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Since the early 1980s, multi-object spectrographs, CCD detectors, and some dedicated telescopes have allowed the mass production of galaxy redshifts. 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 streamers, 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.

We must remember that in dynamical terms, this structure is very young. Galaxies are not highly concentrated into groups and clusters, in the way that luminous stars are largely confined within galaxies. Averaged over a rich cluster like Coma or Perseus, the density is only 10-100 times greater than the cosmic mean. So the gravitational forces binding a galaxy cluster or group together are relatively weak, and the time for an individual galaxy to move across the system is long. We saw in Section 6.5 that since the Big Bang, galaxies in the outskirts of the Coma cluster have not yet had time to travel to its core; in the Local Group, the large galaxies are falling together for the first time.

For all but the youngest stars within a galaxy, the task of finding where a given star was formed is essentially hopeless, because each 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: we see galaxies continually falling in to join them. The regions where mass is presently concentrated reveal where denser material was laid down in the early Universe. The peculiar motions of groups and clusters of galaxies, their speeds relative to the uniformly expanding cosmos, are largely motions of infall toward larger concentrations of mass. So the problem of understanding the large-scale structure that we see today becomes one of explaining small variations in the density of the early Universe.

We begin in Section 7.1 by surveying the galaxies around us, mapping out both the local distribution, and the larger structures stretching over hundreds of megaparsecs. The following section discusses the history of our expanding Universe, within which the observed spongy structures grew. In Section 7.3 we ask how regions where the mass density was higher or lower than average would have developed as the expansion proceeded, how gravity would have pulled surrounding material into regions where mass was already concentrated, and how we can use the observed peculiar motions of galaxies to estimate how much matter is present. The final section discusses various theories for the origin of the initial density fluctuations, and explains why we have no satisfactory explanation for the large-scale structures that we now observe.

Further reading: For a descriptive introduction to this material, T. Padmanabhan, 1998, After the First Three Minutes (Cambridge University Press, Cambridge, UK); an undergraduate-level text is M. Rowan-Robinson, 1996, Cosmology, 3rd edition (Oxford University Press, London); for a graduate text, J.A. Peacock, 1999, Cosmological Physics (Cambridge University Press, Cambridge, UK).

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