Cosmological constraints from the CMB and Ly-alpha forest revisited

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  • 28 February 2003
Abstract
The WMAP team has recently highlighted the usefulness of combining the Ly-alpha forest constraints with those from the cosmic microwave background. This combination is particularly powerful as a probe of the primordial shape of the power spectrum. Converting between the Ly-alpha forest observations and the linear mass power spectrum requires a careful treatment of nuisance parameters and modeling with cosmological simulations. We point out several errors and inconsistencies in the previous treatments that propagate into the estimations and associated errors of cosmological parameters, including those reported by the WMAP team. The two most important are the insufficient range of cosmological parameters explored in simulations used to date and an incorrect treatment of the mean transmitted flux constraints. We employ a likelihood calculator for the current data set based on an extensive 6-dimensional grid of simulations. We show that the current uncertainties in the mean transmission and the flux power spectrum define a degeneracy line in the amplitude-slope plane. The errors are at least a factor of two larger in this plane than previous estimates. The CMB degeneracy due to the primordial power spectrum shape follows a similar relation in this plane. This weakens the statistical significance of the parameter constraints based on combined analysis derived by the WMAP team and makes the evidence for the running of the slope statistically insignificant. The simplest n=1 scale invariant model with dn/dln k=0 and no tensors has a Delta chi^2=4 compared to the best fitting model in which these 3 parameters are free. Current data therefore do not require relaxing these parameters to improve the fit.

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