Once Before Time - Martin Bojowald [74]
Since Einstein’s equations allow one to compute the clumping of matter, starting from an initial configuration as can be inferred from the background radiation, a precise mapping of galaxies provides a further test of the theory. Such wide-ranging mappings have become possible with ever more sensitive telescopes detecting even distant galaxies and their positions. Although existing maps remain incomplete, the resulting images, in particular those of the Sloan Digital Sky Survey (SDSS), are impressive (see figure 19). The SDSS includes millions of galaxies in our cosmic neighborhood, showing indeed how homogeneously matter is distributed in the universe. By comparison, what our naked eyes show us in the sky around Earth is only meaningless detail—a historical chance event in the development of the universe and of our own existence.
This large-scale homogeneity turns out to be an immense stroke of luck for cosmology, for such a high degree of symmetry implies strong simplifications in solving Einstein’s equations. Otherwise one could not see the cosmic forest for all the galactic trees in their complicated evolution, all observations notwithstanding. To model the universe, so many details would have to be taken into account that no satisfactory explanations could be gained. Fortunately, the universe does help (or possibly fool?3) us.
Besides this confirmation of homogeneity, long confidently assumed by cosmologists, galaxy maps provide concrete data supporting conclusions drawn from the cosmic microwave background. Despite the strong homogeneity over large distances, the distribution of galaxies shows small variations on different scales. As in the microwave background, one sees a clear maximum for a certain distance between galaxies: one is much more likely to find a pair of galaxies separated by that distance than by others, an observation announced simultaneously in 2005 by the collaboration of SDSS and 2dFGRS (the Two Degree Field Galaxy Redshift Survey). As before, Einstein’s equations lead to a connection of that distance with the spatial curvature, again showing evidence for a flat space.
19. Galaxy map from SDSS. On large scales, the universe looks much more homogeneous than the close neighborhood of the solar system and the Milky Way would indicate. (http://www.sdss.org/news/releases/galaxies.jpg.)
Such agreements of very different measurements, demonstrating the consistency of the data, always constitute impressive indications that we are on the right track toward understanding the universe. What is more, in this specific case it indicates that the spatial curvature remains constant on long time scales. After all, the tiny density fluctuations from the release time of the microwave background on the one hand, and the distribution seen in galaxy maps on the other, are separated by millions of years. Through all this time, the spatial curvature must have remained unchanged. That the spatial curvature is constant in time is a prediction of general relativity for a universe with nearly homogeneous mass distribution. Thus the theory has passed another test superbly.
SUPERNOVAE: THE WISE LAST WORDS OF DYING STARS
After the first galaxies formed, their stars evolved further. At some time, those living fast approached the end of their nuclear fuel; high enough inner temperatures and pressures to prevent gravitational collapse could no longer be sustained. For some of them, the collapse