Relativistic sound waves raced through the hot plasma that filled the universe for its first 380 000 years. That sound left an imprint that is still discernible in the cosmic microwave background and also in the large-scale distribution of galaxies. Measurements of the sound waves manifested in the CMB, coupled with a detailed understanding of the physics of sound waves in the plasma epoch, provide the foundation of our standard model of cosmology (see Physics Today, April 2003, page 21 ). A new opportunity to track their imprint in the spatial distribution of galaxies as far back as we can see them opens a path to elucidating the mystery of dark energy and its role in the history of the cosmos. 1
Sound waves are oscillations of over- and underpressure that propagate longitudinally through a medium. For example, a loudspeaker creates a local overpressure in air; the overpressure then...
