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The Lyman-alpha Forest as a Cosmological Tool

Abstract
We review recent developments in the theory of the Ly forest and their implications for the role of the forest as a test of cosmological models. Simulations predict a relatively tight correlation between the local Ly optical depth and the local gas or dark matter density. Statistical properties of the transmitted flux can constrain the amplitude and shape of the matter power spectrum at high redshift, test the assumption of Gaussian initial conditions, and probe the evolution of dark energy by measuring the Hubble parameter H(z). Simulations predict increased Ly absorption in the vicinity of galaxies, but observations show a Ly deficit within r ~ 0.5h–1 Mpc (comoving). We investigate idealized models of "winds" and find that they must eliminate neutral hydrogen out to comoving radii ~ 1.5h–1 Mpc to marginally explain the data. Winds of this magnitude suppress the flux power spectrum by ~ 0.1 dex but have little effect on the distribution function or threshold crossing frequency. In light of the stringent demands on winds, we consider the alternative possibility that extended Ly emission from target galaxies replaces absorbed flux, but we conclude that this explanation is unlikely. Taking full advantage of the data coming from large telescopes and from the Sloan Digital Sky Survey will require more complete understanding of the galaxy proximity effect, careful attention to continuum determination, and more accurate numerical predictions, with the goal of reaching 5 – 10% precision on key cosmological quantities.
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Date
2003-01-01
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