Neutrino masses and cosmology with Lyman-alpha forest power spectrum

We present constraints on neutrino masses, the primordial fluctuation spectrum from inflation, and other parameters of the $Lambda$CDM model, using the one-dimensional Ly$alpha$-forest power spectrum measured by Palanque-Delabrouille et al. (2013) from SDSS-III/BOSS, complemented by Planck 2015 cosmic microwave background (CMB) data and other cosmological probes. This paper improves on the previous analysis by Palanque-Delabrouille et al. (2015) by using a more powerful set of calibrating hydrodynamical simulations that reduces uncertainties associated with resolution and box size, by adopting a more flexible set of nuisance parameters for describing the evolution of the intergalactic medium, by including additional freedom to account for systematic uncertainties, and by using Planck 2015 constraints in place of Planck 2013. Fitting Ly$alpha$ data alone leads to cosmological parameters in excellent agreement with the values derived independently from CMB data, except for a weak tension on the scalar index $n_s$. Combining BOSS Ly$alpha$ with Planck CMB constrains the sum of neutrino masses to $sum m_ u < 0.12$ eV (95% C.L.) including all identified systematic uncertainties, tighter than our previous limit (0.15 eV) and more robust. Adding Ly$alpha$ data to CMB data reduces the uncertainties on the optical depth to reionization $tau$, through the correlation of $tau$ with $sigma_8$. Similarly, correlations between cosmological parameters help in constraining the tensor-to-scalar ratio of primordial fluctuations $r$. The tension on $n_s$ can be accommodated by allowing for a running ${mathrm d}n_s/{mathrm d}ln k$. Allowing running as a free parameter in the fits does not change the limit on $sum m_ u$. We discuss possible interpretations of these results in the context of slow-roll inflation.

The one-dimensional Ly-alpha forest power spectrum from BOSS

We have developed two independent methods to measure the one-dimensional power spectrum of the transmitted flux in the Lyman-$alpha$ forest. The first method is based on a Fourier transform, and the second on a maximum likelihood estimator. The two methods are independent and have different systematic uncertainties. The determination of the noise level in the data spectra was subject to a novel treatment, because of its significant impact on the derived power spectrum. We applied the two methods to 13,821 quasar spectra from SDSS-III/BOSS DR9 selected from a larger sample of over 60,000 spectra on the basis of their high quality, large signal-to-noise ratio, and good spectral resolution. The power spectra measured using either approach are in good agreement over all twelve redshift bins from $<z> = 2.2$ to $<z> = 4.4$, and scales from 0.001 $rm(km/s)^{-1}$ to $0.02 rm(km/s)^{-1}$. We determine the methodological and instrumental systematic uncertainties of our measurements. We provide a preliminary cosmological interpretation of our measurements using available hydrodynamical simulations. The improvement in precision over previously published results from SDSS is a factor 2--3 for constraints on relevant cosmological parameters. For a $Lambda$CDM model and using a constraint on $H_0$ that encompasses measurements based on the local distance ladder and on CMB anisotropies, we infer $sigma_8 =0.83pm0.03$ and $n_s= 0.97pm0.02$ based on ion{H}{i} absorption in the range $2.1<z<3.7$.