© NASA, WMAP Science Team.
In our standard model of cosmology, the universe began with a singularity that immediately went through a period of extremely rapid expansion called "inflation." When inflation was finished, the universe as we know it began. Quarks, photons, and other fundamental particles formed a hot, dense soup that cooled and expanded. As the universe cooled, atomic nuclei formed after about 100 seconds, and then light atoms formed after 390,000 years. At that time, the photons decoupled from the matter, traveling nearly uninterrupted from that time until the present. These photons, called the "cosmic microwave background" (CMB), contain a record of how the universe has evolved since the time of decoupling. A careful analysis of the CMB reveals that there is not enough matter in the universe to account for its geometry, providing a second line of evidence for the existence of dark energy. (Unit: 11)