The Cosmic Reality Check; March 2002; Scientific American Magazine; by G¿nther Hasinger and Roberto Gilli; 8 Page(s)

With each day's newspaper seems to arrive a new astronomical discovery: a new celestial body, a new physical process, a new form of matter. Will the revelations ever end? Will there ever come a day when astronomers feel confident that they have made a complete inventory of the universe? If the question is put so broadly, the answer is clearly no: astronomers already know that not everything in the universe can be seen directly, and additional surprises are inevitable. But a somewhat narrower question-will astronomers ever finish their head count of stars, galaxies and other luminous matter?-has a rather different answer. A day will indeed come when astronomers have accounted for the bulk of the light in the universe, and that day is fast approaching.

Over the years, astronomers have developed a type of quality-control check that can signal whether they have missed any important source of light. The idea is to study a phenomenon that most observers consider a nuisance: the so-called background radiation. When scientists in any discipline talk about a "background," they usually mean everything except what they are interested in. A telescope capturing the radiation from a star cannot avoid collecting light from other bodies near and far. This extraneous light serves only to reduce the precision of the desired measurement.