An Outline of the ‘Visible’ Universe
The physics we have dealt with so far has been largely explored and appropriate theories developed using controlled laboratory experiments. These experiments have been used over the years to provide information about a wide variety of physical processes from the sub atomic through to the macroscopic behaviour of complex materials. In turn, many experiments have been specifically designed to test the predictions of new physical theories and, as a result, revolutionary changes have taken place for example, the development of quantum and relativistic mechanics. However, in approaching the structure and behaviour of the universe itself, it is no longer possible to ‘set up’ controlled laboratory experiments. The evolution of the universe and the behaviour of its components stars, galaxies etc-is a natural ongoing physical process over which we have no control. The best that can be done is to design experiments to study particular aspects of these processes. Because of the massive size of the universe and the extreme conditions that exist in stars such study can only be conducted by observing physical processes from a distance. The oldest way of doing this is simply by studying the light emitted from stars through optical telescopes which, over the years, have become increasingly large and sophisticated. The largest are reflecting telescopes using concave mirrors with diameters up to around 10 m which are placed at high altitudes so as to minimize the distorting effects of the earth’s atmosphere. However light is only a minute part of the electromagnetic spectrum and celestial objects emit electromagnetic radiation throughout this spectrum. Such radiation is not ‘visible’ in the strictest sense of the word but is, of course, detectable by appropriate instrumentation. Most parts of this wider spectrum are unable to penetrate the earth’s atmosphere but wavelengths in the millimetre to a few metres range are readily detectable and radio telescopes using one very large or several smaller discs (like large domestic satellite dishes) have provided much useful information. Radiation in the infrared, microwave and x-ray region has to be detected using instruments carried in rockets or satellites above the earth’s atmosphere. Altogether, these different approaches have provided a wide spread of information about the electromagnetic radiation in space and, in turn, about its origins. In addition, there are other forms of radiation which can be detected. Cosmic rays, which consist mostly of protons together with a few alpha particles and light nuclei, some of which can have very high energies, are continually bombarding the earth. They probably originate in supernovae and quasars and lead to the creation of showers of other particles when they collide with atomic nuclei in the atmosphere. There is also a steady stream of neutrinos from the nuclear reactions in stars, which pass through the earth and which hardly interact at all because they only experience the weak interaction. In consequence, they are extremely difficult to detect. Resulting from a study of the electromagnetic radiation detected in space the following outline description of the nature of the physical universe emerges. We live on the earth, a nearly spherical planet with diameter 1.27×10⁴ km, which moves around the sun during the year in an elliptical path at an average distance of 1.5×10⁸km. There are eight other planets of various sizes all orbiting in the same direction and in roughly the same plane-the ecliptic some nearer to (Mercury, Venus) and some further from (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) the sun than the earth. In addition there is an assortment of debris-asteroids (very small planets mostly between Mars and Jupiter), meteoroids and comets also in orbit. The sun is a medium-size star having a diameter of about 1.4×10⁶km and is just one of about 10^l1 stars which are gathered together in the galaxy which we see at night as the Milky Way. This galaxy is in the form of a disc with a diameter around 10^l8 km (10⁵ light years, where 1 light year is the distance travelled by light in one year, namely 9.46×10¹²k m). The detailed shape of the disc is unclear but it has roughly the form shown schematically in figure l.l(a) and is surrounded by a halo of stars. It bulges in the centre and the solar system is about two thirds of the way to the edge. Our nightly view of the Milky Way is 'edge on' and arises from looking inwards towards the centre of the disc, this view being somewhat obscured by the presence of interstellar dust. The component stars of the galaxy are slowly orbiting about its centre taking around lo8 years to complete an orbit. Our galaxy is called a spiral galaxy since, away from the centre the stars are in spiral arms as shown in figure l.l(b). There are many galaxies like this, but there are many others which are not in the form of spiral discs and which are ellipsoidal in shape, and then others fall in between these two shapes. They also vary in size, some containing as few as 10⁶ stars and others as many as 10¹³. There are around 10^l1 galaxies in the visible universe. Many of them are gathered together in clusters, which may contain several thousand galaxies held together by their mutual gravitational attraction. The distribution of galaxies and clusters is rather like that of the material in a sponge: there are sheets and filaments of galaxies and there are also many large gaps. So, although uneven on the small scale, the visible universe appears to be essentially uniform, as with a sponge, on the large scale. This uniformity is referred to as the cosmological principle.

Figure 1.1: A schematic representation of our galaxy: (a) edge on; (b) a full view.
The foregoing brief description of the visible universe is extremely superficial but it does give a framework within which more detailed discussion of its different features can be conducted. The first task is to consider the information that can be derived from the study of the electromagnetic radiation generated within the universe.