One of the driving forces behind astrophysical research is the practical justification that the universe provides us with a magnificent laboratory from which much can be learned that will be useful here on Earth. For example, we can get clues to the origin and nature of life or to new sources of energy, and we can find and study new states of matter and possibly new physical laws that govern the universe. Another, more philosophical, but equally compelling urge is our desire to comprehend the scheme of things. What is our place in this scheme? How did the universe begin, if indeed it had a beginning? Will it end, and, if so, when and how? These questions are the province of cosmology, the study of the universe as a whole: its large-scale structure, its origin, and its ultimate fate.
Since the beginning of X-ray astronomy, it has been apparent that X-ray observations would have an impact on cosmology. In 1962, the first X-ray sources outside our solar system were discovered as a result of a rocket flight that lasted about five minutes. The detectors on this rocket also discovered evidence for an apparently uniform background glow of X-radiation. This background is the reference level against which radiation from stars and galaxies must be observed. If we look at the night sky in visible light, we see a large number of bright spots plus a few diffuse patches of light on a black background. In the X-ray sky, the equivalent of the black night sky does not exist. We see a bright, diffuse emission that is the same in all directions. The uniformity of the background radiation means that it must originate far outside our galaxy. It brings to us information about the depths of space and time, if only we could decode it.
A variety of observations indicate that the universe as we know it began some 15 to 20 billion years ago in an explosion from a hot dense state. This is the big-bang theory for the origin of the universe. The high energy radiation produced when the universe was very young and very hot would have been absorbed and degraded as the universe expanded and cooled. The microwave background radiation first observed by A. Penzias and R. Wilson in 1968 is thought to be a relic of this very early state, when the universe was only about a million years old. The uniformity of microwave background indicates that the universe was homogeneous until it was a few million years old. Then, sometime between a few million and a few billion years after the start of the expansion of the universe, the universe made a transition from a uniform, featureless gas to the present state. In the present....
....state, the matter in the universe is not uniformly distributed, but is in clumps of stars and galaxies. Why and how this transition from smoothness to clumpiness occurred is one of the great unanswered questions of modern cosmology. The X-ray background must have been produced sometime during this period, so studies of the background provide insight into this important phase of the universe.
Two theories currently vie to explain the X-ray background radiation. Both of them have support from the observations, and both have problems. Both cannot be right.
One theory for the origin of the X-ray background is that it comes from a 400 million degree gas spread throughout the space between the galaxies. The results of the HEAO 1 experiments show that the spectrum, that is, the distribution of the radiation according to wavelength, of the X-ray background matches very closely that expected from such a hot gas. Using the theory of how a hot gas radiates X-rays, the amount of intergalactic  hot gas needed to explain the observed intensity of the X-ray background radiation can be computed. It is found that the total mass of this gas would be greater than all the matter in all the galaxies. This is not necessarily as unreasonable as it might sound. Some, and perhaps even a large fraction, of the gas that collapsed to form galaxies might have been left over in the form of a hot intergalactic gas. The density of the hot gas necessary to explain the X-ray background radiation would be very nearly that needed to close the universe, that is, to halt the present cosmic expansion and turn it into a collapse billions of years from now. In this event, the universe would be finite, and the universe as we know it would end in a cosmic fireball produced by the collapse.
There are serious problems with the hot gas theory for the X-ray background, however. The energy required to heat the gas to 400 million degrees would be enormous. Perhaps this energy could be derived from the collapse of matter to form galaxies, or perhaps some other means could be used to heat the gas, such as the energy given off by quasars and exploding galaxies. Furthermore, if the hot intergalactic gas exists, then it should evaporate all the gas trapped in clusters of galaxies. This obviously has not happened, since gas is observed in clusters of galaxies.
The other explanation for the X-ray background radiation is that it is caused by the superposition of many very distant individual sources, such as quasars. The X-ray telescope on HEAO 2 offered a way to decide between the two hypotheses, namely, a diffuse hot gas and distant individual sources. If the background is truly composed of a diffuse hot gas, then no matter how long an X-ray telescope looks at a particular region of the sky, the picture should not change. It should show a diffuse glow at all levels of sensitivity. Suppose, on the other hand, that the background radiation only appears to be smooth and is in reality composed of many individual sources. Then as fainter and fainter limiting fluxes of radiation are reached, more and more sources should appear.
That is what has happened. In two deep surveys of blank fields of the sky, that is, regions empty of known radio, optical, or X-ray sources, dozens of new sources have been found. About two-thirds of these sources are quasars. The surveys analyzed so far indicate that at least 30 percent of the X-ray background is due to individual sources such as quasars. It is known that there were more bright quasars in the past, about the time when galaxies were thought to be forming, so we should find more and more quasars as we look to greater distances, since we are in effect looking back further in time. Studies of the quasar population confirm this trend. This indicates that more than 50 percent and possibly all of the X-ray background could be due to quasars and other active galactic nuclei.
Can the observed spectrum, which is consistent with radiation from a hot gas, be explained in terms of radiation from distant quasars and active galaxies? In an effort to answer this and other questions, HEAO I investigators have taken the spectra of a large number of quasars and active  galaxies. These spectra do not match the X-ray background spectrum. Either the quasars and active galaxies once radiated in such a way as to produce a different spectrum of X-radiation from what they are presently observed to produce, or they are not the source of the X-ray background.
In order to resolve the problems and contradictions, more information is necessary. Instruments even more sensitive than those used in the HEAO experiments will make it possible to distinguish between the alternatives, and we will be closer to understanding such fundamental questions as how and when did galaxies form? How did the universe begin? How will it end, if ever?