THE universe began not with a bang but with a low moan, building into a roar that gave way to a deafening hiss. And those sounds gave birth to the first stars.
Cosmologists don鈥檛 usually think in terms of sound, but this aural picture is a good way to think about the universe鈥檚 beginnings, says astronomer Mark Whittle of the University of Virginia in Charlottesville. Whittle has reconstructed the cosmic cacophony from data teased out over the past couple of years from the high-resolution mapping by NASA鈥檚 WMAP spacecraft of the cosmic microwave background radiation, the afterglow of the hot early universe.
The variations in the cosmic background radiation expose the relative clumpiness of the early cosmos at a variety of different scales. These density variations began as quantum fluctuations in the moments after the big bang, and then propagated out as sonic waves. The denser regions became the seeds of galaxies and stars, which is why astronomers are so interested in them.
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Translating the observed frequency spectrum directly to sound yields tones far too low for ears to hear 鈥� some 50 octaves below middle A 鈥� but transpose the score up all those octaves and you can listen to it. As for volume, the intensity of the variations corresponds to about 110 decibels, as loud as a rock concert. Whittle has also used the best available cosmological models to map the way the vibrations evolved over time, showing how the chords of the big bang changed over the universe鈥檚 first million years or so.
He played the resulting soundtrack at the American Astronomical Society meeting in Denver last week. Contrary to its name, the big bang began in absolute silence. But the sound soon built up into a roar whose broad-peaked notes corresponded, in musical terms, to a 鈥渕ajestic鈥� major third chord, evolving slowly into a 鈥渟adder鈥� minor third, Whittle explained.
For those worried that you can鈥檛 have sounds in space, that is true today, but it wasn鈥檛 so in the universe鈥檚 infancy. For perhaps its first million years, the universe was small and dense enough that sound waves could indeed travel through it 鈥� so efficiently, in fact, that they moved at about half the speed of light.
The new sonic reconstructions don鈥檛 involve any new science, Whittle says, but like a good diagram or 3D visualisation, they may help astronomers teach complex ideas, and maybe understand the observations a bit more clearly themselves.
鈥淓veryone was fascinated,鈥� said Steve Maran, the society鈥檚 press officer, who added that even he had 鈥渓earned from this, that the big bang actually was silent, then things got louder and louder鈥�. Hear it for yourself at , under 鈥淎AS presentation鈥�.
