We develop a finite-temperature hydrodynamic approach for a harmonically trapped one-dimensional quasicondensate and apply it to describe the phenomenon of frequency doubling in the breathing-mode oscillations of the quasicondensate momentum distribution. The doubling here refers to the oscillation frequency relative to the oscillations of the real-space density distribution, invoked by a sudden confinement quench. By constructing a nonequilibrium phase diagram that characterizes the regime of frequency doubling and its gradual disappearance, we find that this crossover is governed by the quench strength and the initial temperature rather than by the equilibrium-state crossover from the quasicondensate to the ideal Bose gas regime. The hydrodynamic predictions are supported by the results of numerical simulations based on a finite-temperature c-field approach and extend the utility of the hydrodynamic theory for low-dimensional quantum gases to the description of finite-temperature systems and their dynamics in momentum space. DOI: 10.1103/PhysRevA.94.051602 Hydrodynamics is a powerful and broadly applicable approach for characterizing the collective nonequilibrium behavior of a wide range of classical and quantum fluids, including Fermi liquids, liquid helium, and ultracold atomic Bose and Fermi gases [1][2][3][4][5][6]. For ultracold gases, the hydrodynamic approach has been particularly successful in describing the breathing (monopole) and higher-order (multipole) collective oscillations of harmonically trapped three-dimensional (3D) Bose-Einstein condensates [2,6,7]. For condensates near zero temperature, the applicability of the approach stems from the fact that for long-wavelength (low-energy) excitations the hydrodynamic equations are essentially equivalent to those of superfluid hydrodynamics, which in turn can be derived from the Gross-Pitaevskii equation for the order parameter. For partially condensed samples at finite temperatures, the hydrodynamic equations should be generalized to the equations of two-fluid hydrodynamics, where the applicability of the approach to the normal (thermal) component of the gas is justified by fast thermalization times due to collisional relaxation [3,8].In contrast to 3D systems, the applicability of the hydrodynamic approach to 1D Bose gases is not well established. First, in the thermodynamic limit 1D Bose gases lack the long-range order required for superfluid hydrodynamics to be a priori applicable. Second, the very notion of local thermalization, required for the validity of collisional hydrodynamics of normal fluids, is questionable due to the underlying integrability of the uniform 1D Bose gas model [9]. Despite these reservations, the hydrodynamic approach has already been applied to zero-temperature (T = 0) dynamics of 1D Bose gases in various scenarios [10][11][12][13][14][15] (for related experiments, see [16][17][18]). The comparison of hydrodynamic predictions with exact theoretical results is challenging. In Ref.[13], timedependent density-matrix renorm...