In 1996 the Cosmic Background Explorer (COBE) revealed the existence of a diffuse cosmic far-IR background at wavelengths ranging from below 100 µim to beyond 1 mm. The FIRB is about 30 times fainter than the cosmic microwave background, which was the COBE’s primary interest. “The FIRB was our most unexpected finding,” recalls COBE team leader John Mather. “It suggested a very dusty universe some 2 or 3 billion years after the Big Bang.” Nurseries crowded with newborn stars tend to be particularly dusty. So it was thought that much of the FIRB would eventually be accounted for by thermal radiation from dust heated by visible and UV light from bright young stars in so-called starburst galaxies experiencing high rates of star formation. Because the most massive and luminous stars die youngest, they are greatly overrepresented in newborn populations.
Peaking at about 200 µm, the FIRB spectrum encompasses about as...
