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Big Bang Nucleosynthesis: the first elements

On the timeline · around In the universe's first three minutes · The First ElementsThe First ElementsBig Bang Nucleosynthesis: the first elements13.8 Ga13.8 Ga13.8 Ga13.8 Ga13.8 Ga13.8 Ga13.8 Ga13.8 Ga13.8 Ga

What happened

In the first seconds after the Big Bang, the universe was a hot, dense soup of protons, neutrons, electrons, and photons, far too hot for atomic nuclei to survive. As it expanded and cooled below about 1.2 billion degrees Kelvin, at an age of roughly 2 minutes, protons and neutrons began fusing into deuterium, an isotope of hydrogen. Within another minute, as the temperature dropped further, most of that deuterium fused into helium, with only trace amounts going on to form lithium. The whole process, physicists call it Big Bang nucleosynthesis, was over within about twenty minutes, once the universe had cooled and thinned out too much for further fusion. It left behind a universe that was roughly 75 percent hydrogen and 25 percent helium by mass, with only faint traces of anything heavier.

Why it matters

That 75-to-25 hydrogen-helium split is not a rough guess, it is a hard prediction that either matches reality or falsifies the whole model, and it matches. Every star, planet, and person that would eventually form drew its raw material from this brief, universal furnace, and the leftover ratio is still measurable today in the oldest, most pristine gas astronomers can find.

How we know

The predicted abundances come directly from nuclear physics: known fusion reaction rates, run forward from a known starting temperature and density, produce a specific hydrogen-helium-lithium ratio. Measurements of the oldest, most metal-poor stars and gas clouds, environments barely touched by later stellar fusion, show almost exactly that predicted 75 percent hydrogen and 25 percent helium mix. A second, independent confirmation of the same hot, expanding origin came in 1929, when Edwin Hubble found that every distant galaxy's light was redshifted, stretched by the same cosmic expansion, in direct proportion to its distance, a relationship now called Hubble's Law.

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Part of a timelineThe Early Universe7 events · The first billion years, from the fusion furnace of the Big Bang's opening minutes to the collision that reshaped the Milky Way, told through the instruments and satellites that found the evidence.View all →