Figure 17.12: The temperature of the cosmic background radiation as measured by the COBE satellite (Bennett et al., 1996). Three maps of the temperature on different scales are shown. The maps are in galactic coordinates, so the plane of the galaxy is the equator. At the top is a picture that could be a map at a temperature resolution of resolution well above a millikelvin. The temperature is completely uniform at 2.73 K. The middle map shows the temperature at a resolution of millikelvin. There is a dipole anisotropy attributable to the motion of the solar system with respect to the rest frame of the background radiation. When this anisotropy is subtracted out, the bottom figure is obtained, which shows the remaining anisotropies at a resolution of a microkelvin. The dark strip through the center is due to residual radiation from the galaxy. The remaining variations correspond to fluctuations at the time the radiation was emitted, some hundreds of thousands of years after the big bang.