The Cosmic Microwave Background (CMB) is a form of electromagnetic radiation dating from an early stage of the universe. It is the oldest known form of radiation and is considered to be evidence for the Big Bang theory of the formation of the Universe.
- With a traditional optical telescope, the space between stars and galaxies is completely dark but a sufficiently sensitive radio telescope reveals the CMB as a faint glow not associated with any star, galaxy or other object.
- The glow of the cosmic microwave background is strongest in the microwave region of the electromagnetic spectrum.
- The CMB is a remnant of the Big Bang and so dates back to the earliest history of the Universe. The radiation that it produced has taken over 13 billion years to reach earth and so to become visible to us today.
- The CMB was initially composed of gamma wave radiation of an infinitely small wavelength, but the expansion of the Universe over time means it appears today as microwave radiation.
- The CMB is detected as an emission of uniform thermal energy, a faint glow coming from all parts of the sky.
A timeline of the Universe. The far-left shows the Big Bang followed by a period of “inflation” that caused exponential growth in the scale of the Universe. The afterglow of the Big Bang, the Cosmic Microwave Background, began to be emitted about 400,000 years later.
Attribution: Adapted from NASA / WMAP Science Team. https://www.nasa.gov/sites/default/files/thumbnails/image/wmap_0.jpg