Cosmic infrared background (CIB) radiation is expected to arise from the cumulative emissions of pregalactic, protogalactic, and evolved galactic systems. It has long been recognized that measurement of the CIB will provide new insight into energetic processes associated with structure formation and chemical evolution following the decoupling of matter from the cosmic microwave background (CMB) radiation (Partridge & Peebles 1967; Harwit 1970; Bond, Carr, & Hogan 1986, 1991; Franceschini et al. 1991, 1994; Fall, Charlot, & Pei 1996). In this paper, the CIB is defined to be extragalactic radiation, exclusive of the CMB, in the wavelength range 1-1000 µm.
Sources of cosmic radiant energy include nuclear processes such as nucleosynthesis in stars; gravitational processes, such as accretion of matter onto black holes; and decaying unstable particles remaining from the Big Bang. Though the primary radiant energy from such processes may not emerge at infrared wavelengths, the combined effects of the cosmic redshift and absorption of some fraction of the primary radiations by dust with re-emission by the dust at long wavelengths will shift much of the energy into the infrared. Hence, the infrared background is expected to be uniquely informative about cosmic history.
Sky brightness measurements with instruments on NASA's Cosmic Background Explorer (COBE) mission have provided the first definitive detections of the CIB. In this paper I focus on the COBE measurements, which initially provided detections only at far infrared and submillimeter wavelengths. I will also describe recent results of analyses based upon COBE data that have provided likely detections in the near infrared and restrictive limits in the mid-infrared, since these results are not covered substantially elsewhere at this conference. I then briefly describe some of the implications of these results. A comprehensive review paper on the infrared background and its implications is in preparation (Hauser & Dwek 2001). Since energy distributions are of primary interest, sky brightness measurements are reported as I in units of nW m-2 sr-1, where I is the spectral intensity at frequency . Conversion to I can be accomplished using the relation I(nW m-2 sr-1) = [3000 / (µm)] I(MJy/sr).