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Diagram showing COBE spectrum of the background radiation with best fit to 2.725 K.The errors are so small that they do not show up on the data.The upper part of the diagram has the best fit spectrum, and the lower part has the error bars on an expanded scale. [NASA Goddard Space Flight Center and the COBE Science Working Group]

Origin of the dipole anisotropy of the cosmic background radiation.We see an observer in the center, moving to the right.This produces a Doppler shift to the blue on the right, to the red on the left.

Origin of the dipole anisotropy of the cosmic background radiation.We see an observer in the center, moving to the right.This produces a Doppler shift to the blue on the right, to the red on the left.

should appear isotropic. The first observations found the radiation to be isotropic, but did not test this to a high degree of accuracy. More sensitive observations have looked for small deviations from perfect isotropy, called anisotropies. In describing any anisotropies in the background radiation, there are two relevant quantities: (1) how strong is the deviation, and (2) over what angular scale on the sky is the deviation seen?

One local source of anisotropy is the combined motion of the Earth, Sun, galaxy, local group and local supercluster with respect to the general Hubble flow, as indicated schematically in Fig. 21.9. This motion produces a Doppler shift, which varies with direction. If our net motion is with a speed v, in a particular direction, then the radial velocity away from the direction we are heading is

In the direction in which we are heading (6 = 0°), we get a maximum blueshift, and the radiation appears slightly hotter. In the opposite direction (6 = 180 °), we get a maximum redshift, and the radiation appears slightly cooler. Since this anisotropy is characterized by a hot pole and a cold pole, with smooth variations in between, we call it the dipole anisotropy. We should point out that such an anisotropy doesn't violate special relativity by providing a preferred reference frame for the universe. We just happen to be measuring our velocity with respect to the matter that last scattered the background radiation. In fact, if we accurately determined Hubble's constant separately in different directions, we should measure the same anisotropy. (In fact, we do, in that this shows up in redshift surveys, primarily as our motion towards the great attractor, discussed in Chapter 18.)

As Fig. 21.10 shows, this dipole anisotropy has been observed by COBE. When we correct for the motions of the Earth and Sun within the galaxy, we find a Doppler shift corresponding to a motion of 600 km/s towards the great attractor.

There have also been searches for anisotropies on smaller angular scales. The goal of these studies is to learn more about the structure of the universe when the temperature was about 3000 K. After all, the background radiation retains an almost perfect record of that era. In looking for these anisotropies, we recognize that the universe

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