Working Notes (First Draft-2): Relativistic Shedding as Threshold Emission into Hidden Sectors

This is a first-pass sketch to capture the idea and obvious checks before polishing. I propose a kinematic-threshold mechanism: when a highly boosted excitation outruns the phase velocity of some field mode in its environment, an emissive channel opens. In ordinary dielectrics this is Cherenkov light; here I hypothesize that, in the early universe and in certain laboratory media, analogous thresholds also open into a hidden sector, potentially producing (i) a stable cold dark-matter remnant χ, (ii) a small amount of hidden radiation (kept under current ∆Neff bounds), and (iii) an ultra-light, shift-symmetric scalar ϕ whose potential energy today behaves like dark energy. The emphasis in this draft is on testability and built-in failure modes; details are provisional and meant to be tightened after basic consistency checks.