These squeezed and stretched matter, heating the compressed regions and cooling the rarefied ones. Even though the universe has been expanding and cooling ever since, the sound waves have left their imprint as temperature variations on the afterglow of the big bang fireball, the so-called cosmic microwave background. Cramer was prompted to recreate the din- last heard13.7 billion years ago- by an11-year-old boy who wanted to know what the big bang sounded like for a school project.
To produce the sound, Cramer took data from NASA's Wilkinson Microwave Anisotropy Probe. Launched in 2001, the probe has been measuring tiny differences in the temperature between different parts of the sky. From these variations, he could calculate the frequencies of the sound waves propagating through the universe during its first 760,000 years, when it was just 18 million light years across. At that time the sound waves were too low in frequency to be audible. To hear them, Cramer had to scale the frequencies 100,000 billion billion times.
Nevertheless, the loudness and pitch of the sound waves reflect what happened in the early universe. During the 100-second recording (http://www.npl.washington.edu/AV/BigBangSound_2.wav), the frequencies fall because the sound waves get stretched as the universe expands. "It becomes more of a bass instrument," says Cramer.
Author: Marcus Chown
New Scientist issue: 1 November 2003
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