Caption: A log-log plot of the cosmic microwave background radiation (CMB) from the COBE spacecraft and other detectors.

The plot covers several energy/frequency/wavelength bands: microwave band 0.1--100 cm, infrared 0.7 μm -- 0.1 cm, radio band 0.1 cm -- 10**5 km. Note the microwave band is actually part of the radio band.

The data is excellently fit by a blackbody spectrum with CMB T = 2.72548(57) K (Fixsen 2009)) (see also Wikipedia: Cosmic microwave background radiation: Features).

Note: If this figure ever goes AWOL, see Wikimedia: CMB black body spectra for similar images.

Credit/Permission: CMB Astrophysics Research Program: COBE site: This is a Lawrence Berkeley Lab site reporting work by George Smoot's (1945--) group / Public domain.
Image link: CMB Astrophysics Research Program: COBE site.
Local file: local link: cmb_2.html.
File: Cosmology file: cmb_2.html.