Tiny amount of ultraviolet light comes from the Big Bang and other stars, galaxies
Your summer suntan is almost entirely locally sourced. But a smidgen of that healthy glow hails not from the sun but from the ultraviolet light of nearby stars and other galaxies: less than one-billionth of 1 percent. Even photons lingering from the Big Bang contribute some: roughly 0.001 percent.
Simon Driver, an astronomer at the University of Western Australia in Crawley, and colleagues calculated these numbers, but not because they’re interested in tanning. They were trying to decipher the extragalactic background light, or EBL, a diffuse glow that fills the universe (SN: 9/7/13, p. 22). Using galaxy observations from multiple telescopes, they assessed the number of EBL photons, from infrared to ultraviolet, that reach Earth. About half originated with the formation of galaxy cores and supermassive black holes