Comment on "Self-interacting warm dark matter" - art. no. 088301

Hannestad and Scherrer analyzed self-interacting warm and hot dark matter ( HDM) and found power spectra in disagreement with previous papers. We argue that they include self-interactions in a weak-coupling approximation, as f or photons after recombination. The "tightly coupled" approximation used fo r prerecombination photons, and previous discussions of self-interacting HD M. would have been more appropriate. Their approximation generates a Boltzm ann hierarchy leading to a stiff system of equations. Furthermore, contamin ation by gauge modes could invalidate their results.