In the first few moments after the Big Bang, the universe was extremely hot and dense. As the universe expanded, it became less dense and it cooled. After only a few seconds, the universe had cooled enough that protons, neutrons, and electrons could form. After a few minutes, the nuclei of atoms could form. The first neutral atoms with neutrons, protons, and electrons, did not form until about 380,000 years after the big bang.

The oldest radiation (light) in the universe, called the cosmic microwave background, also originated about 380,000 years after the big bang. The Wilkinson Microwave Anisotropy Probe (WMAP) has produced a detailed picture of how the early universe looked.

The matter in the early universe was not smoothly distributed across space. Some parts of the universe were denser than others. These clumps of matter were held close together by gravity. Eventually, these clumps became the gas clouds, stars, galaxies, and other structures that we see in the universe today.

The next two pages provide the evidence that scientists use to support the Big Bang theory.

Sources

Teachers' Domain, WMAP: "Baby Picture" of the Universe, published December 17, 2005, retrieved on November 4, 2009, http://www.teachersdomain.org/resource/ess05.sci.ess.eiu.wmap/ (attribution, noncommercial)