HOW FILAMENTS OF GALAXIES ARE WOVEN INTO THE COSMIC WEB

Large-scale structure in the distribution of galaxies is thought to ha ve evolved through gravitational instabilities from small density fluc tuations in the (largely homogeneous) early Universe. This structure o f galaxies consists of rich and poor clusters, connected by filaments and sheets, with regions largely devoid of galaxies (voids) in between (1). Numerical simulations of the growth of initial density fluctuatio ns through a nonlinear regime, motivated by the likely physics of the early Universe, also show a network of filaments and voids(2,3,18), bu t the origin of this picture of filaments as the dominant structure wa s not well understood. Here we show that the 'web' of filaments that d efines the final state in these simulations is present in the initial density fluctuations; the pattern of the web is defined largely by the rare density peaks in the initial fluctuations, with the subsequent n onlinear evolution of the structure bringing the filamentary net work into sharper relief. Applying these results to the observed galaxy dis tribution, we suggest that 'superclusters' are filamentary cluster-clu ster bridges, and we predict that the most pronounced filaments will b e found between clusters of galaxies that are aligned,vith each other and close together.