Cosmic Strings and the Origin of Structure in the Universe

Abstract

The observed universe is homogeneous and isotropic on the largest observable scales. The best evidence for this comes from observations of the cosmic background radiation (CBR). On smaller scales, a striking amount of structure can be seen — galaxies, clusters of galaxies, and the “large scale structure” in the form of possible filaments, bubbles, sheets or voids. The best proximate explanation for this structure is small amplitude perturbations in the early universe, which grew by gravitational instability into the observed large scale structure during the expansion of the universe. At some time in the future when we have a complete theory of the universe and its initial conditions — see James Hartle’s lectures in this volume1 for some promising ideas toward such a theory — both the overall homogeneity and the structure should be a calculable consequence of the theory. Until then, people have made partial progress toward understanding the genesis of structure on a homogeneous background, based on the laws of fundamental physics as currently known. At this time we have at least two possible fundamental mechanisms for generation of the conjectural initial perturbations, namely quantum fluctuations, or thermodynamic fluctuations of a particular sort. My purpose in these lectures is to review and outline the basic physical nature of these two mechanisms, leaving out the details. Both mechanisms are well reviewed in the literature, and the reader will be referred both to more comprehensive reviews and to the primary literature throughout these lectures.