The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

When astronomers using photographic plates hunted for supernovae by eye, they used an optical device called a comparator. By rapidly switching back and forth between two images of a galaxy taken several weeks apart, the comparator would allow an astronomer to see whether any new pinpoint of light had appeared in the Interim. The comparator blinked the two images. New computer technology, however, allowed astronomers to take all the light from the earlier image and remove it from the later one. It subtracted the first image from the second. If the computer signaled that a telltale bit of light remained, then a real live human being analyzed the data. Sometimes the source of the bit of light was “local”—a fluctuation in the output, a cosmic ray from space striking the instrument, a subtraction error. Sometimes the source of the light was “astronomical”—asteroids, comets, variable stars. But once in a while the blip of light was a star erupting in a farewell explosion that stood out even against the background of all the tens or hundreds of billions of other stars in its host galaxy combined: that is, a supernova.