Original: See also Ionizing radiation Non-ionizing radiation Background radiation , which actually refers to the background ionizing radiation Black hole Cosmic microwave background radiation , 3K blackbody radiation that fills the Universe Electromagnetic spectrum Radiant energy , radiation by a source into the surrounding environment .
Simplified: Cosmic microwave background radiation , 3K blackbody radiation that fills the Universe Radiant energy , radiation emitted by a source into its environment .