A MEETING WITH THE UNIVERSE

Chapter 4-3



The New Astronomy

One cannot perform laboratory experiments on stars and galaxies. For this reason, astronomy has always been a science of careful observation. Our two main sources of information about the universe are electromagnetic radiation (light, radio waves, infrared, X-rays, gamma rays, etc.) and cosmic rays, which are atomic particles that have been accelerated to high velocities and carry great amounts of energy. Each kind of electromagnetic radiation moves at the speed of light, while cosmic rays are slightly slower.

Electromagnetic radiation is electrically neutral, but cosmic rays carry an electric charge. As a result, light can travel through space in essentially straight lines, but cosmic rays spiral along the weak lines of magnetic force that permeate space. We can see where a beam of light or of X-rays has come from, but because of their spiralling, cosmic rays cannot be traced to their points of origin.

Before the Space Age, all astronomy was performed on the ground, limited by the Earth's atmosphere. Cosmic rays could not be observed directly, but it was possible to study the showers of energetic charged particles that they produce when they strike the atmosphere. The atmosphere absorbs almost all of the radiation that reaches the Earth from space, so stars and galaxies could only be seen at the limited wavelengths to which the atmosphere is transparent, primarily visible light and radio waves. Large telescopes were built to "see" at these wavelengths. Diameters of optical telescopes are measured in meters, while some radio telescopes are hundreds of meters across. Ground-based observations discovered star clusters, galaxies, cosmic radio sources, and the expansion of the universe. Optical and radio telescopes also discovered quasars and pulsars, two types of energetic objects that have gained in interest through studies from space.

Perhaps the most significant discovery of ground-based astronomy was that the universe is expanding, a result which led directly to the Big Bang theory of creation. As fragments fly apart from any explosion, the faster-moving pieces leave the slower moving pieces behind. A simple law applies to the various exploded parts: the further apart they are the faster they are moving apart. From the ground, we could see that galaxies as far away as 40 million light years were receding from us in accordance with just such a law. This law - that the velocity at which a galaxy recedes from us is 20 to 40 kilometers per second for each million light years of distance away from us - lets us determine the size and age of the universe. At great distances, everything is moving away; nothing is approaching us. About 10 to 20 billion light years away, the receding matter would have the speed of light, and observation to greater distances is impossible.

There are several alternative versions of the Big Bang theory and also some competing theories about the nature of the universe. We hope to learn whether the universe is "open", meaning that the expansion will continue forever, or if it is "closed", in which case the expansion will some day come to an end. In that event, the end of the expansion will be followed by a collapse phase, in which all the galaxies in space approach each other and eventually coalesce in a fiery end to the universe as we know it. To discriminate among these alternatives, we need to see further and make measurements of distant phenomena at appropriate wavelengths and with higher precision than heretofore. Hopefully, we can then resolve the uncertainties we face in applying the laws of physics at the very largest scales of energy and distance. Astronomy will advance greatly in the future when we launch large optical and radio telescopes above the interfering atmosphere, but the main achievement of space astronomy so far has not been in these traditional areas.

The Space Age has made it possible to see the universe in new kinds of light, notjust the visible light and radio waves that reach the ground. The most dramatic discoveries have come from the telescopes that observe ultraviolet radiation, X-rays, and gamma rays. Small telescopes to study cosmic rays have also been flown, and the first large cosmic ray telescopes were launched on the third High Energy Astronomy Observatory (HEAO-3) in September, 1979. From these telescopes in space has come a burst of discovery that rivals the revolution produced by the invention of the telescope itself in the early 1600's.

We are reaching toward a new understanding of the components of the universe: the stars, the galaxies, the strange pulsars and stranger quasars, even the almost-empty space that lies between the stars.



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