The redshifted radiation from the Big Bang which is observed to be nearly isotropic.It was first unambiguously detected by Penzias(1979) and Robert Wilson(1979) in the microwave at a brightness temperatureof 
K.However, evidence for a 3 K background had been seen in the optical and submillimeter by noting excitation of CN rotation states at 3874 Å and 1.32 mm.The temperature require to produce the observed excitation was K. The COBE satellite measured the temperature to be 2.735 K ± 12 
K, with variations of ± 30 K.
The temperature of the CBR as time progresses can be computed starting from the equation of free-streaming photons,
Therefore, if the spectrum starts out as a blackbody spectrum, it will remain a blackbody spectrum, but will have temperature with
entropy is also conserved. Let a be the Stefan-Boltzmann constant.
If the pressure is small,
, so
K.However, evidence for a 3 K background had been seen in the optical and submillimeter by noting excitation of CN rotation states at 3874 Å and 1.32 mm.The temperature require to produce the observed excitation was K. The COBE satellite measured the temperature to be 2.735 K ± 12 K, with variations of ± 30 K.The temperature of the CBR as time progresses can be computed starting from the equation of free-streaming photons,
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Therefore, if the spectrum starts out as a blackbody spectrum, it will remain a blackbody spectrum, but will have temperature with
(8)
entropy is also conserved. Let a be the Stefan-Boltzmann constant.
(9)
(10)
(11)
If the pressure is small,
, so (12)
(13)
(14)
(15)
(16)
(17)
(18)
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