Spherical top-hat collapse model
= Spherical top-hat collapse model
A uniform overdensity evolves as a separate spherical pressureless region until <shell crossing>. In an Einstein-de Sitter background, formal collapse has linearly extrapolated threshold $\delta_c=(3/5)(3\pi/2)^{2/3}\simeq1.686$. This is distinct from the actual nonlinear <density>. Virialization and nonspherical tides require further physics.