The types of topological defects that can appear in gauge theories – domain walls, strings and monopoles – are described in the following. The possibility that such defects were generated at phase transitions in the very early history of the Universe is discussed, in particular the idea that cosmic strings may provide the seeds for the density perturbations from which galaxies form.

Introduction

The subject that I want to discuss did not exist in Erwin Schrödinger's time. But it forms a natural bridge between two of his major interests – the fundamental physics of the smallest and the largest scales.

As Yang describes in this volume (pp. 53–64) Schrödinger contributed very significantly to the early development of gauge theories – the fundamental framework for all our present understanding of elementary particles and their interaction. He also made influential contributions to cosmology.

At the time these two fields were essentially separate. Indeed one might say that our failure so far to reconcile the basic physical theories of the small and the large is the major outstanding problem in physics. Schrödinger was clearly aware of the lack. He was very interested in any attempt to bridge the gap, for example in Eddington's ambitious though unsuccessful fundamental theory (Schrödinger, 1938).

A final reconciliation of relativity and quantum theory still eludes us, but cosmology and particle physics are no longer poles apart.