Speaker
Description
Warm dark matter (WDM) particles lighter than O(keV) are excluded at high significance because their early free-streaming strongly suppresses the formation of small-scale structure. However, recent work has suggested that introducing a small fraction of cold dark matter (CDM) with a blue-tilted isocurvature can counteract this suppression, potentially allowing lighter WDM candidates. In this talk, I will discuss new constraints on this “warm + cold-isocurvature” scenario. We combine galaxy ultraviolet luminosity functions from HST and JWST over redshifts 4 to 11 with cosmological data from the CMB, BAO, and supernovae. For pure WDM, we find a lower bound of m_{WDM} > 1.8 keV. When including a small CDM isocurvature component, this bound is relaxed significantly to m_{WDM} > 0.27 keV. This reflects a partial degeneracy where blue-tilted isocurvature fluctuations compensate for the suppression caused by WDM. Our results quantify how much such isocurvature contributions can hide the small-scale signatures of light dark matter and reassess the viability of sub-keV WDM candidates.