The Sound of the Big Bang

WMAP Version (2003)

John G. Cramer
Emeritus Professor of Physics
University of Washington
Seattle, WA 98195-1560

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.  Here is a plot of the WMAP multipole spectrum.

Plot of the multipole moments of the CMB  from the WMAP Collaboration

    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 time^2/3 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