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.