The Formation of Structure
The matter and radiation of the universe are not so uniformly distributed now as they were earlier. The matter is gathered into galaxies, and the galaxies cluster on still larger scales, although on the largest scales the distribution appears to be approximately uniform. Even the cosmic background radiation displays tiny imperfections—very slight hot and cold spots that probably derive from the same source as the galaxies, although they are larger than structures studied in current galaxy catalogs. The main features of cosmic structure can be explained by adding small variations in density or expansion rate to the Big Bang model and letting gravity (including that of dark matter) do the rest of the job of gathering material together.1
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