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