  We construct a hydrodynamic theory of noisy, apolar active smectics, in bulk
suspension or on a substrate. Our predictions include: quasi-long-ranged
smectic order in dimension d = 2, and long- ranged in d = 3, extending
previously published results to all dynamical regimes; Kosterlitz-Thouless
melting to an active nematic at high and low concentrations in d = 2; nonzero
second-sound speed parallel to the layers; the suppression of giant number
fluctuations by smectic elasticity; instability to spontaneous undulation and
flow in bulk contractile smectics; a layer spacing instability, possibly
oscillatory, for large enough extensile active stresses.
