1 Introducing cosmology
General relativity has a very different conceptual basis from that of Newtonian mechanics. Its success in accounting for the precession of Mercury's orbit, and the bending of light by massive objects like the Sun, gives us confidence that our picture of space and time should be Einstein's rather than Newton's. In this and the following courses, we turn our attention to the study of the large-scale structure of spacetime. We see how spacetime as a whole is curved by the gross distribution of mass and energy in the Universe. This distribution, together with the question of how the Universe has developed over time, is the subject of cosmology.
Astronomy and cosmology are subjects that merge into one another with the single combined aim of understanding the structure and history of the Universe. The basis and motivation for the whole subject area comes ultimately from astronomers’ observations. Since the days of Galileo, optical telescopes of ever greater size have been made. In the last 60 years, it has been possible to study an increasing range of the electromagnetic spectrum as different types of telescope have become available. The first radio telescopes were made just before World War II. Infrared, ultraviolet, X-ray and gamma ray telescopes then followed, often operating from spacecraft above the Earth's atmosphere. These methods have become so complicated that we have attempted to outline only certain results; a far longer course would have been necessary to do anything like justice to the delicacy and sophistication of the techniques involved.
What have we learned from these new techniques? Firstly, matter has been detected in a wide variety of forms – interstellar gas as well as the stars themselves, for instance. Secondly, it has become possible to perform detailed studies of the radiation from the very ancient Universe. These developments have given cosmologists a more comprehensive list of the forms of mass and energy that govern the spacetime of the Universe. Next, matter has been detected at much greater distances – the more distant view providing cosmologists with more telling tests of their models. Then there has been the increasingly detailed information on spectra, with its evidence on the compositions of the emitting bodies. This information is of particular concern to the astrophysicist, who tries to understand the evolution of stars and other types of matter. Such evolution is governed by stellar dynamics and the processes of nuclear physics. The latter will be touched on later, though with reference more to nuclear reactions taking place in the very early Universe than to those occurring in the stars today.
We hope to build a bridge between the two extremes – the raw data obtained by the astronomer, and the metric parameters derived by the cosmologist. Thus, we examine the reasons for believing that the Universe had its origins in the big bang. We shall find that there are three independent pieces of evidence all pointing to the same conclusion. Sections 2 to 4 introduce you to the first of these.