Astroparticle Physics

232 11 The Cosmic Microwave Backgroundphoton contributionto the cosmic energy densityprediction and observationof the CMBearth-based vs.satellite-supportedCMB measurementsa CMB temperature of around 5 K, which turned out to benot far off.Even without invoking BBN one can argue that the con-tribution of photons to the current energy density cannotexceed by much the critical density. If one assumes, say,Ω γ ≤ 1, then this implies T ≤ 32 K.Gamow’s prediction of the CMB was not pursued for anumber of years. In the 1960s, a team at Princeton (Dicke,Peebles, Roll, and Wilkinson) did take the prediction seriouslyand set about building an experiment to look forthe CMB. Unknown to them, a pair of radio astronomers,A. Penzias and R. Wilson at Bell Labs in New Jersey, werecalibrating a radio antenna in preparation for studies unrelatedto the CMB. They reported finding an “effective zenithnoise temperature . . . about 3.5 ◦ K higher than expected.This excess temperature is, within the limits of our observations,isotropic, unpolarized, and free from seasonal variations...” [24]. The Princeton team soon found out aboutPenzias’ and Wilson’s observation and immediately suppliedthe accepted interpretation [25].Although the initial observations of the CMB were consistentwith a blackbody spectrum, the earth-based observationswere only able to measure accurately the radiation atwavelengths of several cm; shorter wavelengths are stronglyabsorbed by the water in the atmosphere. The peak of theblackbody spectrum for a temperature of 3 K, however, isaround 2 mm. It was not until 1992 that the COBE satellitemade accurate measurements of the CMB from space. Thisshowed that the form of the energy distribution is extremelyclose to that of blackbody radiation, i.e., to a Planck distribution,as shown in Fig. 11.1.Wavelength [mm]2 1 0.67 0.5Fig. 11.1The spectrum of the CMBmeasured by the COBE satellitetogether with the blackbody curvefor T = 2.725 K. The error barshave been enlarged by a factor of400; any deviations from thePlanck curve are less than 0.005%(from [26])Intensity [MJy/sr]400300200100FIRAS data with 400σ errorbars2.725 K Blackbody00 5 10 15 20ν [1/cm]