This is a real picture of the cosmic microwave background radiation from all directions in the universe, color coded to depict deviations from average temperature. The temperature fluctuations are about 1 part in 100,000, indicating that the universe on large scales is highly homogeneous and isotropic. Credit: NASA / WMAP Science Team. Image Link.

On the left, we show the cosmic microwave background radiation (CMBR), which is a picture of the baby universe. This consists of light that originates from about 380,000 years after the Big Bang, which is just after the period of recombination---a short period during the early universe when electrons were first able to bond to protons. The CMBR, therefore, is a picture of the universe when it was 380,000 years old. This background radiation was predicted years before it was discovered by accident, and it looks nearly the same from all directions. Like clues scattered throughout a forest, it permeates the universe. In fact, a famous computation in cosmology shows that there are about 400 of these light particles per cubic centimeter. The CMBR has given us a wealth of information, and we compare our scientific models to data collected from it.