The universe began with a sort of cosmic "two-step." In the first step -- known as inflation -- space itself puffed up like a child's balloon connected to a firehose. In the second step -- the Big Bang -- the energy that was powering inflation was converted to particles and radiation.
As inflation ended, it left a critical footprint on the universe. It created tiny "ripples" in space and time. The ripples created slight differences in density in the otherwise smooth mixture of the early universe, like lumps in a cake batter. These lumps exerted a slightly stronger gravitational pull than other regions, so matter congregated around them. Over time, enough matter congregated to form the first galaxies and clusters of galaxies -- the structure of the universe that we see today.
In fact, without inflation, we probably wouldn't even be here. University of Texas physicist Steven Weinberg explains:
WEINBERG: Without inflation, the universe would now presumably be highly chaotic. You wouldn't see the beautiful smooth expansion of galaxies in all directions away from each other.... That lovely grand expansion of the universe would be something more like a boiling kettle with bubbles and everything in turmoil. And I don't think there would be any life because it requires billions of years of fairly smooth, stately expansion of the universe for galaxies and then stars and planets to form and life to arise and then gradually to evolve.
More about inflation tomorrow.
Script by Damond Benningfield, Copyright 2011