The Echo of Einsteins Greatest Blunder

The coupling of baryons and photons by Thomson scattering in the early universe leads to a rich structure in the power spectra of the cosmic microwave background photons and the matter. The study of the former has revolutionized cosmology and allowed recise measurement of a host of important cosmological parameters. The study of the latter is still in its infancy, but holds the potential to constrain the nature of the dark energy believed to be causing the accelerated expansion of the universe. I will discuss how we can measure the cosmic sound, and the theoretical and observational developments that need to be made before we can realize the promise of future missions.