Models for the isotropic velocity divergence power spectrum of matter at z = 0: linear theory (black dashed), an empirical model based on fits to N-body simulations from Bel et al. (2019; green), and a beyond first-order perturbation theory model from Taruya et al. (2012; red). For these two models, we show the power spectrum with a damping function using σ_u = 15 h⁻¹Mpc (dashed lines). Nonlinearities become important at k > 0.1 h Mpc⁻¹.