The strong cosmological motivations for measuring the CIBR have been extensively discussed in the references cited above, and are discussed further by Lonsdale at this Symposium. I therefore restrict my comments to providing some perspective on the observational search. In the past three decades while we have measured the properties of the CMBR in detail, we have only slowly learned something about what the CIBR isn't, but we still can't say what it is.
At the time of the discovery of the CMBR in the mid-1960's, there were
no direct
observational limits on cosmic electromagnetic radiation in the more
than three decades
from optical to millimeter wavelengths (for example, see Figure 6.3 in
Peebles 1993).
In particular, we could not by direct observations of the infrared sky
brightness exclude the
possibility that a radiant energy density as large as that required to
close the Universe
might be present in the spectral range from 1 µm to 1 cm!
When the COBE proposals
were written in 1974, a few pioneering rocket measurements in the
infrared had limited
that unlikely possibility, but one could not rule out a CIBR with an
order of magnitude
more integrated energy density than the CMBR, the dominant known
contribution to
the cosmic electromagnetic radiation. At the time of the COBE
launch in 1989, though
there had been more infrared instruments on rockets and the IRAS
mission, there were
still no definitive detections of a CIBR at any wavelength.
Ressell & Turner (1990)
compiled a comprehensive accounting of the diffuse photon background
from radio to
-ray energies. If
the CIBR were in fact comparable to the measurements or upper limits
compiled by Ressell & Turner, it was still possible that the CIBR might
contain more
cosmic energy density than any other part of the electromagnetic
spectrum. Clearly,
determining the cosmic infrared background is important for sharpening
our knowledge
of the radiant energy content of the Universe, as well as for providing
clues to past and present astrophysical processes.