Astroparticle Physics

238 11 The Cosmic Microwave Backgroundobserved large-scale structure of the universe one expectedto see anisotropies in the CMB temperature at a level ofaround one part in 10 5 .These small-angle anisotropies were finally observed bythe COBE satellite in 1992. COBE had an angular resolu-tion of around 7 ◦ and could therefore determine the powerspectrum up to a multipole number of around l = 20. In thefollowing years, balloon experiments were able to resolvemuch smaller angles but with limited sensitivity. Finally, in2003 the WMAP project made very accurate measurementsof the CMB temperature variations with an angular resolutionof 0.2 ◦ . One of the maps (with the dipole term subtracted)is shown in Fig. 11.3.angular resolutionof COBE and WMAPFig. 11.3Cosmographic map of the CMBtemperature measured by theWMAP satellite with the dipolecomponent subtracted (from [28]){28}Because of the better angular resolution of WMAP comparedto COBE, it was possible to make an accurate measurementof the angular power spectrum up to l ≈ 1000, asshown in Fig. 11.4.11.6 Determinationof Cosmological Parameters“There are probably few features of theoreticalcosmology that could not becompletely upset and rendered uselessby new observational discoveries.”Sir Hermann Bondiconclusions from the CMBangular power spectrumThe angular power spectrum of the CMB can be used tomake accurate determinations of many of the most importantcosmological parameters, including the Hubble constant H ,the baryon-to-photon ratio η, the total energy density over