What is the Flux Density of the Cosmic Microwave Background?

-- | November 7, 2013

Question:  The temperature of the CMB is about 3 Kelvin, but the 3 K here mean the brightness temperature or the antennna temperature ? what is the CMB flux density ? — Hongmin

Answer:  The CMB temperature is the physical temperature that a black body would have in order to emit the radiant flux we measure.  The flux density of the CMB at a given wavelength can be calculated using this fact that it emits as a black body with a temperature of about 3 Kelvin.  That leads to a peak flux density of about 380 million Janskys per steradian at a wavelength of 2mm.

Jeff Mangum