Scientists Recreated the Nuclear Reaction That Happened Right After the Big Bang

Caroline Delbert
·3-min read

From Popular Mechanics

  • Researchers have solved for a key variable in a reaction that happened very early after the Big Bang.

  • By blasting protons into a deuterium cloud, the scientists simulated the way early elements bonded to make a helium isotope.

  • The calculated rate of proton pickup maps with a later measurement and gives scientists new tools to continue study.

In a research laboratory deep beneath a mountain in Italy, scientists have made a new measurement of a nuclear reaction that immediately followed the Big Bang.

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By recreating the conditions with more sophisticated materials and instruments than ever before, scientists say they’ve made the most high-fidelity simulation of this nuclear reaction—one that also maps with our understanding of what happened afterward.

In Nature, the researchers explain their experiment:

“Light elements were produced in the first few minutes of the Universe through a sequence of nuclear reactions known as Big Bang nucleosynthesis (BBN). We bombarded a high-purity deuterium gas target with an intense proton beam and detected the γ-rays from the nuclear reaction. Our experimental results settle the most uncertain nuclear physics input to BBN calculations and substantially improve the reliability of using primordial abundances to probe the physics of the early Universe.”

The researchers used “period-correct” deuterium—one of the very first compounds formed after the Big Bang—and a very specific laser to measure how quickly the deuterium, in turn, bonded to become helium-3. By blasting a cloud of deuterium with a beam of protons, scientists can directly observe how a proton joins with the nucleus of the deuterium in order to form helium-3.

That reaction in particular is key to understanding our very, very young universe. And scientists have relied on extremely educated guesses, because they’ve never understood one critical part of this reaction. How easy is it to get deuterium to even grab the proton in the first place? At what rate do the deuterium molecules absorb protons to become helium-3?

“Most importantly,” Thomas Lewton at Quanta explains, “the uncertainty has prevented physicists from comparing that picture to what the cosmos looked like 380,000 years later, when the universe cooled enough for electrons to begin orbiting atomic nuclei.”

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This means the scientists' experiment had a very simple pass-fail test for its results. Once they blasted deuterium gas with protons and measured the resulting amount of helium, they had to plug that value into existing and more concretely understood numbers for the background radiation and other factors 380,000 years later. And, thankfully, their value fills in that equation nicely.

The research has a ripple effect, too—like solving the first equation in a system, and then using the newfound value to substitute into the others. In this case, knowing deuterium’s rate of proton absorption leads to better understanding of the countless chemical processes that came afterward.

“Our experimental results settle the most uncertain nuclear physics input to BBN calculations and substantially improve the reliability of using primordial abundances to probe the physics of the early Universe,” the researchers conclude.

A big missing piece has been identified, and it’s shaped a lot like helium-3.

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