The acoustic wave equation in the expanding universe. Sachs-Wolfe theorem

In this paper the acoustic field propagating in the early hot ($p=ϵ\mathrm{/}$) universe of arbitrary space curvature ($K=0,\pm 1$) is considered. The field equations are reduced to the d’Alembert equation in an auxiliary static Roberson-Walker space-time. Symbolic computation in {\em Mathematica} is applied.