1
00:00:16,260 --> 00:00:22,060
As the sun dips below the horizon,
its light begins to fade.

2
00:00:22,060 --> 00:00:26,580
Night falls and our world
descends into darkness.

3
00:00:42,380 --> 00:00:45,740
Today, in our street-lit towns
and cities,

4
00:00:45,740 --> 00:00:48,980
we rarely experience true darkness.

5
00:00:48,980 --> 00:00:51,660
But without our eyes to guide us,

6
00:00:51,660 --> 00:00:55,180
the world becomes
a much more mysterious place.

7
00:01:11,700 --> 00:01:13,580
I can't see anything now,

8
00:01:13,580 --> 00:01:17,020
but strangely I can still sense
the presence of the trees

9
00:01:17,020 --> 00:01:18,580
enveloping me in the gloom.

10
00:01:20,340 --> 00:01:23,660
I can't see them, but I know
there's something out there.

11
00:01:31,020 --> 00:01:35,260
And in the same way,
as we've explored the cosmos,

12
00:01:35,260 --> 00:01:37,580
we've come to realise
we can only see

13
00:01:37,580 --> 00:01:39,940
the merest hint of what's out there.

14
00:01:42,620 --> 00:01:47,420
Our best estimate is that more
than 99% of the universe

15
00:01:47,420 --> 00:01:52,100
lies hidden in the dark,
invisible to our telescopes

16
00:01:52,100 --> 00:01:54,140
and beyond our comprehension.

17
00:01:56,660 --> 00:02:00,260
This film is the story
of how we went from thinking

18
00:02:00,260 --> 00:02:03,820
we were close to a complete
understanding of the universe,

19
00:02:03,820 --> 00:02:08,260
to realising we'd seen
almost none of it,

20
00:02:08,260 --> 00:02:12,300
and the extraordinary quest to
uncover what's really out there

21
00:02:12,300 --> 00:02:13,660
in the dark.

22
00:02:17,300 --> 00:02:21,220
It's perhaps the most important
undertaking in science,

23
00:02:21,220 --> 00:02:25,260
because our universe
was forged in darkness.

24
00:02:25,260 --> 00:02:28,860
And darkness will one day
tear it apart.

25
00:02:48,820 --> 00:02:53,220
For centuries, scientists have used
light to build up a seemingly

26
00:02:53,220 --> 00:02:55,860
comprehensive picture
of the universe.

27
00:02:57,620 --> 00:03:00,380
We'd discovered that the Earth
was just one planet

28
00:03:00,380 --> 00:03:01,860
in orbit around the sun.

29
00:03:03,260 --> 00:03:05,860
And that the sun was itself a star,

30
00:03:05,860 --> 00:03:08,900
made of the same stuff
as the billions upon billions

31
00:03:08,900 --> 00:03:13,460
of stars that light up
a vast - perhaps endless - cosmos.

32
00:03:25,460 --> 00:03:28,980
But there was one niggling problem
that had remained unsolved

33
00:03:28,980 --> 00:03:32,140
for over 400 years,
and it was this -

34
00:03:32,140 --> 00:03:35,820
with so many stars out there,
why was there any darkness at all?

35
00:03:38,180 --> 00:03:41,780
The story of the dark begins
with this simple question.

36
00:03:43,420 --> 00:03:46,940
And at its heart
lies a deep paradox.

37
00:03:49,060 --> 00:03:52,700
In the forest, no matter what
direction I point my torch,

38
00:03:52,700 --> 00:03:55,700
the beam will always hit
the trunk of a tree.

39
00:03:57,220 --> 00:04:00,260
And just as everywhere I look I see
a tree,

40
00:04:00,260 --> 00:04:02,820
if the universe
is sufficiently large,

41
00:04:02,820 --> 00:04:07,780
then every line of sight from
Earth should end in a star.

42
00:04:07,780 --> 00:04:10,340
The night sky shouldn't be
black at all,

43
00:04:10,340 --> 00:04:12,420
it should be ablaze with starlight.

44
00:04:15,780 --> 00:04:18,620
First posed in the 1570s,

45
00:04:18,620 --> 00:04:22,700
this question would become
known as Olbers' Paradox.

46
00:04:24,340 --> 00:04:29,220
One possible solution was that
the Earth was surrounded

47
00:04:29,220 --> 00:04:34,220
by dark stuff that obscured our view
of the stars behind.

48
00:04:34,220 --> 00:04:38,580
But it was soon realised that
these dark clouds would absorb

49
00:04:38,580 --> 00:04:42,660
the light from the stars,
heat up and eventually glow

50
00:04:42,660 --> 00:04:45,380
with the same brightness
as the stars they obscured.

51
00:04:52,020 --> 00:04:56,420
The paradox was only satisfactorily
explained in the 20th century.

52
00:04:59,420 --> 00:05:02,580
The answer -
the reason it gets dark at night

53
00:05:02,580 --> 00:05:06,020
is because the universe
had a beginning.

54
00:05:06,020 --> 00:05:10,660
It began with the big bang
13.8 billion years ago, and so

55
00:05:10,660 --> 00:05:15,660
we only see those stars whose light
has had time to reach us since then.

56
00:05:15,660 --> 00:05:19,260
The sky is dark because light
from the most distant stars

57
00:05:19,260 --> 00:05:20,980
hasn't got here yet.

58
00:05:29,380 --> 00:05:33,740
No mysterious stuff was needed
to block out the light.

59
00:05:33,740 --> 00:05:37,620
The dark spaces that starlight
had yet to reach were empty,

60
00:05:37,620 --> 00:05:40,980
and cosmologists could sleep easy
at night.

61
00:05:42,940 --> 00:05:46,020
But before long, we began to see
hints that there might be

62
00:05:46,020 --> 00:05:48,620
more out there than meets the eye,

63
00:05:48,620 --> 00:05:51,940
that the shadowy recesses
of empty space

64
00:05:51,940 --> 00:05:54,420
might not be so empty after all.

65
00:05:56,620 --> 00:06:00,220
The first clues had in fact begun
to emerge from the gloom

66
00:06:00,220 --> 00:06:03,580
some 200 years ago,

67
00:06:03,580 --> 00:06:07,260
not in the depths of the universe,
but in our own back yard.

68
00:06:18,420 --> 00:06:22,740
The invention of the telescope
in the 17th century had allowed us

69
00:06:22,740 --> 00:06:26,900
to see the dimmest light from the
deepest reaches of the solar system.

70
00:06:28,340 --> 00:06:33,820
And in 1781, it had revealed
a seventh planet, Uranus,

71
00:06:33,820 --> 00:06:36,540
the first to be found
since ancient times.

72
00:06:38,380 --> 00:06:41,580
But there was something odd
about this new planet.

73
00:06:41,580 --> 00:06:46,140
Astronomers found that as time
passed, Uranus's actual position

74
00:06:46,140 --> 00:06:49,460
was drifting further and further
away from the position

75
00:06:49,460 --> 00:06:52,580
the laws of gravity predicted
it should be at.

76
00:06:52,580 --> 00:06:56,460
One explanation was that the laws
themselves were wrong,

77
00:06:56,460 --> 00:06:58,580
but working
at the Paris Observatory,

78
00:06:58,580 --> 00:07:01,460
one man came up with
a different solution.

79
00:07:01,460 --> 00:07:03,460
There was something else out there,

80
00:07:03,460 --> 00:07:07,460
something we couldn't see that was
interfering with Uranus's orbit.

81
00:07:18,660 --> 00:07:22,940
In 1846, the mathematician
Urbain Le Verrier

82
00:07:22,940 --> 00:07:27,740
was employed at the observatory
to calculate the orbits of comets

83
00:07:27,740 --> 00:07:29,940
as they wandered through
the solar system...

84
00:07:32,740 --> 00:07:35,820
..and predict when
they would light up the night sky.

85
00:07:39,980 --> 00:07:43,380
Le Verrier has been described
as having an almost pathological

86
00:07:43,380 --> 00:07:47,700
need to impose order on everything
and everyone around him,

87
00:07:47,700 --> 00:07:51,100
and to have made no allowances
for human error or frailty.

88
00:07:52,900 --> 00:07:55,460
When asked what he was like,
a colleague remarked,

89
00:07:55,460 --> 00:07:58,100
"I do not know whether
Monsieur Le Verrier is actually

90
00:07:58,100 --> 00:08:00,460
"the most detestable man in France,

91
00:08:00,460 --> 00:08:03,740
"but I am quite certain
that he is the most detested."

92
00:08:05,700 --> 00:08:08,420
But he was undoubtedly
a mathematical genius,

93
00:08:08,420 --> 00:08:11,100
and he was as harsh on himself
as he was on others.

94
00:08:13,420 --> 00:08:17,300
And because he was a mathematician,
he set about finding the object

95
00:08:17,300 --> 00:08:21,780
he thought was influencing Uranus
not by scouring the skies

96
00:08:21,780 --> 00:08:26,500
with a telescope, but by determining
its position through calculation.

97
00:08:35,740 --> 00:08:40,580
These are Le Verrier's original
hand-written notes from 1846.

98
00:08:40,580 --> 00:08:41,740
This one is called

99
00:08:41,740 --> 00:08:45,260
"Searches of the disturbing body.
Second approximation."

100
00:08:49,460 --> 00:08:51,900
It contains page after page

101
00:08:51,900 --> 00:08:54,500
of complicated mathematical
calculations.

102
00:09:05,300 --> 00:09:07,820
What Le Verrier was attempting
was quite different

103
00:09:07,820 --> 00:09:10,020
to what was normally done
in astronomy,

104
00:09:10,020 --> 00:09:12,220
where you know where an object is -

105
00:09:12,220 --> 00:09:17,180
say a star or planet or comet -
you then use the laws of gravity

106
00:09:17,180 --> 00:09:21,420
to explain its effects on
nearby objects.

107
00:09:21,420 --> 00:09:24,700
Here, he didn't know
where his disturbing body was.

108
00:09:24,700 --> 00:09:29,420
All he had to go by was the effect
it had on the orbit of Uranus.

109
00:09:29,420 --> 00:09:33,580
So he made some starting assumptions
about its position, and then

110
00:09:33,580 --> 00:09:38,500
carried out a calculation to predict
the effect it would have on Uranus.

111
00:09:38,500 --> 00:09:41,500
He then compared that with
what had been observed.

112
00:09:41,500 --> 00:09:45,100
When the two didn't match,
he went back and adjusted

113
00:09:45,100 --> 00:09:48,300
his starting assumptions
and repeated the calculation.

114
00:09:50,140 --> 00:09:52,420
He did this again and again

115
00:09:52,420 --> 00:09:55,300
until his prediction matched
the observation.

116
00:10:00,500 --> 00:10:03,940
On the 31st of August, 1846,

117
00:10:03,940 --> 00:10:06,980
after three months of
painstaking work,

118
00:10:06,980 --> 00:10:10,500
Le Verrier presented his results
to the French Academy.

119
00:10:10,500 --> 00:10:13,180
He announced that his calculations
had revealed

120
00:10:13,180 --> 00:10:16,100
what he believed was a new planet,

121
00:10:16,100 --> 00:10:19,700
and, crucially, that he had
the co-ordinates in the night sky

122
00:10:19,700 --> 00:10:22,020
that showed where it could be found.

123
00:10:26,540 --> 00:10:29,740
And yet, despite this,
he was unable to persuade

124
00:10:29,740 --> 00:10:32,820
any French astronomers
to search for his planet.

125
00:10:37,500 --> 00:10:40,580
Eventually, Le Verrier
sent his calculations

126
00:10:40,580 --> 00:10:43,900
to Johann Galle
at the Berlin Observatory.

127
00:10:43,900 --> 00:10:47,180
His letter arrived
on the 23rd of September,

128
00:10:47,180 --> 00:10:50,140
and the new planet was found
the same evening

129
00:10:50,140 --> 00:10:54,180
within one degree of
Le Verrier's predicted location.

130
00:10:54,180 --> 00:10:56,940
His calculations were so precise,

131
00:10:56,940 --> 00:11:00,260
it took Galle less than an hour
to find it.

132
00:11:07,460 --> 00:11:11,140
Le Verrier and Galle had discovered
the planet Neptune.

133
00:11:12,340 --> 00:11:16,860
A vast ice giant,
17 times heavier than the Earth

134
00:11:16,860 --> 00:11:19,420
and nearly 60 times its volume,

135
00:11:19,420 --> 00:11:23,740
lurking in the shadows some
4 billion kilometres from the sun.

136
00:11:30,300 --> 00:11:32,460
Neptune had been hard to find

137
00:11:32,460 --> 00:11:35,660
not because there was anything
inherently mysterious about it.

138
00:11:35,660 --> 00:11:39,500
It's dark simply because
it's so far from the sun,

139
00:11:39,500 --> 00:11:42,300
there's precious little light
to illuminate it.

140
00:11:43,940 --> 00:11:46,140
And outside our solar system,

141
00:11:46,140 --> 00:11:49,780
this lack of illumination
is an even bigger problem.

142
00:11:51,540 --> 00:11:55,500
And it means even more stuff
is hidden in the dark.

143
00:11:57,700 --> 00:12:03,500
Stars are thought to contain just
11% of the atoms in the universe.

144
00:12:03,500 --> 00:12:08,100
The rest - clouds of gas
and dust, planets, dead stars -

145
00:12:08,100 --> 00:12:11,500
we can't see, because they
give off hardly any light.

146
00:12:12,700 --> 00:12:16,780
The dark spaces between
the stars aren't empty at all.

147
00:12:16,780 --> 00:12:19,540
In fact, they contain the vast
majority

148
00:12:19,540 --> 00:12:21,380
of the stuff that's out there.

149
00:12:26,260 --> 00:12:30,340
Up until the middle of the
20th century, most astronomers

150
00:12:30,340 --> 00:12:35,420
believed that, although they
couldn't see nearly 90% of it,

151
00:12:35,420 --> 00:12:38,820
the universe was still,
theoretically at least,

152
00:12:38,820 --> 00:12:40,180
entirely visible.

153
00:12:43,060 --> 00:12:45,060
But that was about to change.

154
00:12:48,540 --> 00:12:50,700
Welcome to White Sands Missile Range.

155
00:12:58,340 --> 00:13:03,140
In 1964, NASA scientists fitted
an Aerobee rocket

156
00:13:03,140 --> 00:13:05,420
with an X-ray detector...

157
00:13:05,420 --> 00:13:07,060
'..two, one...'

158
00:13:09,340 --> 00:13:12,220
..and blasted it to the edge
of space.

159
00:13:14,660 --> 00:13:18,100
High above the X-ray-absorbing
layers of the atmosphere,

160
00:13:18,100 --> 00:13:21,100
the detector spotted something
extremely bright

161
00:13:21,100 --> 00:13:23,540
in the constellation of Cygnus.

162
00:13:28,220 --> 00:13:31,900
The young British astronomer
Paul Murdin was fascinated by this

163
00:13:31,900 --> 00:13:35,460
mysterious X-ray source,
known as Cygnus X-1.

164
00:13:35,460 --> 00:13:37,860
And when he joined
the Royal Greenwich Observatory

165
00:13:37,860 --> 00:13:39,740
in the summer of 1971,

166
00:13:39,740 --> 00:13:42,940
he was given with the perfect
opportunity to discover what it was.

167
00:13:44,980 --> 00:13:47,540
It was known that X-rays
were produced

168
00:13:47,540 --> 00:13:52,420
when gas was heated to temperatures
upwards of a million degrees.

169
00:13:52,420 --> 00:13:54,180
DOORBELL CHIMES

170
00:13:54,180 --> 00:13:55,940
Hello, Paul!

171
00:13:55,940 --> 00:13:59,580
'But no-one knew for sure
what could produce such extreme

172
00:13:59,580 --> 00:14:01,540
'conditions out in space.'

173
00:14:03,140 --> 00:14:06,140
What was it about X-ray sources
that interested you?

174
00:14:06,140 --> 00:14:09,460
Celestial X-ray sources
had just been discovered.

175
00:14:09,460 --> 00:14:12,500
They were places in the sky
where X-rays came from.

176
00:14:12,500 --> 00:14:14,100
It's a very energetic radiation,

177
00:14:14,100 --> 00:14:17,020
it means something really powerful
is happening there.

178
00:14:17,020 --> 00:14:20,540
I mean, the X-rays are a flag
which the star is waving at you,

179
00:14:20,540 --> 00:14:24,180
saying, "Look at me, look at me,
look at me - I'm really interesting."

180
00:14:26,460 --> 00:14:30,060
But when Paul trained his optical
telescope on the source,

181
00:14:30,060 --> 00:14:33,660
all he saw was an ordinary,
everyday star,

182
00:14:33,660 --> 00:14:36,620
nowhere near hot enough
to produce X-rays.

183
00:14:38,420 --> 00:14:41,900
Most stars are in systems
where there's two stars,

184
00:14:41,900 --> 00:14:44,460
three stars, even five stars
or many more.

185
00:14:44,460 --> 00:14:48,620
It's really unusual to have a
star like our sun that's on its own.

186
00:14:48,620 --> 00:14:50,700
I decided therefore that I'd try

187
00:14:50,700 --> 00:14:54,100
and look for evidence on the star
that I could see, that there

188
00:14:54,100 --> 00:14:57,860
was another star nearby and
that they were circling one another.

189
00:14:57,860 --> 00:15:01,700
By recording its motion
night after night, Paul discovered

190
00:15:01,700 --> 00:15:07,780
the star was orbiting an invisible
partner, once every 5.6 days.

191
00:15:07,780 --> 00:15:12,300
What you can calculate, once
you know the period of a binary star,

192
00:15:12,300 --> 00:15:17,820
is the mass of the system and the
mass of the component parts of it.

193
00:15:17,820 --> 00:15:19,820
And so,
that was the thing to do next.

194
00:15:19,820 --> 00:15:22,460
And then, maybe within an hour,

195
00:15:22,460 --> 00:15:27,140
I knew that the star which
I couldn't see

196
00:15:27,140 --> 00:15:30,380
was four solar masses or more.

197
00:15:33,220 --> 00:15:37,020
Something that heavy
so close to the star he could see

198
00:15:37,020 --> 00:15:40,260
would strip material
from its outer layers, the immense

199
00:15:40,260 --> 00:15:46,100
frictional forces heating the gas to
such an extent it produced X-rays.

200
00:15:49,020 --> 00:15:53,500
But physicists only knew of one
object that could be that massive

201
00:15:53,500 --> 00:15:56,660
and yet remain completely invisible.

202
00:15:56,660 --> 00:15:59,900
It was something that had
only ever existed in theory.

203
00:16:02,220 --> 00:16:05,060
Paul Murdin had discovered
the first black hole.

204
00:16:09,540 --> 00:16:12,260
I was just... I was just elated.

205
00:16:12,260 --> 00:16:16,580
And I had to get up from my desk
and walk about a bit to calm down.

206
00:16:16,580 --> 00:16:18,420
My pulse raced.

207
00:16:18,420 --> 00:16:22,500
I knew it was big, but I was also
a little bit frightened of it,

208
00:16:22,500 --> 00:16:24,260
so I knew I had to check it
very carefully

209
00:16:24,260 --> 00:16:29,460
and go through it all again
and check what I was doing.

210
00:16:29,460 --> 00:16:31,220
But it was... It was a great hour

211
00:16:31,220 --> 00:16:34,340
and I couldn't really do any serious
work for the rest of the day.

212
00:16:35,500 --> 00:16:38,580
And I felt... I felt really happy
with myself, actually.

213
00:16:43,100 --> 00:16:44,780
Thanks to Paul Murdin,

214
00:16:44,780 --> 00:16:49,660
the universe now had a new
and profoundly dark inhabitant.

215
00:16:52,660 --> 00:16:55,300
Black holes are so incredibly dense,

216
00:16:55,300 --> 00:16:59,580
their gravity warps the fabric
of space and time around them

217
00:16:59,580 --> 00:17:04,500
to such an extent that nothing,
not even light, can escape.

218
00:17:12,460 --> 00:17:14,420
As you approach a black hole,

219
00:17:14,420 --> 00:17:16,620
an observer watching you
from a distance

220
00:17:16,620 --> 00:17:20,940
will see the light coming from you
getting redder and redder.

221
00:17:20,940 --> 00:17:24,020
And you will appear to be moving
in slow motion

222
00:17:24,020 --> 00:17:27,780
as the immense gravitational field
of the black hole

223
00:17:27,780 --> 00:17:31,180
stretches both space and time.

224
00:17:31,180 --> 00:17:34,940
And then, as you pass through
the event horizon,

225
00:17:34,940 --> 00:17:38,820
the point of no return that marks
the edge of a black hole,

226
00:17:38,820 --> 00:17:43,820
you simply disappear,
lost from the universe for ever.

227
00:17:54,180 --> 00:17:57,580
Black holes are objects
that would remain dark

228
00:17:57,580 --> 00:18:00,380
no matter how much light
you shone on them.

229
00:18:04,540 --> 00:18:06,540
Through their effects
on other things,

230
00:18:06,540 --> 00:18:10,380
we've now discovered dozens
of black holes in our own galaxy,

231
00:18:10,380 --> 00:18:13,420
and estimate there must be billions
upon billions

232
00:18:13,420 --> 00:18:15,580
of them throughout the universe.

233
00:18:15,580 --> 00:18:18,860
Including huge, supermassive
black holes

234
00:18:18,860 --> 00:18:21,580
millions of times
the mass of the sun

235
00:18:21,580 --> 00:18:24,340
at the heart of nearly every galaxy.

236
00:18:33,780 --> 00:18:35,940
As strange as black holes are,

237
00:18:35,940 --> 00:18:39,580
they were at least something
we'd expected to find.

238
00:18:41,540 --> 00:18:44,260
We had theories
that predicted their existence

239
00:18:44,260 --> 00:18:46,300
and described their properties.

240
00:18:49,340 --> 00:18:51,340
But since the 1930s,

241
00:18:51,340 --> 00:18:56,060
astronomers had seen disturbing
hints of something much stranger.

242
00:18:58,220 --> 00:19:03,540
Stuff that was both completely
invisible and completely unexpected.

243
00:19:08,780 --> 00:19:10,300
FAINT WHISPERING

244
00:19:13,540 --> 00:19:17,940
As a child, Vera Rubin
spent hours awake at night

245
00:19:17,940 --> 00:19:20,740
staring out of the window
above her bed,

246
00:19:20,740 --> 00:19:23,980
gazing at the stars as they moved
across the sky.

247
00:19:28,100 --> 00:19:30,980
Then, in her 30s
and a mother herself,

248
00:19:30,980 --> 00:19:33,860
she decided to realise
her childhood dream

249
00:19:33,860 --> 00:19:36,900
and embark on a career
as an astronomer.

250
00:19:36,900 --> 00:19:38,620
FAINT WHISPERING

251
00:19:46,980 --> 00:19:51,380
In the mid 1960s, the hottest topic
in astronomy was quasars.

252
00:19:54,260 --> 00:19:56,940
But the field was extremely crowded

253
00:19:56,940 --> 00:20:00,620
and because the biggest telescopes
that were needed to study them

254
00:20:00,620 --> 00:20:03,140
were often in the remotest parts
of the world,

255
00:20:03,140 --> 00:20:07,740
working on quasars meant
a lot of time spent away from home.

256
00:20:07,740 --> 00:20:10,140
So Vera needed to find
a research topic

257
00:20:10,140 --> 00:20:13,500
that was more compatible
with being a working mum,

258
00:20:13,500 --> 00:20:16,940
and a smaller field where
she could really make her mark.

259
00:20:23,380 --> 00:20:28,420
So she began a project measuring
the way stars move within galaxies

260
00:20:28,420 --> 00:20:30,340
like our own Milky Way.

261
00:20:56,180 --> 00:20:58,500
Whoa!
HE LAUGHS

262
00:21:05,300 --> 00:21:09,620
Everything in a galaxy
is on the move and rotating.

263
00:21:09,620 --> 00:21:12,420
In one minute, the Earth travels

264
00:21:12,420 --> 00:21:15,700
nearly 2,000 kilometres
around the sun.

265
00:21:18,540 --> 00:21:22,420
But in that same time, the sun
and the entire solar system

266
00:21:22,420 --> 00:21:28,140
travel 12,000 kilometres around
the centre of the Milky Way galaxy.

267
00:21:30,980 --> 00:21:33,260
Ah!

268
00:21:33,260 --> 00:21:35,140
I'm not liking this!

269
00:21:41,500 --> 00:21:44,940
If you think this is spinning fast,
think about this.

270
00:21:44,940 --> 00:21:49,340
The Earth is travelling around the
sun at 108,000 kilometres an hour.

271
00:21:51,420 --> 00:21:54,620
Ha! And the sun and
the entire solar system

272
00:21:54,620 --> 00:21:58,460
are travelling
at 720,000 kilometres an hour

273
00:21:58,460 --> 00:22:00,900
around the centre of the galaxy.

274
00:22:00,900 --> 00:22:02,540
HE LAUGHS

275
00:22:06,820 --> 00:22:08,820
Can we stop it now?

276
00:22:19,460 --> 00:22:21,860
That's done me in, that really has.

277
00:22:21,860 --> 00:22:23,540
Thanks very much.

278
00:22:29,460 --> 00:22:32,540
But when Vera Rubin
measured the speed of stars

279
00:22:32,540 --> 00:22:35,060
orbiting the centre
of the Andromeda Galaxy,

280
00:22:35,060 --> 00:22:37,900
she found something deeply puzzling.

281
00:22:42,020 --> 00:22:44,700
If I plot a graph of the speed

282
00:22:44,700 --> 00:22:47,820
at which planets in our solar system
orbit the sun

283
00:22:47,820 --> 00:22:50,780
against their distance from the sun,

284
00:22:50,780 --> 00:22:56,540
I find that the closest planet,
Mercury, orbits the fastest.

285
00:22:56,540 --> 00:23:02,700
It's then followed by Venus,
Earth, Mars and so on.

286
00:23:02,700 --> 00:23:06,900
The further out you go...

287
00:23:06,900 --> 00:23:08,900
the slower the orbit.

288
00:23:08,900 --> 00:23:12,940
In fact, Neptune moves so slowly
relative to the other planets

289
00:23:12,940 --> 00:23:15,780
and has so far to go in orbit
around the sun,

290
00:23:15,780 --> 00:23:18,500
that it's only
completed one full circuit

291
00:23:18,500 --> 00:23:20,980
since it was discovered
167 years ago.

292
00:23:22,540 --> 00:23:27,900
Now, if I plot the same graph again
of speed against distance,

293
00:23:27,900 --> 00:23:30,500
but this time, the speed

294
00:23:30,500 --> 00:23:34,060
at which the stars orbit
the centre of a galaxy

295
00:23:34,060 --> 00:23:35,940
against their distance
from the centre,

296
00:23:35,940 --> 00:23:38,700
I'd expect to see
for the outer stars,

297
00:23:38,700 --> 00:23:42,580
that the speed drops off with
distance, as it did for the planets.

298
00:23:42,580 --> 00:23:46,060
But when Vera Rubin
plotted her data,

299
00:23:46,060 --> 00:23:48,500
she found that the further out
you went,

300
00:23:48,500 --> 00:23:51,660
the speed of the stars
didn't drop off,

301
00:23:51,660 --> 00:23:54,300
it remained roughly the same.

302
00:23:56,260 --> 00:23:59,220
The planets move more slowly
the further out they are

303
00:23:59,220 --> 00:24:01,100
because the further you go,

304
00:24:01,100 --> 00:24:04,380
the weaker the sun's
gravitational field becomes.

305
00:24:04,380 --> 00:24:09,700
So anything moving too fast would
simply fly off into outer space.

306
00:24:09,700 --> 00:24:12,460
But Vera Rubin's result for galaxies

307
00:24:12,460 --> 00:24:15,380
suggested there must be
an extra source of gravity

308
00:24:15,380 --> 00:24:18,900
holding all those fast-moving stars
in their orbits.

309
00:24:22,820 --> 00:24:24,780
This extra gravity was needed

310
00:24:24,780 --> 00:24:28,500
because when astronomers added up
the gravitational pull

311
00:24:28,500 --> 00:24:32,580
of all the dark things they thought
might be lurking in the galaxy,

312
00:24:32,580 --> 00:24:36,380
planets, clouds of dust,
even black holes,

313
00:24:36,380 --> 00:24:39,660
it always came out
about ten times less

314
00:24:39,660 --> 00:24:41,540
than that needed to account for

315
00:24:41,540 --> 00:24:44,260
the stellar speeds
Vera Rubin had measured.

316
00:24:46,140 --> 00:24:48,420
There were two possible
explanations.

317
00:24:48,420 --> 00:24:51,740
Either Einstein's
theory of gravity was wrong,

318
00:24:51,740 --> 00:24:55,860
or galaxies were full of
a completely new kind of stuff.

319
00:24:55,860 --> 00:24:57,740
Something that wasn't made of atoms,

320
00:24:57,740 --> 00:25:01,060
was completely invisible
and very heavy.

321
00:25:01,060 --> 00:25:03,260
A new form of dark matter.

322
00:25:03,260 --> 00:25:06,020
Something astronomers named...
dark matter.

323
00:25:20,620 --> 00:25:23,220
Unsurprisingly, rather than accept

324
00:25:23,220 --> 00:25:27,460
that galaxies were full
of some mysterious unseen stuff,

325
00:25:27,460 --> 00:25:31,260
some physicists once again
thought tweaking the laws of gravity

326
00:25:31,260 --> 00:25:33,100
might be the simplest solution.

327
00:25:36,060 --> 00:25:39,860
That was until astronomers
captured an astonishing image.

328
00:25:40,980 --> 00:25:42,660
For me, this is one of the most

329
00:25:42,660 --> 00:25:45,060
amazing pictures
in modern astronomy.

330
00:25:45,060 --> 00:25:50,340
It's an image of a cluster of
galaxies called the Bullet Cluster.

331
00:25:50,340 --> 00:25:53,500
It gets its name from this
bullet-shaped cloud of gas,

332
00:25:53,500 --> 00:25:58,340
which is actually a shockwave
caused by the collision

333
00:25:58,340 --> 00:26:02,860
not of just clouds of gas
or stars or even whole galaxies,

334
00:26:02,860 --> 00:26:06,220
but clusters of galaxies
coming together

335
00:26:06,220 --> 00:26:10,380
and passing through each other
at 10-million kilometres an hour.

336
00:26:13,860 --> 00:26:18,020
It almost gives me vertigo trying to
imagine the immensity of the scale.

337
00:26:20,500 --> 00:26:22,940
But it's not the magnitude
of the collision

338
00:26:22,940 --> 00:26:25,420
that makes this image so important.

339
00:26:25,420 --> 00:26:28,820
It's what it did to the clusters'
constituent parts.

340
00:26:30,220 --> 00:26:32,500
As the clusters came together,

341
00:26:32,500 --> 00:26:34,500
the stars and planets
in the galaxies

342
00:26:34,500 --> 00:26:36,460
pretty much passed
through each other

343
00:26:36,460 --> 00:26:40,260
because although they're big, the
distances between them are so vast

344
00:26:40,260 --> 00:26:44,740
that the chances of any two stars
colliding is actually very small.

345
00:26:44,740 --> 00:26:47,500
But that doesn't apply to
the dust and gas

346
00:26:47,500 --> 00:26:52,820
that makes up 90% by mass of all
the stuff we can see in a galaxy.

347
00:26:52,820 --> 00:26:56,660
When these collide,
they create a huge, hot cloud -

348
00:26:56,660 --> 00:26:59,500
these two pink regions
in the centre of the image.

349
00:27:01,020 --> 00:27:04,580
But if most of the mass
is trapped here in the clouds,

350
00:27:04,580 --> 00:27:08,460
then you'd expect most of the
gravity to be centred there, too.

351
00:27:08,460 --> 00:27:10,460
But that's not what you see.

352
00:27:10,460 --> 00:27:15,140
These outer blue regions
show where light has been bent round

353
00:27:15,140 --> 00:27:19,220
as gravity warps
the fabric of space itself.

354
00:27:19,220 --> 00:27:22,820
That means most of the gravity
is centred out here,

355
00:27:22,820 --> 00:27:24,980
rather than in the middle.

356
00:27:24,980 --> 00:27:27,660
The simplest way to explain this

357
00:27:27,660 --> 00:27:30,220
is that it wasn't just stars
and planets

358
00:27:30,220 --> 00:27:33,020
that passed
through as the clusters collided,

359
00:27:33,020 --> 00:27:34,780
something else did, too.

360
00:27:34,780 --> 00:27:37,660
Something massive, yet invisible.

361
00:27:37,660 --> 00:27:41,300
This image is the best evidence
we have yet

362
00:27:41,300 --> 00:27:43,100
for the existence of dark matter.

363
00:27:50,860 --> 00:27:54,460
It's now generally accepted
that dark matter is real,

364
00:27:54,460 --> 00:27:58,580
which means there's far more stuff
in the universe than we'd thought.

365
00:28:00,380 --> 00:28:02,380
In fact, there's four times

366
00:28:02,380 --> 00:28:05,580
as much dark matter
as there is normal matter.

367
00:28:08,220 --> 00:28:13,020
And so vast swathes of the universe
are not just unseen,

368
00:28:13,020 --> 00:28:15,780
they're fundamentally unseeable.

369
00:28:17,460 --> 00:28:19,500
The reason dark matter
is so elusive

370
00:28:19,500 --> 00:28:23,980
is because it doesn't reflect light
and it doesn't emit light.

371
00:28:23,980 --> 00:28:26,620
So we can't see it.

372
00:28:26,620 --> 00:28:30,420
And worse than that, what gives
normal matter its solidity

373
00:28:30,420 --> 00:28:32,980
is the electromagnetic force.

374
00:28:32,980 --> 00:28:36,060
And dark matter particles
don't feel that force,

375
00:28:36,060 --> 00:28:38,700
so they just pass
straight through matter.

376
00:28:38,700 --> 00:28:42,980
The only hope we have is if they
hit an atomic nucleus head-on.

377
00:28:42,980 --> 00:28:46,780
And even if they do,
that's really hard to detect.

378
00:28:51,100 --> 00:28:53,620
And so the hunt for dark matter

379
00:28:53,620 --> 00:28:58,900
has turned from the incredibly large
to the unimaginably small.

380
00:29:04,220 --> 00:29:06,780
From scouring the skies
with telescopes

381
00:29:06,780 --> 00:29:09,540
to detectors buried deep
underground.

382
00:29:10,980 --> 00:29:13,220
When it comes to the search
for dark matter,

383
00:29:13,220 --> 00:29:16,780
the place I'm going to is pretty
much the centre of the universe.

384
00:29:23,980 --> 00:29:26,420
The Gran Sasso National Laboratory

385
00:29:26,420 --> 00:29:30,540
lies beneath almost a kilometre
and a half of solid rock.

386
00:29:33,580 --> 00:29:35,820
And can only be reached
through a tunnel

387
00:29:35,820 --> 00:29:38,340
cut deep into the Italian Apennines.

388
00:29:43,620 --> 00:29:46,980
The reason you'd build a laboratory
underneath a mountain

389
00:29:46,980 --> 00:29:51,940
is because our planet is constantly
being bombarded by cosmic rays.

390
00:29:51,940 --> 00:29:54,340
These collide with the upper
atmosphere,

391
00:29:54,340 --> 00:29:56,620
creating a cascade of particles

392
00:29:56,620 --> 00:29:59,580
that shower down onto the surface
of the Earth.

393
00:29:59,580 --> 00:30:05,140
The rock above me effectively forms
a 1400-metre-thick roof

394
00:30:05,140 --> 00:30:08,140
that absorbs most of these
particles,

395
00:30:08,140 --> 00:30:11,580
shielding and protecting
the equipment below.

396
00:30:11,580 --> 00:30:14,340
But crucially
for dark-matter hunters,

397
00:30:14,340 --> 00:30:16,540
it passes straight through
normal matter,

398
00:30:16,540 --> 00:30:17,900
straight through the rock,

399
00:30:17,900 --> 00:30:20,260
and the hope is,
into their detectors.

400
00:30:25,540 --> 00:30:27,780
Oh!

401
00:30:27,780 --> 00:30:30,100
It looks like a Bond villain's
evil lair.

402
00:30:59,540 --> 00:31:03,180
Gran Sasso is the world's largest
underground laboratory.

403
00:31:08,980 --> 00:31:10,780
And for the last ten years,

404
00:31:10,780 --> 00:31:14,740
it's been home to dark matter
scientists like Dr Chamkaur Ghag,

405
00:31:14,740 --> 00:31:17,540
who works on DarkSide-50,

406
00:31:17,540 --> 00:31:20,700
one of five dark matter experiments
based here.

407
00:31:31,300 --> 00:31:33,580
So hairnet. Hairnet.

408
00:31:33,580 --> 00:31:34,140
Or head net, in my case.

409
00:31:34,740 --> 00:31:35,940
Or head net, in my case.

410
00:31:37,980 --> 00:31:42,940
Milligram levels of dust
can destroy the experiment. Right.

411
00:31:59,340 --> 00:32:01,540
That looks very impressive.

412
00:32:01,540 --> 00:32:03,060
Yep. Very sci-fi.

413
00:32:08,100 --> 00:32:10,340
So tell me,
how does the experiment work?

414
00:32:10,340 --> 00:32:13,100
Well, the entire experiment
is configured like a Russian doll,

415
00:32:13,100 --> 00:32:15,740
where the first outer layer
is the mountain itself,

416
00:32:15,740 --> 00:32:19,340
protecting the experiment
from radiation from space.

417
00:32:19,340 --> 00:32:21,940
Then we have this tank
that we're standing in.

418
00:32:21,940 --> 00:32:24,500
And this tank is going to be
flooded full of water.

419
00:32:24,500 --> 00:32:26,540
What, the whole cylinder?

420
00:32:26,540 --> 00:32:29,220
Absolutely. This is all
completely filled to the brim.

421
00:32:29,220 --> 00:32:32,500
About 750 cubic metres of water
will fill this thing

422
00:32:32,500 --> 00:32:36,420
to stop radiation coming from the
laboratory and the rock around us.

423
00:32:36,420 --> 00:32:39,740
That's protecting this huge metal
sphere right here,

424
00:32:39,740 --> 00:32:41,540
which is the final layer
of protection

425
00:32:41,540 --> 00:32:44,740
before we get to DarkSide itself,
which is inside there right now.

426
00:32:44,740 --> 00:32:47,140
That's the detector,
that's the heart of the experiment.

427
00:32:47,140 --> 00:32:49,340
That's the thing that will be
detecting dark matter.

428
00:32:51,580 --> 00:32:54,580
You haven't got a light switch
up there. No.

429
00:32:54,580 --> 00:32:56,500
I'm going to get up there
and have a look.

430
00:33:03,740 --> 00:33:06,180
DarkSide-50 is designed to detect

431
00:33:06,180 --> 00:33:08,820
a new class of fundamental particles

432
00:33:08,820 --> 00:33:12,020
called weakly interacting
massive particles.

433
00:33:13,420 --> 00:33:17,140
Predicted by theory, it's
thought that these WIMPs

434
00:33:17,140 --> 00:33:20,340
might be the stuff
of which dark matter is made.

435
00:33:23,660 --> 00:33:27,100
So that metal sphere in the centre,
that's DarkSide?

436
00:33:27,100 --> 00:33:30,180
That's right. That's a detector
full of 150kg of liquid argon.

437
00:33:30,180 --> 00:33:31,820
Dark matter particles should be

438
00:33:31,820 --> 00:33:34,140
streaming through the detector
all the time,

439
00:33:34,140 --> 00:33:36,300
but most of them just go
straight through

440
00:33:36,300 --> 00:33:38,820
because they're very
weakly interacting particles.

441
00:33:38,820 --> 00:33:42,180
If we're lucky, one will collide
with the nucleus of an argon atom,

442
00:33:42,180 --> 00:33:44,900
producing flashes of light
that the detector will pick up.

443
00:33:50,700 --> 00:33:53,620
DarkSide is yet to begin its search,

444
00:33:53,620 --> 00:33:56,540
but elsewhere in the laboratory's
labyrinth of tunnels,

445
00:33:56,540 --> 00:34:00,100
they're already seeing
tantalising hints.

446
00:34:02,380 --> 00:34:04,980
This is the XENON100 experiment
that's already running

447
00:34:04,980 --> 00:34:06,820
and taking data
and has been for a while.

448
00:34:06,820 --> 00:34:10,420
It's the most sensitive dark matter
detector in the world right now.

449
00:34:10,420 --> 00:34:14,380
And this is a live feed of dark
matter data coming in right now.

450
00:34:14,380 --> 00:34:17,820
So, what exactly... What sort of
signal or shape are you looking for?

451
00:34:17,820 --> 00:34:20,860
Well, what we're looking for
is an initial flash of light

452
00:34:20,860 --> 00:34:22,740
which will be a very sharp
peak like this,

453
00:34:22,740 --> 00:34:26,300
followed by a much larger
peak like that one,

454
00:34:26,300 --> 00:34:29,500
which is light being generated
in a gas layer

455
00:34:29,500 --> 00:34:31,500
on top of the liquid xenon.

456
00:34:31,500 --> 00:34:33,940
Oh. That could be a good one
as well, actually.

457
00:34:33,940 --> 00:34:35,180
There you go.

458
00:34:35,180 --> 00:34:37,460
So any one of those events,
those spikes,

459
00:34:37,460 --> 00:34:39,340
could be a dark matter particle?

460
00:34:39,340 --> 00:34:41,860
That's right.
Any one of these events

461
00:34:41,860 --> 00:34:44,740
could be the signature
of dark matter

462
00:34:44,740 --> 00:34:46,260
interacting in XENON100.

463
00:34:46,260 --> 00:34:49,060
It's just we won't know for sure
until the data's been analysed.

464
00:34:51,060 --> 00:34:53,100
Because it's so sensitive,

465
00:34:53,100 --> 00:34:55,940
the overwhelming majority
of the spikes

466
00:34:55,940 --> 00:34:58,740
are due to radiation
emitted by the metal

467
00:34:58,740 --> 00:35:00,860
that makes up the detector itself.

468
00:35:06,140 --> 00:35:08,700
But the hope is experiments
like this

469
00:35:08,700 --> 00:35:11,980
will definitively detect
dark matter particles

470
00:35:11,980 --> 00:35:13,660
within the next ten years.

471
00:35:21,140 --> 00:35:24,500
Today, we think that dark matter
not only exists,

472
00:35:24,500 --> 00:35:27,780
but that it is a vital part
of our universe,

473
00:35:27,780 --> 00:35:32,180
because without it, the world that
we can see wouldn't exist

474
00:35:32,180 --> 00:35:36,180
and that's because dark matter
not only holds galaxies together,

475
00:35:36,180 --> 00:35:40,540
it's dark matter that brought
the clouds of gas together

476
00:35:40,540 --> 00:35:45,380
to form the galaxies in which stars
could ignite in the first place.

477
00:35:53,900 --> 00:35:56,860
Dark matter has gone from being
a curious quirk

478
00:35:56,860 --> 00:36:00,420
of the way stars move around
the fringes of galaxies

479
00:36:00,420 --> 00:36:03,860
to the reason there are stars
and galaxies at all.

480
00:36:09,900 --> 00:36:13,740
But in the late 1990s,
scientists attempting

481
00:36:13,740 --> 00:36:16,940
to measure exactly how much
dark matter there was

482
00:36:16,940 --> 00:36:19,540
made an astonishing discovery.

483
00:36:19,540 --> 00:36:22,660
There was something
even more mysterious

484
00:36:22,660 --> 00:36:25,060
and even more elusive out there.

485
00:36:26,660 --> 00:36:28,780
And to understand what that is,

486
00:36:28,780 --> 00:36:33,380
you have to go back to the very
beginning of everything.

487
00:36:33,380 --> 00:36:36,180
The universe began
with a gigantic fireball.

488
00:36:43,780 --> 00:36:48,300
13.8 billion years ago,
the universe was born.

489
00:36:49,380 --> 00:36:50,900
In the so-called big bang,

490
00:36:50,900 --> 00:36:53,300
everything was created
simultaneously.

491
00:36:56,580 --> 00:36:58,300
See that great flash of light?

492
00:36:58,300 --> 00:37:02,220
That's all the pieces of the atoms
joining together to make a gas.

493
00:37:02,220 --> 00:37:04,660
And now the gas is getting lumpy.

494
00:37:04,660 --> 00:37:07,060
It's making the giant galaxies
of stars.

495
00:37:10,620 --> 00:37:13,420
The expansion of the universe
that we now see

496
00:37:13,420 --> 00:37:16,940
is just a remnant of the initial
violent explosion.

497
00:37:23,700 --> 00:37:25,980
The big bang means that in the past,

498
00:37:25,980 --> 00:37:28,820
the universe was much smaller
than it is today.

499
00:37:31,500 --> 00:37:33,860
And it's been getting bigger
ever since.

500
00:37:44,940 --> 00:37:46,460
According to the big bang theory,

501
00:37:46,460 --> 00:37:51,700
the universe has been expanding
for the past 13.8 billion years.

502
00:37:51,700 --> 00:37:53,500
And for most of that time,

503
00:37:53,500 --> 00:37:56,460
you'd expect the expansion
to be slowing down

504
00:37:56,460 --> 00:37:58,860
due to the combined
gravitational attraction

505
00:37:58,860 --> 00:38:01,460
of all the mass in the universe

506
00:38:01,460 --> 00:38:04,140
trying to pull it back together
again.

507
00:38:04,140 --> 00:38:05,460
Now, here's the clever bit,

508
00:38:05,460 --> 00:38:08,020
Cosmologists realised
that by measuring

509
00:38:08,020 --> 00:38:10,300
how much the expansion was slowing,

510
00:38:10,300 --> 00:38:14,260
they could calculate
how much stuff was out there.

511
00:38:14,260 --> 00:38:18,020
In a sense, it would allow them
to weigh the entire universe.

512
00:38:20,820 --> 00:38:24,420
But in order to measure
how the universe is expanding,

513
00:38:24,420 --> 00:38:28,460
you need a reliable way
to measure distances in space.

514
00:38:35,660 --> 00:38:40,860
Something of known brightness,
astronomers call a standard candle.

515
00:38:42,820 --> 00:38:46,460
The flame in this lantern
produces a fixed amount of light.

516
00:38:46,460 --> 00:38:51,180
It has a specific brightness that
I can measure here on the ground.

517
00:38:51,180 --> 00:38:54,460
But if I let the lantern go,
it'll drift away

518
00:38:54,460 --> 00:38:57,220
and the light will appear to get
dimmer and dimmer

519
00:38:57,220 --> 00:38:58,620
the further away it gets.

520
00:39:02,020 --> 00:39:04,300
Because I know how bright
it really is,

521
00:39:04,300 --> 00:39:07,540
by comparing that
with how bright it appears,

522
00:39:07,540 --> 00:39:10,300
I can calculate how far away it is.

523
00:39:33,380 --> 00:39:36,220
And because every lantern's
the same,

524
00:39:36,220 --> 00:39:39,060
I can use the brightness
to calculate the distance

525
00:39:39,060 --> 00:39:41,420
to any lantern I see in the sky.

526
00:39:47,220 --> 00:39:50,300
The astronomical equivalent
of a Chinese lantern

527
00:39:50,300 --> 00:39:56,780
is a particular species of exploding
star called a Type 1a supernova.

528
00:40:10,380 --> 00:40:15,740
These stars always explode when
they reach the same critical mass

529
00:40:15,740 --> 00:40:19,100
and so always explode with
the same brightness.

530
00:40:22,260 --> 00:40:24,900
So by measuring how bright
they appear,

531
00:40:24,900 --> 00:40:27,380
we can tell how far
they are from the Earth.

532
00:40:31,900 --> 00:40:35,020
As well as telling us
how far away they are,

533
00:40:35,020 --> 00:40:39,900
the light reaching us from distant
supernovae tells us something else.

534
00:40:39,900 --> 00:40:43,940
As it travels across the cosmos,
light gets stretched

535
00:40:43,940 --> 00:40:47,820
because the space it's travelling
through is expanding.

536
00:40:47,820 --> 00:40:53,380
And as its wavelength increases,
the light gets redder and redder.

537
00:40:53,380 --> 00:40:58,820
And this red shift tells us
how fast the universe was expanding

538
00:40:58,820 --> 00:41:02,780
when the light left its source,
when the star exploded.

539
00:41:07,020 --> 00:41:11,780
But when scientists analysed light
from the more distant supernovae

540
00:41:11,780 --> 00:41:14,340
they found something strange.

541
00:41:14,340 --> 00:41:16,700
It was less stretched than expected.

542
00:41:19,060 --> 00:41:21,220
It meant that, in the past,

543
00:41:21,220 --> 00:41:25,380
the universe was expanding more
slowly than it is today.

544
00:41:25,380 --> 00:41:30,340
In other words, the expansion of the
universe wasn't slowing down at all,

545
00:41:30,340 --> 00:41:31,660
it was speeding up.

546
00:41:36,500 --> 00:41:40,620
The only way the universe's
expansion could be accelerating...

547
00:41:43,380 --> 00:41:47,820
..was if there was a mysterious new
force pushing it apart.

548
00:41:51,500 --> 00:41:55,260
And just as with dark matter,
physicists thought the key

549
00:41:55,260 --> 00:41:57,500
to understanding this new force

550
00:41:57,500 --> 00:42:00,660
might lie at the smallest possible
scales...

551
00:42:02,940 --> 00:42:07,780
..because quantum physics appeared
to provide a ready-made explanation.

552
00:42:10,420 --> 00:42:15,620
According to quantum field theory,
empty space is anything but empty.

553
00:42:15,620 --> 00:42:19,140
Particles are constantly appearing
and disappearing,

554
00:42:19,140 --> 00:42:22,900
created out of energy borrowed
from the vacuum itself.

555
00:42:24,980 --> 00:42:28,660
The hope was that this theoretical
vacuum energy

556
00:42:28,660 --> 00:42:32,540
might be the very thing that was
pushing the universe apart.

557
00:42:33,580 --> 00:42:36,820
And the theory allows me to
calculate the energy density

558
00:42:36,820 --> 00:42:40,460
of the vacuum, that's the amount of
energy you'd expect to find

559
00:42:40,460 --> 00:42:42,380
in a given volume.

560
00:42:42,380 --> 00:42:46,220
And so if I take
the energy of the vacuum

561
00:42:46,220 --> 00:42:50,980
to be a sum over J
of half h-bar omega J,

562
00:42:50,980 --> 00:42:53,180
and if I take the cut-off energy

563
00:42:53,180 --> 00:42:56,140
to be of the order
of 10 tera electronvolts

564
00:42:56,140 --> 00:42:57,980
which is just above
the known physics

565
00:42:57,980 --> 00:43:01,260
at the Large Hadron Collider,
then the formula for the vacuum...

566
00:43:01,260 --> 00:43:05,860
'All they needed to do was check the
energy density the theory predicted

567
00:43:05,860 --> 00:43:10,500
'matched that needed to drive
the universe's acceleration

568
00:43:10,500 --> 00:43:13,580
'and the mysterious
force would be explained.'

569
00:43:13,580 --> 00:43:20,220
HE MUTTERS EQUATIONS

570
00:43:37,020 --> 00:43:41,980
So that would give me a value
for the energy density

571
00:43:41,980 --> 00:43:47,260
of the vacuum of 10 to the 35
kilograms per cubic metre.

572
00:43:49,220 --> 00:43:52,460
The trouble is, the value observed
by astronomers

573
00:43:52,460 --> 00:43:56,860
is 10 to the minus 27 kilograms
per cubic metre.

574
00:43:56,860 --> 00:44:00,020
That means the theoretical number
and the experimental number

575
00:44:00,020 --> 00:44:04,100
are out by a factor of
10 to the power 62.

576
00:44:04,100 --> 00:44:06,940
That's one followed by 62 zeros.

577
00:44:08,260 --> 00:44:10,780
To give you a sense
of the scale of the error,

578
00:44:10,780 --> 00:44:13,980
there've been only
10 to the 17 seconds

579
00:44:13,980 --> 00:44:18,780
since the big bang and the diameter
of the entire visible universe

580
00:44:18,780 --> 00:44:21,300
is 10 to the 27 metres...

581
00:44:23,500 --> 00:44:24,940
So it's a pretty big error.

582
00:44:26,900 --> 00:44:32,100
And that meant that whatever was
actually pushing the universe apart,

583
00:44:32,100 --> 00:44:34,820
it was something completely new.

584
00:44:39,420 --> 00:44:42,420
The truth is, we know very little
about what's causing

585
00:44:42,420 --> 00:44:45,140
the expansion of the universe
to accelerate,

586
00:44:45,140 --> 00:44:48,940
but we do have a name for it -
dark energy.

587
00:44:48,940 --> 00:44:51,500
And we know that for it to have
the effect that it does,

588
00:44:51,500 --> 00:44:53,500
there must be
an awful lot of it about.

589
00:44:56,700 --> 00:45:00,420
Einstein's famous equation E=mc2

590
00:45:00,420 --> 00:45:05,340
says that energy and matter are
different forms of the same thing.

591
00:45:05,340 --> 00:45:08,780
And the equivalent mass of
dark energy dwarfs that

592
00:45:08,780 --> 00:45:10,980
of everything else in the universe.

593
00:45:13,100 --> 00:45:15,340
And it means that, today,

594
00:45:15,340 --> 00:45:19,260
normal matter makes up
just 4% of the cosmos.

595
00:45:19,260 --> 00:45:22,580
23% of it is elusive dark matter.

596
00:45:24,020 --> 00:45:28,060
And a colossal 73% of the universe

597
00:45:28,060 --> 00:45:31,100
consists of mysterious dark energy.

598
00:45:35,660 --> 00:45:38,020
Just think about it for a moment.

599
00:45:38,020 --> 00:45:40,140
100 billion galaxies,

600
00:45:40,140 --> 00:45:43,980
each one containing more than
100 billion stars,

601
00:45:43,980 --> 00:45:47,980
home in turn to billions upon
billions of planets and moons.

602
00:45:49,140 --> 00:45:55,260
All of that is mere flotsam adrift
on a vast and unfathomable ocean.

603
00:45:55,260 --> 00:45:59,580
Dark matter we can't see and dark
energy we can barely comprehend.

604
00:46:04,700 --> 00:46:09,300
And the very nature of dark energy
means the universe is getting

605
00:46:09,300 --> 00:46:12,100
more unknowable all the time.

606
00:46:15,340 --> 00:46:19,100
As space expands and distances
become bigger,

607
00:46:19,100 --> 00:46:23,420
most forces get weaker, because you
have the same amount of mass

608
00:46:23,420 --> 00:46:27,100
or electric charge,
only now everything's further apart.

609
00:46:28,780 --> 00:46:32,300
But dark energy behaves
completely differently.

610
00:46:32,300 --> 00:46:36,820
As the universe has expanded,
the stronger it's become.

611
00:46:36,820 --> 00:46:39,940
The more space there is,
the more dark energy there is

612
00:46:39,940 --> 00:46:43,300
and so the faster
the universe expands,

613
00:46:43,300 --> 00:46:47,260
creating ever more space
and ever more dark energy.

614
00:46:52,580 --> 00:46:55,860
And that has a profound
consequence.

615
00:46:55,860 --> 00:46:59,260
Just as dark matter pulled
the galaxies together

616
00:46:59,260 --> 00:47:01,420
to create the cosmos
as we know it...

617
00:47:02,980 --> 00:47:06,980
..so dark energy will tear
the universe apart.

618
00:47:09,060 --> 00:47:11,460
In the future, as space gets bigger,

619
00:47:11,460 --> 00:47:14,740
dark energy will become
ever more dominant.

620
00:47:14,740 --> 00:47:18,500
And so it will ultimately shape
the universe's destiny.

621
00:47:18,500 --> 00:47:22,060
And if it continues to increase as
it appears to be doing today,

622
00:47:22,060 --> 00:47:25,580
then it will push the galaxies
further and further apart

623
00:47:25,580 --> 00:47:28,780
until, eventually,
they slip out of view,

624
00:47:28,780 --> 00:47:32,780
creating a cosmos that will become
ever more dark

625
00:47:32,780 --> 00:47:34,140
and ever more desolate.

626
00:47:44,460 --> 00:47:48,660
The ultimate goal of modern
cosmology is to understand

627
00:47:48,660 --> 00:47:52,100
dark energy
and the fate of the universe,

628
00:47:52,100 --> 00:47:56,100
and to witness how dark matter
brought everything together

629
00:47:56,100 --> 00:47:57,460
in the first place.

630
00:48:03,020 --> 00:48:07,860
And so to shed light on both the
beginning and end of the universe,

631
00:48:07,860 --> 00:48:12,500
cosmologists have embarked on
a quest of epic proportions -

632
00:48:12,500 --> 00:48:17,580
to map everywhere in space over
the entire lifespan of the cosmos...

633
00:48:19,140 --> 00:48:24,100
..starting with the darkest
period in its past,

634
00:48:24,100 --> 00:48:28,700
an era that began as the afterglow
of the big bang faded away.

635
00:48:30,580 --> 00:48:33,740
We talk about the ages of
the universe in the same way

636
00:48:33,740 --> 00:48:37,660
that we talk about the stages in our
own lives, from its birth,

637
00:48:37,660 --> 00:48:41,940
through childhood, adolescence,
adulthood and even death.

638
00:48:41,940 --> 00:48:44,780
So mapping the universe
is really about

639
00:48:44,780 --> 00:48:47,500
filling in the photo album
of its life.

640
00:48:48,740 --> 00:48:54,340
Here's a picture of me
from 20 years ago with my children.

641
00:48:54,340 --> 00:48:56,940
I know it because I have a lot
more hair there.

642
00:48:56,940 --> 00:49:01,140
And here's a picture of me
in my early 20s on graduation.

643
00:49:01,140 --> 00:49:03,260
And here's one of me as a teenager.

644
00:49:05,180 --> 00:49:08,260
In the same way,
by looking out into space,

645
00:49:08,260 --> 00:49:10,300
we have good images of the universe

646
00:49:10,300 --> 00:49:12,500
all the way back
to its teenage years,

647
00:49:12,500 --> 00:49:16,100
when large galaxies first formed.

648
00:49:16,100 --> 00:49:20,460
But before that,
we have nothing but a single image -

649
00:49:20,460 --> 00:49:24,340
a picture of the universe
when it was just 400,000 years old,

650
00:49:24,340 --> 00:49:28,940
the cosmic microwave background -
the afterglow of the big bang.

651
00:49:28,940 --> 00:49:31,940
It's as though,
in the photo album of my life,

652
00:49:31,940 --> 00:49:35,580
I have nothing before this picture
of me aged 16,

653
00:49:35,580 --> 00:49:38,900
apart from this one of me and my
parents in Iraq

654
00:49:38,900 --> 00:49:40,740
when I was just a few months old.

655
00:49:42,700 --> 00:49:45,580
This gap in the childhood
of the universe,

656
00:49:45,580 --> 00:49:49,460
the period between its earliest
moments, through the birth

657
00:49:49,460 --> 00:49:53,500
of the first stars to the formation
of large galaxies

658
00:49:53,500 --> 00:49:56,820
is a time known
as the dark ages of the universe.

659
00:50:02,660 --> 00:50:08,060
The universe's dark ages
lasted for around a billion years

660
00:50:08,060 --> 00:50:11,460
and they get their name because
there were precious few stars

661
00:50:11,460 --> 00:50:13,020
to illuminate them.

662
00:50:17,900 --> 00:50:22,660
So to fill in those pages in the
cosmic photo album, we'd need

663
00:50:22,660 --> 00:50:26,820
something capable of seeing where
there was next to no light.

664
00:50:33,700 --> 00:50:38,180
During the Second World War, Bernard
Lovell had developed a machine

665
00:50:38,180 --> 00:50:40,460
that could see in the dark.

666
00:50:40,460 --> 00:50:42,420
He'd worked on airborne radar

667
00:50:42,420 --> 00:50:45,180
that mapped bombers' targets
on the ground.

668
00:50:48,580 --> 00:50:50,660
But his real ambition was to build

669
00:50:50,660 --> 00:50:53,300
something capable of
mapping the heavens.

670
00:51:26,140 --> 00:51:29,900
The giant dish at Jodrell Bank
was Bernard Lovell's baby.

671
00:51:29,900 --> 00:51:34,300
It was designed to be the world's
largest fully manoeuvrable

672
00:51:34,300 --> 00:51:37,540
radio telescope,
capable of scouring the entire sky

673
00:51:37,540 --> 00:51:40,900
and picking up the
longest-wavelength radio signals

674
00:51:40,900 --> 00:51:43,820
coming from the deepest
recesses of space.

675
00:52:06,500 --> 00:52:08,980
The Lovell Telescope
has a collecting area

676
00:52:08,980 --> 00:52:12,980
of 4,560 square metres,

677
00:52:12,980 --> 00:52:18,460
made up of more than
2,400 galvanised steel plates.

678
00:52:23,660 --> 00:52:27,460
In the original designs,
this bowl of the telescope

679
00:52:27,460 --> 00:52:30,100
wasn't meant to be
solid like this.

680
00:52:30,100 --> 00:52:33,740
The plan was for it to be built of
much lighter wire mesh.

681
00:52:37,180 --> 00:52:39,180
The dish was redesigned

682
00:52:39,180 --> 00:52:43,780
because astronomers had discovered
a new way of seeing in the dark,

683
00:52:43,780 --> 00:52:46,660
something that might ultimately
allow them

684
00:52:46,660 --> 00:52:49,380
to map the universe's dark ages.

685
00:52:51,820 --> 00:52:55,180
Hydrogen permeates every galaxy.

686
00:52:55,180 --> 00:52:56,660
It was produced in the big bang

687
00:52:56,660 --> 00:53:00,460
and is the basic constituent of all
normal matter, including us.

688
00:53:00,460 --> 00:53:02,780
And like most normal matter,

689
00:53:02,780 --> 00:53:05,860
it wasn't thought
to give off any light.

690
00:53:05,860 --> 00:53:09,220
But then astronomers discovered
something remarkable.

691
00:53:09,220 --> 00:53:11,260
As it floats around in space,

692
00:53:11,260 --> 00:53:15,780
neutral hydrogen gas is constantly
producing radio waves

693
00:53:15,780 --> 00:53:22,220
and, crucially, those waves are
always the same wavelength - 21cm.

694
00:53:22,220 --> 00:53:25,860
And this meant that hydrogen
could be used to map

695
00:53:25,860 --> 00:53:27,300
the galaxies that it fills.

696
00:53:30,220 --> 00:53:35,460
By detecting the 21cm signal,
the Lovell Telescope helped reveal

697
00:53:35,460 --> 00:53:39,220
the spiral structure
of the Milky Way

698
00:53:39,220 --> 00:53:42,860
and produced detailed maps
of distant galaxies.

699
00:53:47,420 --> 00:53:51,620
But galaxies aren't the only place
in the cosmos you find hydrogen gas.

700
00:53:51,620 --> 00:53:53,500
During the dark ages
of the universe,

701
00:53:53,500 --> 00:53:57,460
there were no galaxies,
but there was plenty of hydrogen.

702
00:53:57,460 --> 00:54:02,460
So by detecting the 21cm signal
from these primordial gas clouds,

703
00:54:02,460 --> 00:54:05,060
you could see the universe
in its infancy

704
00:54:05,060 --> 00:54:07,820
and peer into the dark ages
themselves.

705
00:54:14,540 --> 00:54:18,340
And by doing so, we'll be able
to watch dark matter

706
00:54:18,340 --> 00:54:20,220
pull the cosmos together...

707
00:54:22,180 --> 00:54:23,980
..and light up the heavens.

708
00:54:27,540 --> 00:54:30,740
It was during the dark ages
that the hydrogen gas created

709
00:54:30,740 --> 00:54:35,780
in the big bang was compressed into
stars and moulded into galaxies.

710
00:54:35,780 --> 00:54:40,540
It was in this era that
the cosmos as we know it was born,

711
00:54:40,540 --> 00:54:44,100
sculpted by the gravitational
pull of dark matter.

712
00:54:50,580 --> 00:54:54,380
But the machine scientists
are building to map the dark ages

713
00:54:54,380 --> 00:54:56,500
will see far more.

714
00:54:58,020 --> 00:55:02,620
With an effective collecting area
of more than 200 times that

715
00:55:02,620 --> 00:55:06,540
of the Lovell Telescope,
the square kilometre array

716
00:55:06,540 --> 00:55:10,140
will be capable of mapping
a billion galaxies,

717
00:55:10,140 --> 00:55:14,420
tracking the expansion and evolution
of the entire universe

718
00:55:14,420 --> 00:55:16,540
more accurately than ever before.

719
00:55:22,340 --> 00:55:24,740
And the hope is, that by doing so,

720
00:55:24,740 --> 00:55:28,380
it will provide clues
to the nature of dark energy

721
00:55:28,380 --> 00:55:30,940
and the universe's
ultimate fate.

722
00:55:50,780 --> 00:55:55,860
Using hydrogen to map the cosmos
might just represent the final

723
00:55:55,860 --> 00:56:01,460
chapter of humankind's exploration
of the universe using light,

724
00:56:01,460 --> 00:56:06,660
a journey that began in earnest
some 400 years ago.

725
00:56:06,660 --> 00:56:11,620
In December 1609, Galileo Galilei
began making observations

726
00:56:11,620 --> 00:56:13,660
of the night sky.

727
00:56:13,660 --> 00:56:16,740
Before then, what was thought to be
out there was essentially

728
00:56:16,740 --> 00:56:18,580
a matter of faith.

729
00:56:18,580 --> 00:56:22,340
The universe at large
lay unseen and unseeable.

730
00:56:22,340 --> 00:56:24,140
But now, for the first time,

731
00:56:24,140 --> 00:56:27,260
the nature of the heavens
was something knowable -

732
00:56:27,260 --> 00:56:30,460
you simply had to look up
and see it.

733
00:56:30,460 --> 00:56:33,620
The light captured in Galileo's
simple telescope

734
00:56:33,620 --> 00:56:36,940
began a chain of discoveries
that would reveal

735
00:56:36,940 --> 00:56:39,100
the true nature of the cosmos.

736
00:56:47,620 --> 00:56:48,900
We've seen galaxies

737
00:56:48,900 --> 00:56:51,820
billions of light years'
distance from the Earth.

738
00:56:54,380 --> 00:56:57,740
And as we've come to understand
light's properties,

739
00:56:57,740 --> 00:57:00,820
we've discovered the stuff
of which stars are made...

740
00:57:04,700 --> 00:57:07,980
..and glimpsed the beginning
of the universe itself.

741
00:57:15,180 --> 00:57:21,020
But the realisation that most
normal matter can't be seen

742
00:57:21,020 --> 00:57:26,020
and the discovery of dark matter
and dark energy

743
00:57:26,020 --> 00:57:31,740
mean that more than 99% of the
universe lies hidden in the shadows.

744
00:57:34,780 --> 00:57:39,740
And as dark energy pushes
the galaxies ever further apart,

745
00:57:39,740 --> 00:57:43,020
what few lights there are
will begin to go out.

746
00:57:44,780 --> 00:57:47,660
As the universe expands ever faster,

747
00:57:47,660 --> 00:57:52,140
one by one the galaxies
will disappear from view.

748
00:57:52,140 --> 00:57:56,540
All that will remain visible will be
the stars in our own galaxy.

749
00:57:56,540 --> 00:57:59,740
It would be almost as if we'd never
invented the telescope at all.

750
00:58:01,220 --> 00:58:04,020
For the vast majority
of the universe's life,

751
00:58:04,020 --> 00:58:07,700
there'll be no way of discovering
all the things we have about it.

752
00:58:09,540 --> 00:58:13,540
So I don't feel disheartened
that so much of the cosmos

753
00:58:13,540 --> 00:58:15,780
is hidden in the shadows.

754
00:58:15,780 --> 00:58:16,940
The real miracle is

755
00:58:16,940 --> 00:58:20,180
that when we first looked out into
the depths of space

756
00:58:20,180 --> 00:58:22,820
there was any light to see at all.

757
00:58:33,220 --> 00:58:35,820
Whether you want to
step into the light

758
00:58:35,820 --> 00:58:38,340
or explore
the mysteries of the dark,

759
00:58:38,340 --> 00:58:40,900
let the Open University
inspire you.

760
00:58:40,900 --> 00:58:42,180
Go to...

761
00:58:44,780 --> 00:58:47,580
..and follow links to
The Open University.

762
00:58:47,580 --> 00:58:50,580
Subtitles by Red Bee Media Ltd