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Editor's Note: We are posting this feature from our March 2002 issue because of news from the annual meeting of the American Astronomical Society about the phenomenon discussed here.
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.
Those of us who study the background radiation focus our attention precisely on what our colleagues try to ignore. We first add up all the light coming from a given region of space. Then we systematically subtract the contributions from known objects such as stars, galaxies and gas clouds—collectively, the “foreground.” If something is left over, some diffuse glow of indeterminate origin, we know that our census of heavenly objects must still be incomplete.
Sometimes a diffuse glow is observed when objects are very closely spaced and the telescope lacks sufficient angular resolution to pick them apart. Take, for example, the Milky Way, which is a blur to the naked eye. With a simple pair of binoculars, you can see that the blur consists of millions of individual points of light. At other times, a diffuse glow comes from a source that truly is diffuse, such as the zodiacal dust of our own solar system or the gaseous supernova remnants of our galaxy. Many (but by no means all) of these sources within our galaxy and nearby galaxies have been identified, so they can be considered part of the foreground. The radiation that comes from far outside our galaxy, filling the whole universe, is the cosmic background.
In the past half a decade, as the sensitivity and resolution of telescopes have improved dramatically, astronomers have accounted for more and more of the background glow. In so doing, we have discovered that our previous inventories of the universe were incomplete: for instance, we had badly underestimated the prevalence of supermassive black holes. Far from being isolated oddities, as was once thought, they are everywhere. Earlier studies had missed them because they are cloaked by prodigious quantities of dust. With these holes now unveiled, we may soon explain the background fully.
That is not to say we will have seen everything there is to see. We can no more catalogue every celestial body than a biologist can count every beetle. But just as biologists can fairly claim to know all the major types of, say, land mammals, astronomers are on the verge of identifying all the major classes of light-emitting objects.