Astroparticle Physics

11.6 Determination of Cosmological Parameters 243are shown in Table 11.1. Among the most important of these,one sees that the Hubble constant is now finally known toan accuracy of several percent, and the value obtained is ingood agreement with the previous average (h = 0.7 ± 0.1).Furthermore, the universe is flat, i.e., Ω is so close to unitythat surely this cannot be a coincidence.value andparameterexperimentalerrorHubble constant h 0.71 +0.04−0.03ratio of total energy density to ϱ c , Ω 1.02 ± 0.02baryon-to-photon ratio η 6.1 +0.3−0.2 × 10−10baryon energy density over ϱ c , Ω b 0.044 ± 0.004matter energy density over ϱ c , Ω m 0.27 ± 0.04vacuum energy density over ϱ c , Ω Λ 0.73 ± 0.04Table 11.1Some of the cosmologicalparameters determined by theWMAP experiment throughmeasurement of the CMB angularpower spectrumData from the Cosmic Background Imager (CBI) [33]– an array of antennae operating at frequencies from 26 to36 GHz in the Atacama Desert – confirm the picture of a flatuniverse, giving Ω = 0.99 ± 0.12 in agreement with resultsfrom the Boomerang 2 and Maxima 3 experiments, and, ofcourse, with WMAP.In addition, WMAP has determined the baryon-to-photonratio η to an accuracy of several percent. The value foundis a bit higher than that obtained from the deuterium abundance,η = (5.1 ± 0.5) × 10 −10 , but the latter measurement’serror did not represent the entire systematic uncertaintyand, in fact, the agreement between the two is quitereasonable. The fact that two completely independent measurementsyield such close values is surely a good sign. Thedensities from non-relativistic matter Ω m and from vacuumenergy Ω Λ are similarly well determined, and these measurementsconfirm earlier values based on completely differentobservables.Further improvement in the determination of theseand other parameters is expected from the PLANCKproject (formerly called COBRAS/SAMBA), scheduled tobe launched by the European Space Agency in 2007 [34].2 Boomerang – Balloon Observations of Millimetric ExtragalacticRadiation and Geophysics3 Maxima – Millimeter Anisotropy Experiment Imaging Arraybaryon-to-photon ratiomeasured by WMAPimproved measurementsin future