The Sound of the Big Bang
Temperature variations of the cosmic background radiation as
measured by WMAP.
I'm a
Professor of Physics at the University
of Washington in Seattle. I do basic
research in ultra-relativistic heavy ion physics with the
STAR experiment, using the RHIC facility at
Brookhaven National Laboratory, colliding gold nuclei to produce systems
that look something like the first microsecond of the Big Bang.
I do not
work much in cosmology and astrophysics, although I've published a paper or
two in those areas, but I do write a bi-monthly science column for
Analog
Science Fiction/Fact Magazine. One
of my columns was entitled "BOOMERanG
and the Sound of the Big Bang" and was published in the January-2001 issue
of Analog. It described the
then-recent Antarctic balloon flight that mapped the small-angle temperature
variations of
the cosmic background radiation.
Following the lead of the scientists involved in the project, I described
the temperature variations they observed as, in effect, a recording of the "sound of the
Big Bang" when the universe was 376 thousand years old.
That column was published several
years ago and is available on the
web .
About a month ago,
I received an E-mail from a mother, who said that her 11 year
old son Daniel was doing a school project on the Big Bang. They had found
my column on the Web, and she was
wondering if the sound of the Big Bang, mentioned there, was actually
recorded anywhere, so that he could play it for his class.
My answer was
"no", but her question caused me to consider the problem.
With the
available data from BOOMERanG and more recently from WMAP, it would not be
very difficult to simulate the sound, using the symbolic algebra program
Mathematica (Wolfram Research). I'm
a fairly skilled user of Mathematica,
and it includes the feature of rendering mathematical functions as sound
that can be captured as .wav files.
The idea of synthesizing the Big Bang
sound fascinated me. It ran
around in my head for a day or so, and I
had a growing desire to hear just what the Big Bang sounded like. So
one Saturday morning, when I should
have been doing something else, I sat down and wrote a 16-line Mathematica
program that produced the sound and saved it as .wav
files. I downloaded the frequency
spectrum measured by WMAP and
used it as input data for
the program.
My PC has a good sound card and a substantial sub-woofer, so it reproduced the
.wav file well. When I ran the program for the first time and the sound started
in my office, our two male Shetland Sheepdogs, Alex and Lance, came running into
the room, barking with agitation. After they had looked around and determined that nothing
terrible was happening, they lay down on the floor and listened attentively,
giving the Sheltie Stare to my
sub-woofer.
My Mathematica program (or notebook) combines the WMAP
measured frequencies, appropriately
scaled for the human ear, assuming that all the sinusoids start at a maximum
at t=0 (the start of the Big Bang), and frequency-shifts them downward as
time2/3 as the universe expands and becomes more of a "bass
instrument".
The simulation lasts 100 seconds representing the first 760 thousand
years
of evolution of the universe, and varies the sound intensity to match the
cosmic microwave which, according to WMAP, peaked at 379 thousand years and
dropped to 60% intensity in 110 thousand years before and after the peak time.
The sound frequencies used in the simulation must be boosted upward by a
huge factor (about 10 to the 26 power) to match the response of the human
ear, because the actual Big Bang frequencies, which had wavelengths on the
order of a fraction of the size of the universe, were far too low to be heard by
humans (even had any been around).
After
I produced the .wav file, I E-mailed a copy to Daniel.
His mother reports that his science project was a great success. I
also linked the file to my BOOMERanG column and mentioned it to my daughter,
Kathryn, who put it on her blog.
I also mentioned it to Marcus Chown, who writes for New
Scienist. He wrote a piece about
it, and it was publicized in a
press release about their new issue. The result was an amazing media
explosion. The story was picked up by newspapers and news services around the world, including Aljazeera,
the Telegraph
(UK), Ananova
(UK), news.com.au
(Australia), The
National Post (Canada), EurekaAlert!,
The
Mirror (UK), The
Australian, The
Brisbane Courier Mail, and The
Frankfurter Allgemeine (Germany).
It was also the feature front page story in the Turkish newspaper
Sabah, crowding out
the California fires. I was interviewed by three different parts of the
BBC in the same 24 hour period, and I did several other radio interviews
including a "Living
on Earth" segment for National Public Radio.
Many thousands of file requests hit our
local web
server, which was making the audio file available, and filled its hard disk,
causing an ugly system crash. We had to move the file to a more robust
system to deal with the traffic.
Originally I produced only one Sound of
the Big Bang file, which had a duration of 100
seconds. However, in response
to various requests I have produced files with more durations, including 20
seconds, 50
seconds, 100
seconds, 200
seconds, and 500
seconds. I still recommend the 100 second
file, but the others are
available. These audio files can be downloaded and used for any purpose,
provided the text "(c) John G. Cramer - 2003" is used in text related
to the file. References to my website,
my Analog Columns,
and my two hard science fiction novels Twistor
and Einstein's
Bridge are also encouraged and would be appreciated.
Here are
the answers to two recurring questions that were asked repeatedly following the New Scientist
article:
(1) The Big Bang Sound in the
simulation is derived from the sound propagating as compression waves through the plasma/hydrogen medium
of the early universe some 100 to 700 thousand years after the initial Big Bang.
The density of this medium was changing as the universe expanded, but
should have been considerably more dense than air on our little planet.
One does NOT need air to have sound, only some medium in which
compression/rarefaction waves can propagate. The sound waves were
very low in frequency and had wavelengths comparable to some fraction of the
size of the universe. For the convenience of humans, who could not hear
such low frequencies, I have increased them to the audio range of the human ear.
(2) The VISIBLE radius of the
universe at age 760,000 years is, of course, 760,000 light years.
However, even at this early age the scale radius of the universe is
already larger than its visible radius, with regions already out of causal
contact. The SCALE radius of 18
million light years mentioned in the New Scientist article is an estimate based
on assuming that the universe expands as time to the 2/3 power (as it would for
a flat Robertson-Walker metric with zero cosmological constant) and propagating
backwards 3/2 of the present visible radius of the universe to its scale radius
at age 760,000 years.
John G. Cramer
November 10, 2003
