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Time English Language 2 Action

00:04

You may have heard the phrase

00:05

"in space, no one can hear you scream."

00:09

(Laughter)

00:10

But that's true only for human ears.

00:13

The universe is filled with tenuous atmospheres of one kind or another,

00:18

and through those atmospheres, pressure waves --

00:20

sound -- moves.

00:22

Through stars, across galaxies, even between galaxies.

00:27

One of the most wonderful

00:29

and recently discovered examples of cosmic sound

00:32

is in the very young universe, shortly after the Big Bang.

00:36

Now you may be wondering, how would we know that?

00:41

Well famously, as you look very far out into space,

00:44

you also look back in time,

00:46

simply because it takes time for the light to get to you.

00:50

Remarkably, if you look far enough past all the galaxies,

00:55

you can see to a time before any galaxies had yet formed,

00:59

even to a time when the universe itself had only just been born.

01:05

So what do you see coming from this newborn universe?

01:09

You see the light of the Big Bang's hot, glowing youth.

01:14

At that time, the universe was filled with an almost uniform,

01:19

hot, glowing gas

01:20

of atomic nuclei and electrons and intense light.

01:25

That light, on its way to us, has crossed an expanding universe.

01:31

That expansion not only carried those regions far away from us,

01:35

but also as the light waves crossed an expanding space,

01:40

they were stretched from micron-sized waves

01:44

to millimeter-sized waves.

01:47

So although it is light that leaves the young universe,

01:51

it’s microwaves that arrive.

01:54

And they are the famous cosmic microwave background.

01:58

Now because every direction in which you look

02:01

ends back in the young universe,

02:04

then the microwave radiation comes to us from all directions,

02:08

and for many years,

02:10

its brightness was thought to be extremely uniform,

02:13

around the full spherical dome of the sky,

02:16

[a] bit like this image here.

02:18

But as microwave telescopes became more sensitive,

02:22

they began to see slight variations in brightness from place to place,

02:26

so more recent images of the microwave sky

02:30

appear completely covered in very slight patches.

02:34

You can see them more clearly

02:36

in this blowup of a region that's about the size of your fist,

02:39

held at arm's length.

02:41

It spans eight million light-years and shows us the newborn universe

02:46

when it was only 400,000 years old.

02:49

Now that's equivalent to a one-day-old human.

02:54

So what are these patches?

02:56

Why is it slightly brighter and darker in different places?

03:00

Well, these are places where the temperature

03:02

and pressure in the glowing gas are slightly higher and slightly lower,

03:07

or stated differently,

03:08

the bright and dark patches are the peaks and troughs

03:11

of huge sound waves

03:13

moving through the primordial atmosphere.

03:16

The brightness contrast reveals 90 decibels --

03:19

that's rock-concert loudness,

03:22

and their gigantic size reveals a frequency or pitch,

03:26

a whopping 50 octaves below the human range.

03:31

One wave might pass you by in 50,000 years.

03:36

So what causes this sound?

03:39

Now don’t be fooled.

03:41

It's not the bang of the Big Bang.

03:44

No, it's a slowly growing sound driven by gravity.

03:48

The distribution of matter at that time was slightly uneven,

03:52

and where there was a denser region,

03:54

its stronger gravity pulled in the surrounding gas,

03:57

which compressed and bounced back out again,

03:59

only to fall back in again,

04:01

creating an approximately spherical sound wave.

04:05

Now the landscape included small and medium and large regions,

04:09

and so, a bit like a set of organ pipes of different sizes.

04:13

Together, they create a wide range of pitch.

04:17

So what did the primordial sound sound like?

04:21

Well to find out, you must first measure the sounds spectrum.

04:25

Now a computer can do that.

04:27

And here is the remarkable result.

04:29

There is a fundamental tone and several higher harmonics.

04:34

And just for comparison,

04:35

here is the sound spectrum of a flute playing a single note

04:39

with its fundamental and harmonics.

04:43

Now although these two sets of harmonics arise for somewhat different reasons,

04:48

nevertheless, it does seem that the young universe had qualities

04:52

similar to a musical instrument or even a human voice singing.

04:57

But notice how those cosmic harmonics are a little bit broad.

05:01

So please don't expect the primordial sound to be too clean

05:06

or musical to your human ears.

05:09

Now before I play you the sound,

05:11

take a look at that green line that goes right through all the data.

05:15

It comes from a detailed computer calculation

05:17

that aims to include all the relevant physics.

05:20

The fit is amazingly good

05:23

and shows that we really do understand what’s happening in the young universe.

05:29

In fact, the task of matching those computer calculations to the data

05:33

basically measures quite accurately many of the fundamental properties

05:38

of our universe.

05:40

OK, let's now listen to the sound after up-shifting by 50 octaves

05:45

so that we can hear it.

05:47

Now I’ve also used those computer calculations

05:50

to track the sound forward in time,

05:52

starting at the Big Bang and spanning the full acoustic era,

05:56

which ends at 400,000 years.

05:59

The sound is accompanied by a graphic that's a bit busy,

06:04

but just focus on the green line,

06:07

which tracks the changing harmonics

06:09

as the young universe begins to expand and age.

06:13

Also, don't worry,

06:15

I’ve compressed those 400,000 years into just 10 seconds.

06:20

(Laughter)

06:22

Here we go.

06:34

OK, thank you.

06:36

(Applause)

06:41

If you weren't quite awake before that, you are now.

06:46

Well as I warned you,

06:48

those broad harmonics make a sound that's more like noise

06:51

than music to our ears.

06:53

And I'll return to that in just a minute.

06:55

You obviously noticed the drop in pitch,

06:57

and that's because as time passes,

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