The Sound of Cosmic Evolution

The ripples in the early Universe were produced by a kind of sound wave. When the Universe was very hot, with a temperatur of several thousand degress, it was ringing with sound waves travelling backwards and forwards. The surface of the Sun is at a similiar temperature and is vibrating in a similiar way. Because of its poor resolution COBE was able to detect only those ripples that have a very long wavelength. These represent sound waves of very low pitch, the bass notes of cosmic evolution. The information contained in these waves is important but not very detailed; their sound is rather dull.

On the other hand, the Universe should also produce sound of higher pitch, like the Sun or a bell, and this is much more interesting. The ripples belonging to these very short sound waves are measured with very high accuracy by WMAP 2003!

Sound waves travel with a particular speed. In air, for example, this is around 300 metres per second. In the early Universe the sound speed is much greater, approaching the speed of light!

By the time the microwave background is produced the Universe is about 380000 years old. In the time up to then since the Big Bang, which is presumably when the sound waves were excited in the first place, they can have travelled only about 300000 lightyears. Oscillations with this wavelength produce a characteristic "note", like the fundamental tone of a musical instrument. It is no coincidence that superclusters of galaxies are roughly of this size; they result from this resounding cosmic fanfare.

This characteristic wavelength of the early Universe reveal itself in the pattern of hot and dark spots an the microwave sky (the wavelength of the red light is stretched out by cosmic expansion by a factor of 1100 to microwaves, this is why the space is dark in visible lght but not for microwave-Instruments).

The detailed analysis of the cosmic sound answers many of the major questions facing modern cosmology. The spectrum of sound contains information about how much mass there is, whether there is Dark Energy (about 70%!), what the Hubble number is, whether space is curved and perhaps even whether inflation happend or not.

after Peter Coles Cosmology, Oxford University Press