 Solution

Divide the physical sound radius by the angular diameter distance $a_{\ast }{r}_{\ast }$. The sound-horizon angle in an open matter universe isThe characteristic acoustic multipole is approximately half a wavelength across this angle:Thus negative curvature moves the acoustic feature to smaller angular scales and larger multipoles. The first temperature peak is of order $200/\sqrt{{\Omega }_m}$ in this approximation; precise peak positions include the sound-speed history, radiation effects and acoustic phase shifts, so the acoustic-scale multipole is not an exact peak-location formula.