Cosmic Microwave Background Radiation

The early universe was hot enough for matter and radiation to interact strongly. As the universe expanded and cooled, conditions eventually allowed electrons and nuclei to combine into neutral atoms. Photons could then travel much more freely. Those ancient photons have been stretched by cosmic expansion to microwave wavelengths. Today, the CMB has a temperature of approximately: 2.725 K The CMB: comes from all directions; has an extremely accurate thermal spectrum; is highly uniform; contains tiny temperature variations. Those tiny variations provide information about early density differences that later grew into cosmic structure.

**T_CMB ≈ 2.725 K For blackbody radiation: λ_max T = b** where `b` is Wien's displacement constant.