Comments submitted to Physics Today at http://www.physicstoday.org/pt/contactus.jsp on April 26, 2008.

Sticky questions arise about "Cosmic sound waves rule" (feature article by Eisenstein and Bennett in April 2008 Physics Today).  What about viscous damping?

The acoustic peak of the cosmic microwave background is huge as the authors rightly emphasize.  Converted to sound waves in air creating temperature fluctuations dT/T values of 10^-5 in the audible range would be at the threshold of pain.  This is a loud sound and requires a powerful source, not a peppering of "slightly overdense, overpressurized regions" provided by cold dark matter condensations as illustrated in the figures a-g of the "Emergence of the baryonic acoustic peak" box on page 48 of the article.

The source of the cosmic sound is the formation of the first structures, driven by masses of protosuperclusters as the plasma epoch began its self gravitational fragmentation 10^12 seconds after the big bang.  At this time the horizon scale of causal connection ct just grew with time t to match the viscous gravitational scale (gamma nu / rho G)^1/2 so density minima in the plasma could begin to form voids, where c is the speed of light, gamma = 1/t is the rate of strain of the expanding universe, nu is the kinematic viscosity of the plasma, rho is its density, and G is Newton's gravitational constant.

Momentum in the plasma is transmitted by photons colliding with electrons, which drag adjacent ions to maintain electrical neutrality.  The photon viscosity nu at first structure was enormous, about 10^5 larger than that of the viscous layer of the upper earth, accounting for the 30,000 year delay in gravitational structure formation and the lack of turbulence.

The speed limit for rarefaction waves is the speed of sound, which is why the CMB exhibits its sonic peak.

A tutorial on self gravitational structure formation is found in Gibson, C. H. "The first turbulent combustion", Combustion Science and Technology, 177: 1049–1071, 2005.