Cocktail party at the beginning of the universe

I’ve just finished reading the feature article by Daniel Eisenstein and Charles Bennett about cosmic sound waves (Physics Today, April 2008, page 44 ). As an acoustical engineer, I am especially drawn to the 1 part in 10⁵ smoothness of the cosmic microwave background (CMB) fluctuations and the attendant “sound wave” analogy. I’m wondering whether your readers appreciate the elegance of this analogy.

Although 1/100 000 may at first seem tiny, acousticians deal in such ratios daily. Consider that an atmospheric pressure fluctuation of 1 bar, expressed in decibels, is approximately 194 dB, while a typical sound level measured in a crowded room of moderate size full of loudly talking people might easily approach 80 dB, a pressure ratio well below 10⁻⁵. Thus lively conversation superimposed on atmospheric pressure looks exactly like the Wilkinson Microwave Anisotropy Probe’s CMB anisotropy. So a proper analogy for the CMB fluctuations might be the cocktail party at the beginning of the universe. Acoustically, one part in 10⁵ smoothness is not a terribly small variation but rather should be regarded as quite normal. We should expect to be able, in a sense, to extract portions of the intelligible conversation from among the din. CMB analysts are working with what would be analogous to a snapshot of an instant in that cocktail party conversation rather than having to wrestle, as acousticians must, with a fully dynamic situation. In Eisenstein and Bennett’s figure 2, the three peaks in the power spectral density function at 0.6°, 0.4°, and 0.2° reveal hints of that conversation. It is interesting to consider where this line of thought might lead.