Sounds from the Infant Universe

    Abstract for talk at AAS session on public outreach (6/3/04)
       Two paragraph summary of the Big Bang Acoustics Project

                          Mark Whittle
                     University of Virginia


One of the most impressive developments in modern cosmology has been the
measurement and analysis of the tiny fluctuations seen in the cosmic
microwave background (CMB) radiation -- the omni-directional wall of
hot glowing gas which dates from when the universe was only 400,000
years old.  When discussing these fluctuations, cosmologists frequently
refer to their acoustic nature -- sound waves move through the hot gas
and are seen as peaks and troughs when they cross the glowing wall.
As is now well known, the most recent observations (culminating with
the February 2003 WMAP results) quantify the amplitudes of these waves,
revealing both a fundamental tone and several harmonics, whose relative
strengths have played a key role in determining a number of fundamental
cosmological parameters.  Not surprisingly, these recent results have
wonderful pedagogical value in educating and inspiring both students and
the wider public, and indeed many excellent non-specialist articles
have already been written about the CMB. To further enhance this
opportunity to communicate the field, I have attempted what might seem
rather obvious: to reproduce the CMB power spectrum as an audible sound,
preserving both volume and sound quality while shifting the frequency
up by the necessary 50 or so octaves to bring it into the human range.
By choosing the fundamental to fall at 200 Hz (matching its harmonic "l"
value), the resulting sound is a rather loud hissing roar, of about 90
decibels volume.

Matching the progress in observational results has been an equally
impressive  development of the theoretical treatment of CMB fluctuations,
culminating in highly sophisticated computer simulations which can
accurately reproduce the observations, once the various fundamental
parameters are set.  Using these simulations it is possible to recreate
the sound generated by various types of universe with, for example,
different curvature (yielding sounds of different pitch) or different
baryon content (yielding different higher harmonics).  Pushing further,
one can generate the "true" sound, characterized by P(k), rather than
the "observed" sound, characterized by C(l).  From P(k), we learn
that the fundamental is offset from the higher harmonics, yielding a
chord somewhere between a major and minor third.  Finally, tracking P(k)
forward in time one can listen to the development of cosmic sound from the
Big Bang, through recombination, and beyond.  This sound sequence can be
loosely described as a descending scream, changing into a deepening roar,
with subsequent growing hiss, nicely matching the increase in wavelengths
caused by universal expansion, followed by the post recombination flow
of baryons into the small scale potential wells created by dark matter.
This final sound, of course, sets the stage for all subsequent growth of
cosmic structure, from stars (hiss) through galaxies (mid-range tones)
to large scale structure (bass notes).

Although popular presentations of CMB studies already make use of many
visual and conceptual aids, introducing sound into the pedagogical mix
can significantly enhance both the intellectual and the emotional impact
of the subject on its audience, without sacrificing scientific honesty.