Cosmic sound waves rule

© 2008 American Institute of Physics, S-0031-9228-0804-010-6 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 overand underpressure that propagate longitudinally through a medium. For example, a loudspeaker creates a local overpressure in air; the overpressure then causes the compressed air to expand. That, in turn, compresses the neighboring air, so that the sound wave propagates. The primordial cosmos was peppered with slightly overdense, overpressured regions that initiated the propagation of sound waves of all wavelengths. Much as the resonant sounds of a drum inform us about its shape and size, so can the details of cosmic density fluctuations tell us about the evolution of the universe.