5The Cosmic Microwave Background
The cosmic microwave background radiation is the oldest fossil we have ever found, and it has much to tell us about the origin of the Universe. The cosmic microwave background (CMB) is a bath of photons coming from every direction. These photons are the afterglow of the Big Bang, and the oldest photons we can observe. Their long journey towards us has lasted more than 99.99% of the age of the Universe and began when the Universe was one thousand times smaller than it is today. The CMB was emitted by the hot plasma of the Universe long before there were planets, stars or galaxies. The CMB is an isotropic field of electromagnetic radiation-the redshifted relic of the hot Big Bang.