**Energy as a guiding principle in nonlinear structure formation **

Abstract: One goal of studies of large scale structure formation is to understand why the dense, virialized clumps which host galaxies form where they do. In cold dark matter cosmologies, the late time field retains some memory of the initial conditions, which models of dark matter halo formation try to exploit. The simplest models are motivated by a spherical collapse calculation which dates back to the early 1970s. In the late 1980s, this approximation for the physics of collapse was coupled with the heuristic assumption that collapse occurs around regions that are maxima of the initial matter density fluctuation field. The density-peaks model of initial `protohalo’ patches has been very influential since it was first proposed. I will show the benefits of replacing this notion with the dynamical requirement that protohalos are regions where the local gravitational flow converges to a point. If time permits, I will discuss how a similar approach can be used to estimate the abundance of primordial black holes which form during the epoch when the energy density was dominated by radiation rather than matter.