How Were Atoms Made

When we looked at fusion in the core of stars (see Chapters 16, "Our Star," and 19, "Black Holes: One-Way Tickets to Eternity"), we noted that hydrogen served as the nuclear fuel in a process that produced helium. Yet there is so much helium in the universe—helium accounts for more than 23 percent of the mass of the universe— that stellar fusion cannot have produced it all. It turns out that most of the helium in the universe was created in the moments following the Big Bang—when the universe had cooled sufficiently for nuclei to hold together.

Hydrogen fusion in the early universe proceeded much as it does in the cores of stars. A proton and a neutron come together to form deuterium (an isotope of hydrogen), and two deuterium atoms combine to form a helium nucleus. Careful calculations show that between 100 and 120 seconds after the Big Bang, deuterium would be torn asunder by gamma rays as soon as it was formed. However, after about two minutes, the universe had sufficiently cooled to allow the deuterium to remain intact long enough to be converted into helium. As the universe continued to cool, temperatures fell below the critical temperature required for fusion, and primordial synthesis ended.

Only after about 300,000 years had passed would the universe expand and cool sufficiently to allow electrons and nuclei to combine into atoms of hydrogen and helium. It is at this point, with the temperature of the universe at some 5,000 K, that photons could first move freely through the universe. The cosmic microwave background we now measure consists of photons that became free to move into the universe at this early time.

The rest of the periodic table—the other elements of the universe—beyond beryllium would be filled out by fusion reactions in the cores of stars and supernova explosions.

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