detailed chemical analysis provided by WFMOS,we will, for the first time, be able to reunite thelong-dispersed stars from ancient accretion events,completely dissecting the Milky Way and layingbare its history . We will then be able to directlydetermine to what extent the Galaxy was builtfrom dwarf galaxies that fell in through the localcosmic web.What are the future challenges in understand-ing our local cosmology? Clearly , we are enteringthe age where near-field archaeo logists will beawash with observational data, and we will befaced with the kinematics of a b illion stars anddetailed chemistry of several million stars notonly in the Milky Way but also within our nearestneighbors. 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Astron.Astrophys. 40, 487 (2002).10.1126/science.1152008PERSPECTIVENumerical Simulations Unravelthe Cosmic WebClaude-André Faucher-Giguère,* Adam Lidz, Lars HernquistThe universe is permeated by a network of filaments, sheets, and knots collectively forming a“cosmic web.” The discovery of the cosmic web, especially through its signature of absorption oflight from distant sources by neutral hydrogen in the intervening intergalactic medium, exemplifiesthe interplay between theory and experiment that drives science and is one of the great examplesin which numerical simulations have played a key and decisive role. We recount the milestonesin our understanding of cosmic structure; summarize its impact on astronomy, cosmology, andphysics; and look ahead by outlining the challenges faced as we prepare to probe the cosmicweb at new wavelengths.Cosmologists envision a universe made upof filaments, knots, and sheets reminis-cent of pancakes (1), dubbed the “cosmicweb” by Bond and collaborators (2). This pictureof the cosmos—now well entrenched even inpopular culture—is, however , much more thanmere fantasy . Indeed, it is one of the best-established results of cosmological research andunderpins much of our contemporary understand-ing of large-scale structure and galaxy formation.After Schmidt’s spectroscopic observations ofa distant quasar (a fantastically bright point sourcelocated at a cosmological distance, later under-stood to be powered by a supermassive black holeaccreting from its host galaxy) in 1965 (3), Gunnand Peterson (4) and (independently) Scheuer (5)and Shklovski (6) pointed out that the spectra ofdistant quasars should show absorption by neutralhydrogen along the line of sight blueward of theirLyman-a (Lya) emission line. Refining this cal-culation, Bahcall and Salpeter (7) argued that theclumpy intergalactic gas should give rise to acollection of discrete absorption lines, termed the“Lya forest.” The following year, observers, par-ticularly L ynds and co-workers (8), indeed sawthose lines.Their exact nature, however , gave rise to a moreextended and heated debate. Were the lines—then poorly resolved—really of cosmologicalorigin, or were they instead associated with thequasars themselves? The extreme conditionsneeded for quasar ejecta to produce the Lyaforest (9), the random redshift distribution of theabsorption lines (10), as well as the association ofmetal-rich systems with intervening galaxies (11),eventually left no doubt that most of the L yaclouds are in fact cosmological.Inspired by research on the interstellar medium,the L ya absorbers w ere originally e nvisioned asde n se , cool clouds confined by pressure within ahotter , tenuous intercloud background (12, 13).However , this model raised questions of its own,namely, how did the clouds form in the firstplace? Ultimately, the model simply failed toaccount for the observed distribution of neutralhydrogen column densities, and other confine-ment mechanisms involving gravity were alsofound unsatisfactory.The key breakthrough in elucidating the na-ture of the L y a forest came as astrophysiciststried to understand the formation of structure inthe universe. In our current model of structureformation, tiny fluctuations in the primordialplasma grew through the gravitational instability,eventually forming a cosmic network of knots,filaments, and sheets. Because gravity is a