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Big Bang Acoustics :
The Sound of Creation
Mark Whittle
University of Virginia
Setting the Stage
Studies of the Cosmic Microwave Background (CMB)
have revealed SOUND WAVES in the early Universe.
It is possible to make this sound audible to humans.
These sounds offer a wonderful opportunity for teaching:
• Cosmology: structure formation
• Acoustics: science of sound
• Music: musical instruments; harmonics; scales
Much of astronomy is visual.
This gives us auditory access to Nature…. and Creation.
1. The Microwave Background
Look far away and we see back in time.
Look far enough, and you can see the “Big Flash”
red-shift
Our view of the Universe
includes space and time
The Early Universe was
filled with hot glowing gas
We cannot see the Big Bang itself
It is hidden behind 400,000 light years
of dense fog
= 4ft in 26 mile marathon
= 12 hours in human lifetime
ionized
foggy
atomic
transparent
hot glowing fog
we see a glowing
wall of bright fog
Bell Labs
(1963)
Observing the
Microwave Background
(highlights, there are many others)
COBE satellite
(1992)
WMAP satellite
(2003)
The
Celestial
Sphere
Optical
Microwave
Sky
Sky
Stretched
2. Raw Sound from the CMB
Measure the pressure waves in the CMB:
• The amplitudes (Δ P/P) ~ 10-4 ≡ 110 dB !
• The pitch is 50 octaves below A440 !
• The sound spectrum has harmonics !
Loudness
sound
The CMB Sound Spectrum
Lower
Pitch
Frequency
Higher
Wavelength Pitch
Why is primordial sound so
Because the Universe is so:
BIG
deeper
An Ultra-Bass
Piano
7 octaves
cosmic
concerto
7 octaves
7 octaves
7 octaves
7 octaves
7 octaves
Transpose up by ~50 octaves = 7 pianos (~7 octaves each)
7 octaves
human
concerto
How does sound get to us ?
Consider listening to a live concert on the radio:
Bow+string
radio waves
sound microphone
amplifier
antenna
Concert hall
gravity + sound
hills/valleys
Big Bang
sound
waves
glow
few 100 miles
few µsec delay
light
microwaves
very long way !
14 Gyr delay !
aerial sound
amplifier
speakers
your
ears
Listener
telescope sound
computer
speakers
Listener
your
ears
3. Sound as Diagnostic
Quality of sound reveals the nature of an object
True also for the Universe:
• The sound spectrum reveals many properties
• Use computer simulations to match data
• Two examples: baryon fraction; total density
Fraction of Baryons (atoms)
reality
Fraction of Baryons (atoms)
reality
Total Cosmic Density
reality
The Concordance Model
•Age of Universe
13.7 Gyr
•Flatness
1.02
•Atoms
4.4%
•Dark matter
23%
•Dark energy
73%
•Hubble constant (km/s/Mpc)
71
•Photon/proton ratio
1.6x109
•Time of first stars
180 Myr
•Time of CMB
380,000yr
(2%)
(2%)
(9%)
(15%)
(5%)
(6%)
(5%)
(50%)
(2%)
4. Removing Distortion: C(ℓ)  P(k)
The Universe is a poor concert hall !
Distortions are present in the sound spectrum.
One can “remove” distortions by using robust
computer simulations (eg CMBFAST)
5. Into the Fog: Evolving Sound
The CMB gives only a snapshot of the sound
Use CMBFAST to reconstruct sound from earlier.
Evolution of Sound Spectra : P(k,t)
The First 400,000 years
Movie 1
1.5 Mb
5. Post Recombination:
From Sound to Stars
After recombination, pressure drops to ~zero
Cavity oscillations in time cease: true sound dies
However, the roughness grows quickly:
• the baryons fall into dark matter “valleys”
• the dark matter itself gets more clumpy
• after 100 Myr the peaks collapse  stars !
Continue to create sound from P(k)
From Sound to Stars
4. First stars form
star
no-star
100 million years
star
no-star
3. Collapse begins
2. Contrast grows
1. Sound wave
peak
peak
trough
trough
The Growing Hiss
1
3
5
7
tapestry
clusters
galaxies
stars
Contrast reaches this level
after ~100 Myr, when
rapid collapse takes over.
Stars/Galaxies/Clusters
form from:
small  big wavelengths.
Large scale tapestry
forms from:
1st & 2nd harmonics.
First 100 Million yrs
Movie time flow exponential : 2s for each 10x increase in real (cosmic) time:
0-2s = 102-103 yrs; 2-4s = 103-104 yrs etc; waveform and cosmic color also shown
Movie 5: volume increase included, but artificially reduced so we can hear it all.
Movie 5
1.5 Mb
Linear frequency x-axis
Log (decibel) y-axis
Varying (fake) volume
Log frequency x-axis
Log (decibel) y-axis
Constant volume
Movie 6
1.5 Mb
7. The Harmonics
These are remarkable features of the sound.
Harmonics usually arise in objects of finite size
The Universe is bounded in time
The harmonics are not as sharp as musical tones
Why does it sound so “unmusical”?
Because the Universe is not a good resonator
 the harmonics are broad (fuzzy)
 we do not easily notice the hidden notes
Compare the Universe with a flute:
C(ℓ)
P(k)
decibel scales
fundamental
Universe
Flute
h a r m o n i c s
What’s the Chord ?
Between major & minor 3rd
C(ℓ) as observed
P(k) undistorted
P(k) pure tones
Creation’s Opening Chord
raw
pure
both
All sounds: 104 – 106.5 years
exponential time
constant volume
microtonal
tempered
one octave
Chord Analysis
The Opening Crescendo
4×105 yrs
raw
pure
both
microtonal
tempered
8. In the Beginning: Quantum Hiss
The Big “Bang” was born in utter silence !!
Sound grew later as gas fell into a lumpy landscape
What made the initial lumpiness ?
Quantum fluctuations amplified by inflation
What did this truly primordial sound “sound” like?
Low pitch
Long wavelength
High pitch
Short wavelength
9. Closing thoughts
The physics of the early Universe can be cast in
terms which are bizarrely familiar, allowing us
access to what must surely be one of the most
remote but important of times.
Strangely, much of this was anticipated in 1680
by poet John Dryden:
John Dryden (1687)
From Harmony, from heavenly harmony,
This universal frame began:
When nature underneath a heap
Of jarring atoms lay,
And could not heave her head,
The tuneful voice was heard from high,
“Arise, ye more than dead.”
Then cold, and hot, and moist, and dry,
In order to their stations leap,
And Music’s power obye.
From harmony, from heavenly harmony,
This universal frame began;
From harmony to harmony
Through all the compass of the notes it ran
The diapason closing full in man.
http://www.astro.virginia.edu/~dmw8f