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We revisit the theoretical analysis of an expanding ring-shapedBose-Einstein condensate. Starting from the action and integrating overdimensions orthogonal to the phonon’s direction of travel, we derive aneffective one-dimensional wave equation for azimuthally-travellingphonons. This wave equation shows that expansion redshifts the phononfrequency at a rate determined by the effective azimuthal sound speed,and damps the amplitude of the phonons at a rate given by \dot{\mathcal{V}}/{\mathcal{V}} 𝒱 ̇ / 𝒱 ,where \mathcal{V} 𝒱 is the volume of the background condensate. This behavior is analogousto the redshifting and ``Hubble friction’’ for quantum fields in theexpanding universe and, given the scalings with radius determined by theshape of the ring potential, is consistent with recent experimental andtheoretical results. The action-based dimensional reduction methods usedhere should be applicable in a variety of settings, and are well suitedfor systematic perturbation expansions.