


Just 370,000 years after the universe began in a big bang, all that existed was a hot plasma, similar to a candle flame. Protons and electrons, seen as the red and green balls, were bouncing around scattering the light. The particles of light, called photons (shown in blue), couldn't go far without colliding with an electron.

As the universe cooled, the protons and electrons could pair up, forming hydrogen atoms, and the light was free to travel. It's been traveling freely ever since. Through the dark ages before there were stars ... then past the formation of the first stars.

As the universe expanded the photons lost energy, changing color. They went past clusters of galaxies. The path of the photon is slightly bent by the gravity of the clusters. Now and then, going through a cluster an electron (that green ball) would collide with some of the photons. They would change their path. Past more matter ... More little wiggles due to gravity, mass.

The photons traveled for 13.8 billion years before they reached the Planck detectors and died a glorious death, giving up the information that they had gleaned passing through the entire universe to our instruments and enabling us to make this beautiful map of the universe.
View all Videos