Doubtful Elements

Astronomers think they know the chemical history of the universe, more or less. In the first few minutes after the Big Bang, the story goes, more than 99 percent of the universe’s mass condensed into nuclei of hydrogen and helium, the two lightest elements; the rest became lithium, the next element in line. Because the universe was expanding rapidly, the primordial plasma quickly became too diffuse to fuse together the heavier, more complex nuclei. Those elements were made long after the Big Bang, by nuclear fusion in the hot, dense cores of stars. Later they were scattered into space by exploding supernovas.