Proton the particles in the solar wind are electrons and protons, in approximately equal numbers. Protons also have extrasolar origin from galactic cosmic rays, where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, and their intensity is far more uniform and less variable than protons coming from the Sun, the production of which is heavily affected by solar proton events such as coronal mass ejections. Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health. To be more specific, there
Baryon acoustic oscillations interacting with the plasma via Thomson scattering. As the universe expanded, the plasma cooled to below 3000 K—a low enough energy such that the electrons and protons in the plasma could combine to form neutral hydrogen atoms. This recombination happened when the universe was around 379,000 years old, or at a redshift of . Photons interact to a much lesser degree with neutral matter, and therefore at recombination the universe became transparent to photons, allowing them to decouple from the matter and free-stream through the universe. Technically speaking, the mean free path of the photons became of order the size