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One of the tenets of the Big Bang theory is that the universe began as a smooth and homogeneous fireball. So how did the universe get to be so lumpy? Visible matter is clumped into galaxies, clusters of galaxies, and clusters of galaxy clusters, or superclusters. The superclusters are arranged in great sheets or filaments spanning 500 million light years. Scattered amongst these superclusters are great voids containing very little visible matter, as large as 400 million light years in diameter.
The extreme uniformity of the cosmic background radiation, first detected in 1964, puzzled cosmologists. This radiation, a relic from the Big Bang, reflected the state of the universe roughly 300,000 years after the Big Bang when radiation separated from matter. Cosmologists reasoned that at that time there must have been at least some irregularities , however slight, to have sown the seeds of the astounding structures in our present-day universe.
In 1992, exhaustive analysis of data from NASA's Cosmic Background Explorer (COBE) satellite revealed minute irregularities or variations in the matter-energy density (only 17 parts in 1,000,000) of the universe. The variations are actually detected as temperature variations.
COBE Sky Map
The regions which were slightly more dense gravitationally attracted photons and,
as the universe expanded, caused them to lose some energy, or heat. The less dense regions are slightly warmer.
The map below shows "hot" (magenta) and "cold" (blue) regions in the radiation detected in the portion of the sky observed.
JPEG Image (25.7 KB); Caption ; Credits
One theory for the origin of these irregularities is that spontaneous fluctuations in the pre-inflationary epoch were greatly magnified by inflation. In the post-inflationary cosmos, these fluctuations produced regions just slightly denser than their surroundings. The differences in density are in turn amplified by gravity, which pulls matter into the denser regions. This process of amplification, cosmologists believe, sowed the "seeds" on which our present-day structures--including the enormous sheets of galaxies--could have formed.
Cosmologists have finally found tangible evidence for theories seeking to explain how an almost perfectly smooth cosmos could have become so "lumpy."
Now the challenge is to link these minute fluctuations to the formation of structures we see today. COBE was able to measure density fluctuations over an angular scale of seven degrees--about the size of 14 moons lined up, side by side, in the sky. But that patch of sky, zoomed back to the epoch of recombination, corresponds to a size larger than the superclusters we see today. In order to correlate density fluctuations with smaller structures like galaxies or clusters of galaxies, cosmologists must detect much finer density fluctuations--within one part in 1,000,000--over scales as small as one degree or less.
In fact, scientific balloon-borne instruments have measured fluctuations--between one and three parts in 100,000--over angular scales between 0.5 and three degrees. So far, however, these experiments have focused on only a few small slices of the sky.
These new measurements of density fluctuations are the hard data cosmologists need to construct more accurate models of the evolution of our universe. For many years, cosmologists who simulate the universe's birth and history using intensive computer models had to guess at the starting conditions for their simulations. They would, of course, prefer measurements of density fluctuations on all scales--those corresponding to the largest superclusters and voids down to individual galaxies. But, for the time being anyway, they're combining COBE's measurements of large-scale fluctuations with physical theory to figure out the starting conditions for their simulations.
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