This article considers the acoustic field propagating in the radiation-dominated () universe of arbitrary space curvature (). The field equations are reduced to the d’Alembert equation in an auxiliary static Robertson-Walker spacetime and dispersion relations are discussed. Introduction The Sachs-Wolfe theorem ([1], pp. 76-77) contains two separate results formulated for two different equations of state: … Continue reading The Acoustic Wave Equation in the Expanding Universe: Sachs-Wolfe Theorem