Free-Streaming of Dark Matter in the Limit Frame: Masses That Preserve Observed Small-Scale Structure

A dark-matter particle that was once fast erases structure below its free-streaming length. This paper computes that length, under a stated thermal history, for masses spanning the carrier ladder and the warm-dark-matter range, and compares the results with the scale below which structure is observed to exist in the Lyman-alpha forest and in dwarf galaxy counts. Candidates lighter than a few kiloelectronvolts erase too much. The electron and heavier ladder carriers stream negligibly, while a sub-electronvolt neutrino-like carrier streams over cosmological distances and cannot form the observed halos. The frame therefore inherits the standard mass floor, and any light candidate it proposes must be produced cold, with a production mechanism that suppresses its velocity, stated and computed. Headline values are: lambda 0p1eV Mpc = 5.000e+03, lambda 5p3keV Mpc = 9.434e-02, lambda electron kpc = 9.785e-01, mass floor keV = 5.3. This study is simulated and computed from stated assumptions; it is not a measurement.