TESTING THE FROZEN-FLOW APPROXIMATION

We investigate the accuracy of the frozen-flow approximation (FFA), re cently proposed by Matarrese et al., for tracing of the non-linear evo lution of cosmological density fluctuations under gravitational instab ility. We compare a number of statistics between results of the FFA an d N-body simulations, including those used by Melott, Pellman & Shanda rin to test the Zel'dovich approximation. The FFA performs reasonably well in a statistical sense (for example, in reproducing the counts-in -cells distribution) at small scales, but it does poorly in the cross- correlation with N-body simulations, which means that it is generally not moving mass to the right place, especially in models with high sma ll-scale power.