Constraints on Cosmological Parameters from the Lyα Forest Power Spectrum andCOBEDMR

We combine COBE-DMR measurements of cosmic microwave background anisotropywith a recent measurement of the mass power spectrum at redshift z=2.5 from Lyaforest data to derive constraints on cosmological parameters and test theinflation+CDM scenario of structure formation. By treating the inflationaryspectral index n as a free parameter, we can find successful fits to the COBEand Lya forest constraints in Omega_m=1 models with and without massiveneutrinos and in low-Omega_m models with and without a cosmological constant.Within each class of model, the combination of COBE and the Lya forest P(k)constrains a parameter combination of the form (Omega_m h^a n^b Omega_b^c),with different indices for each case. This new constraint breaks some of thedegeneracies in cosmological parameter determinations from other measurements.The Lya forest P(k) provides the first measurement of the slope of the linearmass power spectrum on ~Mpc scales, and it confirms a basic prediction of theinflationary CDM scenario: a nearly scale-invariant spectrum of primevalfluctuations (n~1) that bends towards k^{n-4} on small scales. Consideringadditional observational data, we find that COBE-normalized, Omega_m=1 modelsthat match the Lya forest P(k) do not match the observed masses of rich galaxyclusters and that a low-Omega_m model with a cosmological constant provides thebest overall fit, even without the direct evidence for cosmic acceleration fromsupernovae. Modest improvements in the Lya forest P(k) measurement couldgreatly restrict the allowable region of parameter space for CDM models,constrain the contribution of tensor fluctuations to CMB anisotropy, andachieve a more stringent test of the current consensus model of structureformation.Comment: 26 pages, 6 figs, ApJ, in press (10/01). New version corrects some numerical results for Lambda-CDM models, updates discussion in light of new observational developments (especially CMB). Main conclusions unchange