= Cosmic microwave background
{title2=CMB}
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= CMB
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The <cosmic microwave background> is nearly isotropic thermal <electromagnetic radiation>. Its spectrum connects <radio receiver> measurements with the universe's thermal history: <redshift preservation of a thermal photon spectrum> cools a <blackbody> spectrum as space expands. The 1965 Princeton analysis described this relation.<Cosmic Black-Body Radiation by R. H. Dicke and others>[pp.415–416: expansion cools radiation while preserving a thermal spectrum]{ref}
A <radiometer> cannot label a received <photon> as cosmic, atmospheric or instrumental. Its measured <power> combines the incoming radiation with the receiver response. The useful question is which comparisons and controls can isolate a sky contribution. This makes the <Penzias-Wilson microwave background experiment> an example of both <observational cosmology> and <measurement calibration>.
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