The punctuated Zel’dovich approximation

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We study a phenomenological approximation to the full fluid-dynamical equations for non-linear gravitational clustering, based on an extension of the well-known Zel’dovich approximation. In this approach, called the Punctuated Zel’dovich Approximation (PZA), fluid elements move according to the Zel’dovich prescription until they reach a critical distance from each other. At this stage, particles stick with each other by conserving mass and momentum but not kinetic energy, simulating the effects of strong non-linear gravitational clustering. The PZA is then compared with pure Zel’dovich dynamics and with the adhesion approximation. PZA turns out to work at least as well as the adhesion approach, but is more flexible and physically more justified.