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Astronomy · L5 · What Sets the Sign of the 21 cm Brightness Temperature

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The spin-temperature logic that determines whether the redshifted 21 cm line appears in absorption, emission, or is invisible against the CMB.

by @openstemUpdated Astronomy
Neutral hydrogen spintemperature T_SCompare T_S toCMB temperature T_CMBAbsorption:T_b < 0No signal:T_b = 0Emission:T_b > 0Wouthuysen-Field effect:Lyman-α photons from first starscouple T_S to gas temperature T_KT_S < T_CMBT_S = T_CMBT_S > T_CMB

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