text: In the early Universe matter was so hot, that protons and electrons were still roaming freely and had not yet combined into atoms. This state is called the plasma state of matter, and can be found on the sun too.\n\nIn a plasma, photons are constantly absorbed and reemitted in a process called Compton scattering. The sun is therefore opaque to light: we can not see *inside* the sun, we see only its surface. The early universe was opaque to light too. However, after the Big Bang the Universe was cooling down rapidly due to the expansion. Eventually, when the temperature fall below 3,000 K electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This process is called as photon decoupling. \nWith only neutral atoms present the photons can travel largely unimpeded. \n\nThe photons that were emitted at recombination, the first that can travel freely are called First light.