The History of the Universe, This diagram, read from the bottom to the top, traces the evolution of the universe from the Big Bang to the present. Two watershed epochs are shown. Not long after the Big Bang, light from the first stars burned off a fog of cold hydrogen in a process called
re-ionization. At a later epoch, quasars, the black-hole-powered cores of active galaxies, pumped out enough ultraviolet light to
re-ionize the primordial helium. Image courtesy
NASA, ESA, and A. Feild (STScI). Graphic and caption copied from
Space com
