Cosmic microwave background observations: implications for Hubble’s constant and the spectral parameters n andQ in cold dark matter critical density universes

Recent cosmic microwave background (CMB) measurements over a large range of angular scales have become sensitive enough to provide interesting constraints on cosmological parameters within a restricted class of models. We use the CMB measurements to study inflation-based, cold darkmatter (CDM) critical density universes.We explore the 4-dimensional parameter space having as free parameters, Hubble’s constant Ho, baryonic fraction Ωb, the spectral slope of scalar perturbations n and the power spectrum quadrupole normalizationQ. We calculate χ2 minimization values and likelihood intervals for these parameters. Within the models considered, a low value for the Hubble constant is preferred:Ho = 30+ 18 −7 km s −1Mpc−1. The baryonic fraction is not as well-constrained by the CMB data: Ωb = 0.07+ 0.24 −0.07 . The power spectrum slope is n = 0.91 + 0.20 −0.12 . The power spectrum normalization isQ = 18±2.5μK. The error bars on each parameter are approximately 1σ and are for the casewhere the other 3 parameters have beenmarginalized. If we condition onn = 1weobtain the normalizationQ = 17±1.0μK. The permitted regions of the 4-D parameter space are presented in a series of 2-D projections. In the context of the CDM critical density universes considered here, current CMBdata favor a low value for the Hubble constant. Such low-Ho models are consistent with Big Bang nucleosynthesis, cluster baryonic fractions, the large-scale distribution of galaxies and the ages of globular clusters; although in disagreement with direct determinations of the Hubble constant.