Particle Physics and Cosmology

In these lectures on the early Universe I will discuss some recent developments in particle cosmology, taking particular care to highlight the r^ ole of particle physics in our understanding of cosmology. I will assume that the reader is familiar with basic particle physics, but not necessarily basic astronomy. Before starting, I would like to discuss the motivation for particle physicists to be interested in cosmology. The aim of modern cosmology is to understand the origin and the large-scale structure of the Universe on the basis of physical law. The modern framework for this eeort is the hot big-bang model. With knowledge of the laws of physics, the fundamental forces, and the fundamental particles, in principle the model should be able to explain the gross features of our Universe. It is also possible tòreverse engineer' this standard approach: by observations of the outcome, we might be able to tell something about the fundamental ingredients that went in. Therefore we might be able to discover something about particle physics by studying cosmology. Let me also describe my own approach to cosmology. How should one approach the study of the history of the ancient Universe? There are two types of people who study old things, antiquarians and historians. An antiquarian is interested in things that are old simply because they are old. They do not attempt to diierentiate between the relative importance of objects from antiquity. In the extreme, an antiquarian would see no diierence between a grocery shopping list from 1215 and the Magna Carta. A historian on the other hand is interested in events and objects from the past because they have a bearing upon the present. It is the job of the historian to sort through the past to nd the objects and events that had an impact upon the future development of history. I consider it the job of the cosmologist to be a historian of the Universe. The cosmologist should not be interested in the early Universe because it was very old, or very hot, or very dense. Rather a cosmologist studies the early Universe because he or she has the faith that events in the early Universe are responsible for shaping the present Universe, and that it is impossible to understand the Universe today without an understanding of the early Universe. Therefore in these lectures I will concentrate on events in the early Universe that