Astroparticle Physics

11.5 The Monopole and Dipole Terms 237by the WMAP experiment, which finds a temperature differenceofTT = 1.23 × 10−3 . (11.15)The temperature appears highest in the direction (right ascensionand declination, see Appendix C for the definition ofthese coordinates) (α, δ) = (11.20 h , −7.22 ◦ ). This temperaturevariation has a simple interpretation, namely, the movementof the Earth through the (local) unique reference framein which the CMB has no dipole anisotropy. This frame is insome sense the (local) ‘rest frame’ of the universe. The solarsystem and with it the Earth are moving through it with aspeed of v = 371 km/s towards the constellation Crater (betweenVirgo and Hydra). The CMB is blueshifted to slightlyhigher temperature in the direction of motion and redshiftedin the opposite direction. This dipole pattern in the map ofthe CMB temperature is shown in Fig. 11.2. The map is anequal-area projection in galactic coordinates with the planeof the Milky Way running horizontally through the plot.dipole anisotropylocal rest frameFig. 11.2Map of the CMB temperaturemeasured by the COBE satellite.The dipole pattern is due to themotion of the Earth through theCMB (from [27]) {27}11.5.1 Small-Angle AnisotropyIf small density fluctuations were to exist in the early universe,then one would expect these to be amplified by gravity,with more dense regions attracting even more matter, untilthe matter of the universe was separated into clumps. Thisis how galaxy formation is expected to have taken place.Given that one sees a certain amount of clumpiness today,one can predict what density variations must have existedat the time of last scattering. These variations would correspondto regions of different temperature, and so from thefluctuationsand galaxy formation