Cosmological parameters from combining the Lyman-alpha forest with CMB, galaxy clustering and SN constraints
Uros Seljak, Anze Slosar, Patrick McDonald
TL;DR
The paper addresses tightening cosmological parameter constraints by combining the Ly-α forest power spectrum with CMB, SN, and galaxy clustering data, exploiting a PCA-based external constraint on the mean flux decrement to sharpen Ly-α information. Using a broad cosmological parameter space and robust modeling of nonlinearities, bias, BAO, and redshift-space effects, the authors find that Ly-α data raise the inferred fluctuation amplitude to about σ8 ≈ 0.85 and slightly increase the optical depth τ, while ns remains near 0.965 and there is no strong evidence for running. The analysis yields tight limits on neutrino properties (Σmν < 0.17 eV at 95%), a modest preference for extra relativistic degrees (N_eff ≈ 5.3), a nearly flat universe with small negative curvature (Ωk ≈ -0.003), and a dark energy equation of state close to w ≈ -1.04, with Gμ constraints on cosmic strings. A mild tension exists between Ly-α and WMAP3 amplitudes, though the inclusion of additional data reduces the discrepancy; overall, the results significantly improve small-scale power and neutrino constraints, and they demonstrate the value of integrating Ly-α forest information into joint cosmological analyses.
Abstract
We combine the Ly-alpha forest power spectrum (LYA) from the Sloan Digital Sky Survey (SDSS) and high resolution spectra with cosmic microwave background (CMB) including 3-year WMAP, and supernovae (SN) and galaxy clustering constraints to derive new constraints on cosmological parameters. The existing LYA power spectrum analysis is supplemented by constraints on the mean flux decrement derived using a principle component analysis for quasar continua, which improves the LYA constraints on the linear power. We find some tension between the WMAP3 and LYA power spectrum amplitudes, at the ~2 sigma level, which is partially alleviated by the inclusion of other observations: we find σ_8=0.85\pm 0.02 compared to sigma_8=0.80 \pm 0.03 without LYA. For the slope we find ns=0.965\pm0.012. We find no evidence for the running of the spectral index in the combined analysis, dn/dln k=-(1.5\pm 1.2) x 10^{-2}, in agreement with inflation. The limits on the sum of neutrino masses are significantly improved: $\sum m_ν<0.17\eV$ at 95% (<0.32eV at 99.9%). This result, when combined with atmospheric and solar neutrino mixing constraints, requires that the neutrino masses cannot be degenerate, m_3/m_1>1.3 (95% c.l.). Assuming a thermalized fourth neutrino we find m_s<0.26\eV at 95% c.l. and such neutrino cannot be an explanation for the LSND results. In the limits of massless neutrinos we obtain the effective number of neutrinos N_ν^{\rm eff}=5.3^{+0.4}_{-0.6}{}^{+2.1}_{-1.7}{}^{+3.8}_{-2.5} and N_ν^{\rm eff}=3.04 is allowed only at 2.4-sigma. The constraint on the dark energy equation of state is w=-1.04\pm 0.06. The constraint on curvature is Omega_k=-0.003\pm 0.006. Cosmic strings limits are Gμ<2.3 x 10^{-7} at 95% c.l. and correlated isocurvature models are also tightly constrained.