The search for cosmic infrared background radiation (CIBR) is a relatively new field of observational cosmology. Measurement of this distinct radiative background, expected to arise from the cumulative emissions of pregalactic, protogalactic, and galactic systems, would provide new insight into the cosmic `dark ages' following the decoupling of matter from the cosmic microwave background radiation (see, for example, early papers by Partridge & Peebles 1967; Low & Tucker 1968; Peebles 1969; Harwit 1970; Kaufman 1976; and more recent discussions by Bond, Carr, & Hogan 1986, 1991). Observationally, there have been no corroborated detections of the CIBR, though possible evidence for an isotropic infrared background in data from rocket experiments has been reported (Matsumoto et al. 1988; Matsumoto 1990; Noda et al. 1992). The Diffuse Infrared Background Experiment (DIRBE) on the COBE spacecraft is the first satellite instrument designed specifically to carry out a systematic search for the CIBR. The FIRAS instrument on the COBE spacecraft, designed primarily to make a precise measurement of the spectrum of the cosmic microwave background radiation, is also a powerful instrument for the CIBR search. As described by Mather at this Symposium, the FIRAS measures the absolute sky brightness with a 7°-diameter beam from a wavelength of 100 µm to 1 cm.
The search for the CIBR is the most exploratory of the COBE objectives. Even from a spaceborne instrument, this cosmic fossil is far more difficult to observe than the cosmic microwave background radiation (CMBR). Whereas the CMBR is the dominant diffuse celestial radiation at millimeter wavelengths, the local infrared foregrounds from interplanetary dust and the Galaxy are far brighter than expected CIBR levels. Preliminary conservative limits on the CIBR based on the DIRBE data at the south ecliptic pole, and a comparison of DIRBE measurements with those from instruments on sounding rockets and the Infrared Astronomical Satellite (IRAS), were presented by Hauser et al. (1991). The DIRBE investigation is still in its data collection and data reduction phase. In this talk, I report updated CIBR limits in the DIRBE spectral range based upon the faintest sky brightness observed at each wavelength and a more recent calibration of the DIRBE data than that used by Hauser et al. (1991), and note the new stringent limits at submillimeter wavelengths implied by the FIRAS data.