Acoustic Signatures in the Cosmic Microwave Background

We study the uniqueness and robustness of acoustic signatures in the cosmic microwave background by allowing for the possibility that they are generated by some as yet unknown source of gravitational perturbations. The acoustic pattern of peak locations and relative heights predicted by the standard inÑationary cold dark matter model is essentially unique, and its conÐrmation would have deep implications for the causal structure of the early universe. A generic pattern for isocurvature initial conditions arises due to backreaction e†ects but is not robust to exotic source behavior inside the horizon. If present, the acoustic pattern contains unambiguous information on the curvature of the universe even in the general case. By classifying the behavior of the unknown source, we determine the minimal observations necessary for robust constraints on the curvature. The di†usion damping scale provides an entirely modelindependent cornerstone upon which to build such a measurement. The peak spacing, if regular, supplies a precision test. Subject headings: cosmic microwave background È cosmology: theory 1.