To be sure, some researchers think we may still be missing some crucial piece of the puzzle, such as galaxies that are too spread out to see directly or stars that formed before galaxies did [see “The First Stars in the Universe,” by Richard B. Larson and Volker Bromm; Scientific American, December 2001]. Sources other than AGNs have been proposed for the very high-energy tail of the CXB. For example, a significant fraction of the gamma rays could be produced by electrons catapulted to immense speeds during the formation of the large-scale structure of the universe.
Further intensive surveys are needed to disentangle the various processes that contribute to the background, and future observatories—such as the Space Infrared Telescope Facility, the Herschel Far-Infrared Telescope, the Next Generation Space Telescope and the Atacama Large Millimeter Array—will be required to study some of the objects that x-ray satellites have detected. X-ray spectrometry by the planned XEUS mission could be crucial because it might be able to estimate redshifts from x-ray data alone, thereby allowing observation of objects too heavily obscured to be visible in the optical at all. Such work might finally explain the mysterious link between galaxies and the black holes at their centers, deduce which formed first, and describe how star formation relates to black-hole activity.
The Bright Night Sky The study of the background is a classic example of how nothing in astronomy is quite what it appears to be. The mere presence of the background indicates that, despite first appearances, the night sky is not completely dark. For most of human history, the darkness of the night sky was taken for granted, and the question was why it was so. In an infinite universe filled with stars, every line of sight should eventually meet the surface of a star. The dimming of starlight with distance should be exactly canceled by the increase in the number of stars you see as you look farther out, so the night sky should appear as bright as the surface of the sun. Day and night should blend into one.
This puzzle, known as Olber’s paradox, was solved in 1848 by Edgar Allan Poe. In his prose poem Eureka, he argued that the stars must not have had enough time to fill the universe with light. The darkness of the night sky, then, tells us that the universe has not existed forever. Not only has that hypothesis stood the test of time, it eventually proved crucial to formulating the big bang theory.
Still, the night is not pitch-black; it is pervaded by the cosmic background. Although we have made much progress in explaining it, we have much left to do. Whereas 19th-century thinkers had to explain why the night sky isn’t bright, modern cosmologists must figure out why it isn’t completely dark.