Page 17, Cosmology: [Previous Page] [Final Review!] [Class Home Page] Text Pages 432-435

Astronomers observe considerable structure in the universe, from stars to galaxies to clusters and superclusters of galaxies. Most cosmologists believe that the galaxies that we observe today grew out of small gravitational fluctuations in the nearly-uniform density of the early universe, possibly helped by structures in the cold dark matter. These early density fluctuations leave an imprint in the cosmic microwave background radiation in the form of temperature fluctuations from point to point across the sky.

This is a computer simulation showing early structures which collapsed out of the rather smooth initial universe. The filamentary like structures also resemble "bubbles". These simulations try to match the kinds of structures seen in large scale galaxy surveys which show such bubbles or voids between large sheet-like structures of galaxies and galaxy clusters. The brightest dots in this simulation are called galaxy seeds. Click HERE for a movie showing the development of such structures from an initially very smooth mass distribution.

The density of matter in the region of space that now contains the Milky Way was perhaps 0.5% higher than in adjacent regions. Because its density was higher, this region of space expanded more slowly than surrounding regions. As a result of this slower expansion, its relative over-density grew. After a billion years of this slow expansion, our region of space was roughly twice the density of neighboring regions. This, about 1 billion years after the Big Bang, was the time in which galaxies formed, as seen in the most distant HST Deep Field.

Inflation theory to the rescue again!
Subatomic quantum fluctuations would be stretched by inflation to create subtle, but large scale density variations. Later in the history of the Universe these density fluctuations can help provide seeds to cause matter to begin to clump together to form the galaxies and other observed structures we see today.