Perhaps the most striking illustration of the true unity of science is the development of the interdisciplinary field of ``particle cosmology.'' Particle physics examines nature on the smallest scales, while cosmology studies the universe on the largest scales. Although the two fields are separated by the scales of the objects they study, they are unified because it is impossible to understand the origin and evolution of large-scale structures in the universe without understanding the ``initial conditions'' that led to the structures. The initial data was set in the very early universe when the fundamental particles and forces acted to produce the perturbations in the cosmic density field. A complete understanding of the present structure of the universe will also be impossible without accounting for the dark component in the density field. The most likely possibility is that this ubiquitous dark component is an elementary particle relic from the early universe.
The study of the structure of the present universe may reveal insights into events which occurred in the early universe, and hence, into the nature of the fundamental forces and particles at an energy scale far beyond the reach of terrestrial accelerators. Perhaps the early universe was the ultimate particle accelerator, and will provide the first glimpse of physics at the scale of Grand Unified Theories (GUTs), or even the Planck scale.
As a cosmologist I am interested in events that happened a long time ago. But in studying the past, I believe it is best to take the approach of a historian rather than an antiquarian. Now an antiquarian and a historian are both interested in things from the past. But an antiquarian is interested in old things just because they are old. To an antiquarian, there is no difference between a laundry list from June 1215 and the Magna Carta: they are both equally old. A historian, on the other hand, is interested in the past because it shapes the present. The job of a historian is to sort through events of the past and see which are important and which are not. I am not interested in the early universe just because it happened a long time ago, or it was really hot, or it was a bang (a really, really big one). The real reason I study the early universe is that events which occurred in the early universe left an imprint upon the present universe.
In these lectures I will concentrate on two events which occurred in the early universe. The first is the generation of perturbation in the density field during an early period of rapid expansion known as cosmic inflation. The second is the genesis of dark matter. The record of these events is written in the arrangement of galaxies, galaxy clusters, and imperfection in the isotropy of the cosmic microwave background radiation. If we really understood particle physics, we could predict the nature of those patterns. If we really knew how to read the story in the structures, we would learn something about particle physics. The story is there on the sky, patiently waiting for our wits to become sharp enough to read it.
In these lectures I will discuss the early universe. So the first thing we must do is to follow the procedure outlined by William Shakespeare :
Now entertain conjecture of a time
When creeping murmur and the poring dark
Fills the wide vessel of the universe.
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Last modified: 3-Nov-98.