The Universe — Explain

Explain what the cosmic microwave background radiation (CMBR) is and why it is now observed as microwaves.

Shortly after the Universe was formed (the Big Bang), it was extremely hot and dense, filled with intense electromagnetic radiation of very short wavelength. As the Universe has continued to expand ever since, the wavelength of this radiation has been stretched (increased) along with the expansion of space itself. This radiation now reaches Earth from all directions with a much longer wavelength, in the microwave region of the electromagnetic spectrum, and is called the cosmic microwave background radiation (CMBR). Its detection provides strong evidence supporting the Big Bang theory and the expansion of the Universe.