Galaxies in the Universe An Introduction

Since the early 1980s, multi-object spectrographs, CCD detectors, and some dedicated telescopes have allowed the mass production of galaxy red shifts. These large surveys have revealed a very surprising picture of the luminous matter in the Universe. Many astronomers had imagined roughly spherical galaxy clusters floating amongst randomly scattered field galaxies, like meatballs in sauce. Instead, they saw galaxies concentrated into enormous walls and long filaments, surrounding huge voids that appear largely empty. The galaxy distribution has been compared to walls of soapy water, surrounding bubbles of air in a basinful of suds; linear filaments appear where the walls of different soap bubbles join, and rich clusters where three or more walls run into each other. A more accurate metaphor is that of a sponge; the voids are interlinked by low-density ‘holes’ in the walls. Sometimes we think of the filaments as forming a cosmic web.

For a star like the Sun in the disk of our Milky Way, the task of finding where it formed is essentially hopeless, because it has already made many orbits about the galaxy, and the memory of its birthplace is largely lost. But the large structures that we discuss in this chapter are still under construction, and the regions where mass is presently concentrated reveal where denser material was laid down in the early Universe.