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Fig 21.10.

COBE measurements of the dipole anisotropy in the cosmic background radiation.This plot shows the whole sky on a single plot, with the galactic plane going horizontally across the middle. Bright regions are slightly hotter than the average temperature; dark regions are slightly cooler than the average temperature. [NASA Goddard Space Flight Center and the COBE Science Working Group]

Fig 21.10.

COBE measurements of the dipole anisotropy in the cosmic background radiation.This plot shows the whole sky on a single plot, with the galactic plane going horizontally across the middle. Bright regions are slightly hotter than the average temperature; dark regions are slightly cooler than the average temperature. [NASA Goddard Space Flight Center and the COBE Science Working Group]

is not perfectly homogeneous. We are here, and our galaxy, cluster and supercluster are here. We can also see many other galaxies and clusters. For these objects to exist, there must have been very small seeds out of which they grew. That is, there must have been small concentrations of material around which more material could be attracted gravitationally. These small concentrations would also have been slightly hotter than their surroundings. This means that the radiation from these regions should appear slightly stronger than that from the surroundings. Theoreticians have suggested that these fluctuations in brightness will be very small, only about 10 the radiation itself.

of the strength of

Searches for these fluctuations have been carried out almost since the background radiation was discovered. However, the fact that they are so weak has made them quite elusive. The situation was resolved by COBE. In addition to measuring the spectrum of the radiation, COBE was also able to measure the distribution of radiation on the sky. The result of that map is shown in Fig. 21.11. In it, red regions are cooler than average, and blue regions are hotter than average. You can see that there is a mottled appearance, with structure all over the map. However, the signals are so weak, and the analysis is so difficult, that each bright spot you see does not exactly correspond to one bright area in the early universe. Each spot still has some important experimental uncertainties. However, the experimenters can use a statistical analysis of all of the bright spots, and tell us about their average properties. When this result was announced, cosmologists and the general public were quite excited. These fluctuations are so small that they are even a challenge to a satellite designed for the purpose. So, while the spectrum of the background radiation (like that in Fig. 21.8) was measured with great accuracy very early in the mission, the world had to wait another year to hear about the fluctuations. Of course, COBE can only sample on relatively large angular scales.

In order to study the anisotropies on a smaller scale, a balloon mission was launched over Antarctica in 1998. The project was known as Balloon Observations of Millimetric Extragalactic

Fig 21.11.

North Galactic Hemisphere

South Galactic Hemisphere I +100 nK

North Galactic Hemisphere

South Galactic Hemisphere I +100 nK

Fig 21.11.

COBE measurements of the small-scale anisotropies in the cosmic background radiation. The plot shows the whole sky viewed as two hemispheres. In this false color image, red areas are slightly cooler than the average and blue areas are slightly hotter. Notice that the largest fluctuations are 100 ^K, out of a total 2.7 K signal. [NASA Goddard Space Flight Center and the COBE Science Working Group]

Radiation and Geophysics, BOOMERANG. The results are shown in Fig. 21.12. In Fig. 21.12(a), we show the map of roughly 3% of the sky that was observed. In Fig. 21.12(b), we look at how theoretical simulations show how the fluctuations should look for different geometries of the universe (closed, flat, open). A more detailed analysis of this type provides very strong evidence that the universe is flat. In Fig. 21.12(c), we look at the relative amplitude of fluctuations on different angular scales. Extrapolating these fluctuations back to the time of decoupling, this tells us about the seeds of various scales of structure that we see, as described in Chapter 18. In Fig. 21.12(d), we see how the BOOMERANG data fit into various other constraints on various cosmological parameters. Remember, flat universes are ones for which

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