Dark Matter Free-Streaming: 2D Power-Spectrum Lab

This is the 2D counterpart to the 3D free-streaming particle simulator, and it works by a genuinely different mechanism: rather than moving discrete pre-placed clumps of particles, it builds a real Gaussian random density field directly in Fourier space and suppresses it with a linear transfer function set by the dark-matter particle's free-streaming length — exactly the mathematics cosmologists use to compute cold, warm and hot dark-matter power spectra. Watch the field evolve under linear growth and cross the spherical-collapse threshold to see, cell by cell, which regions actually bind into dwarf-scale or cluster-scale halos, and which get washed out before they ever get the chance.