We measure the filling factor, correlation function, and power spectrum of transmitted flux in a large sample of Lyα forest spectra, comprised of 30 Keck HIRES spectra and 23 Keck LRIS spectra. We infer the linear matter power spectrum P (k) from the flux power spectrum P F (k), using an improved version of the method of Croft et al. (1998) that accounts for the influence of redshift-space distortions, non-linearity, and thermal broadening on the shape of P F (k). The evolution of the shape and amplitude of P (k) over the redshift range of the sample (z ≈ 2−4) is consistent with the predictions of gravitational instability, implying that non-gravitational fluctuations do not make a large contribution to structure in the Lyα forest. Our fiducial measurement of P (k) comes from a subset of the data with 2.3 < z < 2.9, mean absorption redshift z = 2.72, and total path length ∆z ≈ 25. It has a dimensionless amplitude ∆ 2 (k p ) = 0.74 +0.20 −0.16 at wavenumber k p = 0.03(km s −1 ) −1 and is well described by a power-law of index ν = −2.43±0.06 or by a CDM-like power spectrum with shape parameter Γ ′ = 1.3 +0.7 −0.5 ×10 −3 (km s −1 ) −1 at z = 2.72 (all error bars 1σ). The correspondence to present day P (k) parameters depends on the adopted cosmology. For Ω m = 0.4, Ω Λ = 0.6, the best-fit shape parameter is Γ = 0.16h Mpc −1 , in good agreement with measurements from the 2dF Galaxy Redshift Survey, and the best-fit normalization is σ 8 = 0.82(Γ/0.15) −0.44 . Matching the observed cluster mass function and our measured ∆ 2 (k p ) in spatially flat cosmological models requires Ω m = 0.38 +0.10 −0.08 + 2.2(Γ − 0.15). Matching ∆ 2 (k p ) in COBEnormalized, flat CDM models with no tensor fluctuations requires Ω m = (0.29 ± 0.04)n −2.89 h −1.9 65 , and models that satisfy this constraint are also consistent with our measured logarithmic slope. The Lyα forest complements other observational probes of the linear matter power spectrum by constraining a regime of redshift and lengthscale not accessible by other means, and the consistency of these inferred parameters with independent estimates provides further support for a cosmological model based on inflation, cold dark matter, and vacuum energy.
Most of the matter in the Universe is not luminous, and can be observed only through its gravitational influence on the appearance of luminous matter. Weak gravitational lensing is a technique that uses the distortions of the images of distant galaxies as a tracer of dark matter: such distortions are induced as the light passes through large-scale distributions of dark matter in the foreground. The patterns of the induced distortions reflect the density of mass along the line of sight and its distribution, and the resulting 'cosmic shear' can be used to distinguish between alternative cosmologies. But previous attempts to measure this effect have been inconclusive. Here we report the detection of cosmic shear on angular scales of up to half a degree using 145,000 galaxies and along three separate lines of sight. We find that the dark matter is distributed in a manner consistent with either an open universe, or a flat universe that is dominated by a cosmological constant. Our results are inconsistent with the standard cold-dark-matter model.
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