• In 1970, P. James Peebles and Jer Yu showed that the early universe must have harbored sound waves. • Sound – an oscillating change in the density of a gas • Faster particles not only cause a region to compress, they also cause it to heat up and shine slightly brighter and “bluer”. Expanded regions shine redder and are cooler. • Only integer multiples of a particular frequency would have maximally compressed or expanded a region right at the time of decoupling, those these are the regions that show up red or blue on the CMB map. • Wavelength = speed/frequency • Sound Horizon – the fixed distance over which a sound wave could have traveled during the formation of the CMB (379,000 years). • Silk Damping – when sound waves dissipate whenever the medium they are traveling through has a low density such that the typical distance between collisions with other particles is larger than the sound’s wavelength. • This simply means that you cannot hear sound in space!
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