As everyone except movie directors knows, sound doesn’t travel in outer space. But there was a time, spanning the first 380 000 years after the Big Bang, when acoustic waves did traverse the cosmos, which was filled with hot, dense plasma. As the plasma cooled and neutral atoms formed, the waves suddenly stopped propagating. Their mass-density pattern froze in place, and the denser regions eventually contracted under their own weight to form galaxy clusters.
Cosmologists are interested in studying that density pattern—the so-called baryonic acoustic oscillations (BAOs)—as a way of tracing the universe’s expansion. (See the article by Daniel Eisenstein and Charles Bennett, Physics Today, April 2008, page 44.) Observations of distant supernovae have led to the surprising conclusion that the expansion is accelerating under the influence of an enigmatic dark energy. (See the article by Saul Perlmutter, Physics Today, April 2003, page 53.) But the...
