Dark Ages HI 21cm signal z > 200: T  = T K = T s due to collisions + Thomson scattering => No signal z ~ 30 to 200: T K decouples from T , but collisions keep T s ~ T K => absorption signal z ~ 20 to 30: Density drops  T s ~ T  => No signal Barkana & Loeb: Richest of all cosmological data sets Three dimensional in linear regime Probe to k ~ 10^3 /Mpc vs.
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