Constraints on Cosmological Parameters from the Lyα Forest Power Spectrum and COBE-DMR

We combine COBE-DMR measurements of cosmic microwave background (CMB) anisotropy with a recent measurement of the mass power spectrum at redshift z = 2.5 from Lyα forest data to derive constraints on cosmological parameters and test the inflationary cold dark matter (CDM) scenario of structure formation. By treating the inflationary spectral index n as a free parameter, we are able to find successful fits to the COBE and Lyα forest constraints in Ωm = 1 models with and without massive neutrinos and in low-Ωm models with and without a cosmological constant. Within each class of model, the combination of COBE and the Lyα forest P (k) constrains a parameter combination of the form Ωmh nΩγb , with different indices for each case. This new constraint breaks some of the degeneracies in cosmological parameter determinations from other measurements of large scale structure and CMB anisotropy. The Lyα forest P (k) provides the first measurement of the slope of the linear mass power spectrum on ∼ Mpc scales, ν = −2.25± 0.18, and it confirms a basic prediction of the inflationary CDM scenario: an approximately scale-invariant spectrum of primeval fluctuations (n ≈ 1) modulated by a transfer function that bends P (k) towards kn−4 on small scales. Considering additional observational data, we find that COBE-normalized, Ωm = 1 models that match the Lyα forest P (k) do not match the observed masses of rich galaxy clusters and that a low-Ωm model with a cosmological constant provides the best overall fit to the available data, even without the direct evidence for cosmic acceleration from Type Ia supernovae. Modest improvements in the Lyα forest P (k) measurement and the CMB anisotropy constraint on n could greatly restrict the allowable region of parameter space for CDM models, distinguish between models with no tensor fluctuations and models with the power-law inflation Department of Physics, The Ohio State University, Columbus, OH 43210; [email protected] Department of Astronomy, The Ohio State University, Columbus, OH 43210; [email protected] Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138; rcroft,[email protected] Lick Observatory, University of California, Santa Cruz, CA 95064 Department of Physics and Astronomy, University of Massachusetts, Amherst, MA, 01003; [email protected] Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK; [email protected]