However, surveys of galaxies out to just 5 percent of the distancerepresented by the CMBR show the universe to be clumpy and uneven(see Figure 2). How much farther must astronomers go to find smoothness, on average?Assuming that the homogeneous scale is finally found, what accountsfor the complex structure graded from smooth to rough, and how didthe universe evolve to this state? Cosmologists must rely heavilyon special instrumentation to make very large sky surveys (encompassinga million galaxies!) to obtain the data to test theoretical predictions. Those predictions are based on theidea that feeble variations (“bumps”) in the CMBR show us the seeds ofthe structure-formation process. Important data on the bumps willcome from new CMBR maps showing even finer details than those revealedby the COBE satellite. Understanding the connections between CMBRbumps and the large-scale structure seen now will challenge observersand theorists for some time to come.

Figure 2. Data from surveys of galaxies show the universe to be clumpy anduneven. The voids and “walls” that from the large-scale structure are mappedhere by 11,000 galaxies. Our galaxy, the Milky Way, is at the centerof the figure. The outer radius is at a distance of approximately450 million light-years. Obscuration by the plane of the Milky Wayis responsible for the missing pie-shaped sectors. North is at thetop. (Courtesy of Smithsonian Astrophysical Observatory, 1993. Northerndata—Margaret Geller and John Huchra, southern data—Luiz da Costa et al.)

  • What can we learn about physical laws from relics of the Big Bang? When the universe was only about a minute old, the density and temperaturewere just right to quickly generate most of the helium and all ofthe deuterium present in the universe today. The remarkable thingis that our physics seems to work in a time and place so distantfrom us, a striking confirmation that physics as we know it can beused to study the early universe, and vice versa. There are otherways to improve our understanding of physics using other relics ofthe Big Bang. A current example is the effort of elementary-particlephysicists to understand the observed ratio (with a value of 1010) of photons from the CMBR to hydrogen atoms in the universe. Theanswer may be found in elementary-particle theory.

  • Did the universe undergo inflation at a very early stage? Despite many successes, the standard, simplest Big Bang model hasserious problems. For example, large regions of the universe observedtoday (using the CMBR) were not in causal contact at the time theearly universe was dense enough to settle down to a uniform temperature.That is, they were not close enough together to have ever exchangedlight signals, or information of any sort. If the regions were neverin causal contact, how did they manage to reach the same temperature,as indicated by the all-sky CMBR measurements? Also, why is the densityof the universe so close (within a



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