08:15I'm talking about the mystery of dark matter.0:08:160:08:19We think that dark matter exists because of some strange phenomena0:08:220:08:26we've observed. Some of the first evidence came in from0:08:260:08:28studying distant galaxies and how they rotate.0:08:280:08:32Now, it's not a strictly accurate analogy,0:08:320:08:34but imagine this turntable is a distant galaxy.0:08:340:08:37As the turntable spins, the marble will fly off.0:08:400:08:44With galaxies, a similar thing should happen.0:08:460:08:49As they rotate, the stars within them0:08:490:08:52should fly off into deep space.0:08:520:08:55But they don't.0:08:550:08:56It appears that gravity keeps them in place.0:08:580:09:02But the problem is, there simply isn't enough mass visible0:09:020:09:05in the galaxy to produce this much gravity.0:09:050:09:08Something else must be providing this extra gravity0:09:100:09:14to hold the stars in place.0:09:140:09:16So, scientists came up with the idea of dark matter,0:09:180:09:21a substance that has mass, so affects the gravity,0:09:210:09:24and holds the galaxy together.0:09:240:09:25They suspect dark matter is made of heavy subatomic particles0:09:270:09:32affected by gravity, but little else.0:09:320:09:36Making it totally invisible.0:09:360:09:37So, what is dark matter,0:09:390:09:41and how do we go about finding it?0:09:410:09:44Chris went to visit one of the biggest dark matter laboratories0:09:450:09:48in the world.0:09:480:09:49This is a wonderful place to be this time of year,0:09:530:09:55Gran Sasso, in the heart of the Italian Apennines.0:09:550:09:59But we're not here to admire the view,0:09:590:10:01we're heading underground in search of dark matter.0:10:010:10:06This laboratory isn't up in the mountains because of the clear air,0:10:060:10:09it's actually underground.0:10:090:10:11And that's because we need to shield ourselves from cosmic rays,0:10:110:10:14particles that are raining down on us all the time.0:10:140:10:18And so, out here, we're being hit by them every second,0:10:180:10:22but, once we go into the tunnel, disappear underground0:10:220:10:25where the laboratory is, we get a million times fewer.0:10:250:10:28That means we can see the more subtle signals0:10:280:10:30that we're looking for, for dark matter among the cosmic particles.0:10:300:10:36This is amazing. We're driving down0:10:360:10:38a secret tunnel underneath a mountain, just like in0:10:380:10:41a James Bond film. But this, this is where physics gets done.0:10:410:10:45I'm here to meet Ranny Budnik, scientist on XENON 1,0:10:490:10:53a detector designed to find the most elusive particles in the universe.0:10:530:10:58This is the XENON Experiment.0:10:590:11:01This is amazing. This place is enormous.0:11:010:11:03-Yeah, it's really large space, spacious.-Yeah.0:11:030:11:07I reckon if you got somebody to design what they think a physics0:11:070:11:10-experiment would look like... -Yeah, this is... -..this is pretty close.0:11:100:11:14Here, deep under the mountain itself,0:11:170:11:19the detector is shielded from cosmic rays and from surface radiation.0:11:190:11:24But dark matter should pass right through.0:11:240:11:27We have 1,400 metres of rock just to protect us from the universe...0:11:270:11:31-OK.-..which is barely enough.-OK.0:11:310:11:34The detector itself is 3.5 tonnes of the inert gas Xenon,0:11:360:11:41held within this huge tank.0:11:410:11:42The hope is that dark matter is made of WIMPS,0:11:440:11:48Weakly Interacting Massive Particles,0:11:480:11:50which will pass straight through the mountain,0:11:500:11:53but then, just occasionally, score a rare direct hit on a Xenon nucleus.0:11:530:11:58Which Ranny should be able to detect.0:11:580:12:01What do we know about dark matter?0:12:040:12:06We don't know much0:12:060:12:07about what this particle could do,0:12:070:12:10but we do know many things0:12:100:12:12about what it cannot do.0:12:120:12:14So, we know it doesn't interact strongly with light,0:12:140:12:19or with any matter that we know.0:12:190:12:21So, this is a problem, because you're trying to detect it,0:12:210:12:24so how on earth do you build a dark matter detector?0:12:240:12:26You need to look for a very rare interaction.0:12:260:12:29If the interaction strength is very, very weak,0:12:290:12:32that means that they do interact, but rarely.0:12:320:12:36When you say interact,0:12:360:12:37what should I be imagining, what's actually happening?0:12:370:12:40What we're looking for is, basically,0:12:400:12:42kind of a billiard ball interaction.0:12:420:12:44They just knock something, and then our particles,0:12:440:12:48the normal particle, is knocked,0:12:480:12:51and then this particle gets some energy,0:12:510:12:54-our nucleus...-Yeah. -..our Xenon nucleus, basically,0:12:540:12:57is being kicked, and then the Xenon nucleus0:12:570:13:01deposits the energy inside our detector.0:13:010:13:03OK. So, you're looking for these direct hits.0:13:030:13:07-Exactly.-These rare cases where the dark matter particle0:13:070:13:10-happens to hit directly a Xenon nucleus.-Yes.0:13:100:13:13And how often do we think that happens?0:13:130:13:16So, we know that in this detector,0:13:160:13:18in Xenon 1 tonne, we expect, at most,0:13:180:13:23let's say, uh, in the low number of tens of events per year.0:13:230:13:28If you see a signal, what will it tell us?0:13:280:13:31We're going to see, if we see something,0:13:310:13:34that would be a bunch of events,0:13:340:13:36let's say around five,0:13:360:13:38that are consistent with being dark matter,0:13:380:13:43and, more importantly, very inconsistent0:13:430:13:46with being anything normal, that we do expect to see in0:13:460:13:49-the experiment.-Well, that's the cheerful possibility.0:13:490:13:52You could press the button and see nothing.0:13:520:13:54-Exactly. Actually, what usually happens.-Yeah.0:13:540:13:58Or what happened all the time so far.0:13:580:14:01So, what does that tell us?0:14:010:14:03Each time we look at data,0:14:030:14:05and that happened in the past, and don't find anything,0:14:050:14:08that means we can rule out,0:14:080:14:10we can just send them back to the drawing board0:14:100:14:13and look for explanations0:14:130:14:16on what could be dark matter that is not seen0:14:160:14:19by our experiment and by other experiments.0:14:190:14:21Well, whatever the results are,0:14:210:14:22I hope you'll come back and tell us about them.0:14:220:14:24-Thank you very much.-You're welcome.0:14:240:14:26It's wonderful to be here,0:14:300:14:32and genuinely exciting to see these marvellous experiments in action.0:14:320:14:36But with all this effort, they still haven't found anything.0:14:360:14:40And that makes me wonder,0:14:400:14:41why are astronomers so sure that dark matter exists?0:14:410:14:45Back in the UK, I met up with cosmologist Andrew Pontzen,0:14:490:14:53who is convinced that evidence for dark matter can be glimpsed at0:14:530:14:57the beginning of the universe.0:14:570:14:59In the same cosmic microwave background0:15:000:15:02that Maggie encountered before.0:15:020:15:04Particle physicists haven't found dark matter.0:15:060:15:09What is it that makes astronomers so convinced that it exists?0:15:090:15:12We've been able to take pictures of the universe when it was very young,0:15:120:15:15using specialist telescopes0:15:150:15:17that just look back through time, by looking to0:15:170:15:20extraordinarily large distances.0:15:200:15:22This is the cosmic microwave background?0:15:220:15:24The cosmic microwave background, exactly.0:15:240:15:27And people had predictions for what that should look like,0:15:270:15:30long before detailed observations0:15:300:15:33were actually technologically possible.0:15:330:15:35And those predictions are based on a kind of competition.0:15:350:15:39There's a competition between gravity pulling stuff together0:15:390:15:42and pressure pushing things apart.0:15:420:15:45That's going to give rise to kind of ripples going through0:15:450:15:48the early universe,0:15:480:15:49and what we're able to do, using these satellite pictures,0:15:490:15:53is to actually measure how strong0:15:530:15:56are the ripples as a function of scale.0:15:560:15:59So, you can have a look at how ripply the universe is0:15:590:16:03on, say, small scales, versus how ripply it is on large scales.0:16:030:16:08And depending on how that balance between gravity0:16:080:16:11and pressure plays out,0:16:110:16:13that gives you a very distinctive set of patterns.0:16:130:16:15So, these patterns tell you that the dark matter exists?0:16:150:16:19The more dark matter you have, the more there's a sort of0:16:190:16:22tendency to make big ripples,0:16:220:16:24and the more you have of normal matter,0:16:240:16:26the more pressure there is that's able to resist that.0:16:260:16:29And so we get the right amount of dark matter?0:16:290:16:31Yeah, we get the right amount of dark matter,0:16:310:16:33based on the entire universe,0:16:330:16:35and that's a genuine prediction coming from dark matter theory.0:16:350:16:39So, if the detectors aren't sensitive enough to find the dark matter,0:16:390:16:42is there a point we get to where we should be worried,0:16:420:16:44where a non-detection would start to question0:16:440:16:47what we know from looking at the universe?0:16:470:16:50There's certainly a point coming up where we should start0:16:500:16:53to worry about our simplest and, in a sense, most compelling,0:16:530:16:57explanation of what particle is responsible for dark matter.0:16:570:17:01These are the so-called WIMPS,0:17:010:17:03or weakly interactive massive particles.0:17:030:17:06And there's a very natural sort of set of expectations0:17:060:17:11for how big and sensitive a detector you need to build0:17:110:17:14before you'll be able to find it,0:17:140:17:16and we are actually reaching that kind of level of sensitivity and,0:17:160:17:20so far, of course, haven't found anything.0:17:200:17:23So, in the next few years, at least as far as that simplest0:17:230:17:27and most favoured explanation for what dark matter actually is,0:17:270:17:32yeah, we should start to get a bit concerned if we don't0:17:320:17:35hear anything pretty soon.0:17:350:17:36Well, there's lots more to do either way. Andrew, thank you very much.0:17:360:17:40Dark matter is, of course,0:17:410:17:43as invisible to the amateur as it is to the professional.0:17:430:17:47But anyone can look up and glimpse objects in the night sky0:17:470:17:51that tell us about this elusive material.0:17:510:17:54Pete Lawrence takes a look at a few of the objects0:17:540:17:57that fascinate dark matter scientists.0:17:570:18:01And he shows us how photography can help us see more.0:18:010:18:05The idea of the invisible universe0:18:050:18:07isn't really news to the amateur astronomer.0:18:070:18:09We make the invisible visible every time we use our telescopes0:18:090:18:13to look at the stars.0:18:130:18:14Take, for example, the Andromeda Galaxy.0:18:160:18:19It's between the constellation of Cassiopeia and Andromeda,0:18:190:18:23just up from the star Mirach.0:18:230:18:26So, what at first appears to be just a faint, fuzzy,0:18:260:18:30elongated blob,0:18:300:18:32with a telescope is revealed to be something far more complex.0:18:320:18:36It's a spiral galaxy, made up of an estimated trillion stars0:18:360:18:41and huge clouds of gas,0:18:410:18:42all revolving around a supermassive black hole.0:18:420:18:46We now believe the shape is due to dark matter0:18:470:18:50which infuses the galaxy,0:18:500:18:52holding the stars in position through gravity.0:18:520:18:54Spiral galaxies like Andromeda are interesting for other reasons, too,0:18:560:19:00and I managed to get a really quick photograph of it just now through0:19:000:19:04a gap in the clouds, and if you look at the shot,0:19:040:19:06you can see there's a little star-like dot0:19:060:19:08very close to the centre of the main Andromeda galaxy.0:19:080:19:11Now, that's not a star, that's actually a dwarf galaxy0:19:110:19:14which is in orbit around the main galaxy.0:19:140:19:16These satellite galaxies are fascinating for astronomers,0:19:180:19:21because they're thought to contain proportionately more0:19:210:19:24dark matter than the larger galaxies.0:19:240:19:26They're a bit of a puzzle, too,0:19:260:19:28because if current theories on dark matter are correct,0:19:280:19:31we should be seeing more of them than we've so far found.0:19:310:19:35There are other dwarf galaxies out there, too,0:19:350:19:38which you can try and photograph.0:19:380:19:40At this time of year, there's a wonderful dwarf galaxy0:19:400:19:43visible to a camera in the constellation of Leo.0:19:430:19:47It's close to the bright star Regulus,0:19:470:19:50and is called Leo I.0:19:500:19:52Now, the length of exposure you need to use will depend on the quality0:19:540:19:58of your skies. If you use a long exposure under light-polluted skies,0:19:580:20:02the image will come out just pure orange.0:20:020:20:04So you then need to knock it back a little bit.0:20:040:20:07Now, the longer your exposure,0:20:070:20:09the more influence you're going to get from the rotation of the earth,0:20:090:20:12so the more star trailing you will have on a fixed platform.0:20:120:20:16So, then, you may need to consider going to a tracking platform,0:20:160:20:20like I've got here.0:20:200:20:22And here's one final example of how to use photography0:20:220:20:25to make the invisible visible.0:20:250:20:28Nothing to do with dark matter,0:20:280:20:30but it's a stunning object if you can find it.0:20:300:20:33It's in Orion, which rises soon after sunset0:20:330:20:36in the eastern sky at this time of year.0:20:360:20:39Orion hanging in the night sky is a wonderful sight0:20:400:20:44in its own right.0:20:440:20:45But it's when you apply long exposure photography0:20:450:20:48that you make the invisible visible,0:20:480:20:50and reveal the beautiful Barnard's Loop.0:20:500:20:53The loop is brightest on its eastern side,0:20:540:20:57and appears as a beautiful red semicircle of gas,0:20:570:21:00glowing due to ionisation.0:21:000:21:02There are many other objects you can reveal using photography0:21:050:21:08in the night sky. So, take a look at our website and we'll show you0:21:080:21:12how to find some of them. And if you do get any photos,0:21:120:21:16don't forget to add them to our Flickr page,0:21:160:21:18because we'd love to see them.0:21:180:21:19As we explore the invisible universe,0:21:260:21:29we finally come to our most problematic mystery.0:21:290:21:32It seems there's something else we need to explain,0:21:330:21:36something we know very little about,0:21:360:21:39but which might control the entire fate of the universe.0:21:390:21:43Jim Al-Khalili explains.0:21:430:21:44Take a moment to consider what astronomy and physics have achieved.0:21:540:21:58Sitting on our small rock, in an unremarkable part of an apparently0:21:580:22:03unimportant galaxy,0:22:030:22:04we've looked out and seen back to the very beginning of time.0:22:040:22:09We've peered into the furthest corners of the universe.0:22:110:22:15And we've uncovered the fundamental laws that govern the behaviour0:22:150:22:20of energy and matter.0:22:200:22:21And yet there is a problem.0:22:230:22:26A big puzzle that remains unsolved.0:22:260:22:29Let me first explain why this puzzle even exists.0:22:320:22:35See what happens when I throw a stone into the water.0:22:350:22:37The ripples spread outwards at a constant speed.0:22:420:22:46Now, consider matter moving outwards from the Big Bang.0:22:460:22:49Imagine - hypothetically, of course -0:22:490:22:52that we could switch off the force of gravity.0:22:520:22:54Then, with nothing out there to slow them down,0:22:560:22:59all the galaxies should move away from each other0:22:590:23:02at a constant speed, just like the ripples.0:23:020:23:05But, in reality, gravity from their combined mass0:23:070:23:11slows down the expansion.0:23:110:23:13We now know how much normal matter there is in the universe,0:23:160:23:19and we have a good idea how much dark matter there is out there, too,0:23:190:23:23so we should know how the gravity of all the stuff influences0:23:230:23:27the way the universe is expanding.0:23:270:23:29And, at this point in its evolution,0:23:290:23:32our calculations suggest that this expansion should be slowing down.0:23:320:23:36But very slowly.0:23:360:23:38However, astronomers have discovered that this isn't the case.0:23:380:23:42To get a sense of how we know this,0:23:450:23:48and why it's a problem,0:23:480:23:50I've come to the Rawlings Array at the Chilbolton Observatory.0:23:500:23:54In August last year, this was one of the many radio telescopes0:23:550:23:59around the world that observed the biggest astronomical event of 2017.0:23:590:24:04Deep in space, two neutron stars collided,0:24:060:24:11causing a stellar explosion of incredible violence.0:24:110:24:14This so-called kilonova0:24:160:24:18unleashed a massive burst of gamma rays,0:24:180:24:22and a powerful gravitational wave,0:24:220:24:24both of which were measured here on Earth.0:24:240:24:26Scientists have now used this event0:24:290:24:31to measure the expansion of the universe.0:24:310:24:34This is how.0:24:360:24:37The gravitational wave told them how much energy0:24:370:24:41the kilonova produced.0:24:410:24:42And how far away it was.0:24:460:24:48They also worked out from the gamma rays0:24:490:24:52how fast the galaxy was moving,0:24:520:24:55by measuring their red shift.0:24:550:24:57So, we now have a measure of how far away the kilonova was when0:24:590:25:02it exploded, and how fast it was moving.0:25:020:25:06So, here's the 64 million question.0:25:060:25:08Would this galaxy move at the speed they just measured0:25:110:25:15if gravity were the only force acting on it?0:25:150:25:19And here are the results.0:25:190:25:20Obviously, I'm ignoring lots of subtleties and complexities,0:25:200:25:24but the speed, if gravity were the only influence, would be this.0:25:240:25:281,600 kilometres per second.0:25:280:25:31But the speed, according to the neutron star collision measurement...0:25:310:25:35..is this. 3,000 kilometres per second - almost double.0:25:380:25:42Now, these two numbers are different, very different.0:25:420:25:45It's more evidence for a startling conclusion.0:25:450:25:48The universe's expansion can't be slowing down.0:25:500:25:53In fact, it's speeding up.0:25:530:25:55So, if the universe is expanding0:25:570:26:00faster and faster,0:26:000:26:02what's making it accelerate?0:26:020:26:04Well, the truth is, we don't know.0:26:080:26:10But at least we've given it a name - dark energy,0:26:100:26:13a weird new force that pushes the universe faster and faster.0:26:130:26:18This is the ultimate invisible something in the universe,0:26:180:26:21because there has to be a hell of a lot of it out there, somewhere.0:26:210:26:25And all this matters because dark energy0:26:300:26:32could be the key to explaining how the universe will end.0:26:320:26:36Without dark energy, gravity is the most significant0:26:360:26:39force dictating the fate of the universe.0:26:390:26:42If gravity is the dominant force,0:26:430:26:46then it means that, one day,0:26:460:26:48the universe might stop expanding and start contracting.0:26:480:26:52Eventually, it'll collapse together,0:26:520:26:55in what's known as the big crunch.0:26:550:26:58But if dark energy turns out to dominate,0:27:000:27:03then the end of the universe could be much lonelier.0:27:030:27:06As the universe spreads out,0:27:070:27:09the influence of gravity becomes weaker,0:27:090:27:11until everything's too far apart for it to have any effect.0:27:110:27:15Then dark energy will be the only player in town.0:27:150:27:18As dark energy keeps pushing the universe apart,0:27:240:27:28eventually, all galaxies will move so far away0:27:280:27:31they'll become invisible to each other.0:27:310:27:34The distances between them will become so great that light0:27:340:27:39from one would never reach the others.0:27:390:27:42And the universe would disappear into darkness for ever.0:27:420:27:46A sobering thought.0:27:490:27:51So, there's a lot to play for over the next few years.0:27:510:27:54But don't panic, none of this is due for another 20 billion years0:27:540:27:58or more. And who knows, it may all turn out to be wrong, anyway.0:27:580:28:02That's it for this month. But do join us for the next programme.0:28:080:28:12Meanwhile, don't forget to check out the website with Pete's star guide,0:28:120:28:15our special weather forecast and all the extra material0:28:150:28:18that we just couldn't fit into the programme.0:28:180:28:21In the meantime, The Sky at Night reports on one of the most unnerving discoveries in space science - that most of the universe is missing.We live in a material world, so instinctively we know what normal matter is - the world around us, the planets, stars and interstellar dust. But scientists currently estimate that 95 per cent of everything in the universe is actually - one way or another - invisible. Some of this is ordinary matter that we just can't easily see. But there's also stuff that's much more weird. For instance, there's a new kind of matter we think is out there, but whose very existence is still largely hypothetical - dark matter.And most mysteriously of all, scientists think there is an unknown form of energy pervading the universe that we know so little about, all it has so far is a name - dark energy.The Sky at Night takes you on a tour of this invisible universe, and shows how its existence - or lack of it - will define the fate of the entire universe.Download SubtitlesSRTDownload the-invisible-universe.srt ⇓ASSDownload the-invisible-universe.ass ⇓Browse Site ContentFormatsAnimationAppealsBulletinsDiscussion & TalkDocudramasDocumentariesFilmsGames & QuizzesMagazines & ReviewsMakeoversPerformances & EventsPhone-insReadingsRealityTalent ShowsGenresChildren'sDramaLearningFactualMusicNewsReligion & EthicsSportWeatherComedyEntertainmentQuick LinksHomepageAboutContact UsTerms of ServicePrivacy PolicyRemoval / DMCA RequestsSearch