B.4 COBE

The COBE satellite was developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early Universe to the limits set by our astrophysical environment. Even if it was not conceived to study point or extended sources but rather the integrated emission of the sky background at IR and sub-mm wavelengths, its results have had a long-lasting impact on the former studies, henceforth its inclusion here. COBE was launched on 18 November 1989 and carried three instruments: the Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background (CIRB) radiation; the Differential Microwave Radiometer (DMR) to map the cosmic microwave background (CMB) radiation extremely sensitively; the Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody. Each instrument yielded a major cosmological discovery:

• DIRBE - Infrared absolute sky brightness maps in the wavelength range 1.25-240 $\mu$m were obtained to carry out a search for the CIRB, which was originally detected in the two longest DIRBE wavelength bands, 140 and 240 $\mu$m, and in the short-wavelength end of the FIRAS spectrum. Subsequent analyses have yielded detections of the CIRB in the near-infrared DIRBE sky maps as well. The CIRB represents the cumulative emission of stars and galaxies dating back to the epoch when these objects first began to form. The COBE CIRB measurements constrain models of the cosmological history of star formation and the buildup over time of dust and elements heavier than hydrogen.
• DMR - The CMB was found to have intrinsic anisotropy for the first time, at a level of a part in 100000. These tiny variations in the intensity of the CMB over the sky show how matter and energy was distributed when the Universe was still very young. Later, through a process still poorly understood, the early structures seen by DMR developed into galaxies, galaxy clusters, and the large scale structure that we see in the Universe today.
• FIRAS - The cosmic microwave background (CMB) spectrum is that of a nearly perfect blackbody with a temperature of 2.725 $\pm$ 0.002 K. This observation matches the predictions of the hot Big Bang theory extraordinarily well, and indicates that nearly all of the radiant energy of the Universe was released right after the Big Bang.