THE COSMIC NEUTRINO BACKGROUND can in principle be detected. There are almost as many neutrinos loose in the universe as photons, and almost as much energy vested in neutrinos as in photons. Yet, owing to the extreme reticence of neutrinos to interact with other particles, detecting the neutrino background is not easy as detecting the cosmic photon (microwave) background. Indeed, dedicated neutrino detectors struggle just to record a handful of incoming neutrinos from potent nearby sources like the sun. Nevertheless, there might be a chance to map the background indirectly. The pattern of lumps in the microwave background, which will be measured by the upcoming MAP and PLANCK orbiting detectors, encodes information about the neutrino background. Scott Dodelson of Fermilab (630-840-2426), Michael Turner and Robert Lopez of the University of Chicago, and Andrew Heckler of Ohio State show these measurements will accurately establish the time at which slow-moving matter (protons and later atoms) became predominant over fast-moving radiation (photons and neutrinos), and that this in turn determines precisely how much early annihilation energy (arising from electrons and positrons smashing up) was apportioned among photons and neutrinos. (Lopez et al., Physical Review Letters, 17 May 1999.)