Physicists Pin Down the Nuclear Reaction Just After the Big Bang

In a secluded laboratory buried under a mountain in Italy, physicists have re-created a nuclear reaction that happened between two and three minutes after the Big Bang.

Their measurement of the reaction rate, published on November 11 in Nature, nails down the most uncertain factor in a sequence of steps known as Big Bang nucleosynthesis that forged the universe’s first atomic nuclei.

Researchers are “over the moon” about the result, according to Ryan Cooke, an astrophysicist at Durham University in the United Kingdom who wasn’t involved in the work. “There’ll be a lot of people who are interested from particle physics, nuclear physics, cosmology, and astronomy,” he said.

The reaction involves deuterium, a form of hydrogen consisting of one proton and one neutron that fused within the cosmos’s first three minutes. Most of the deuterium quickly fused into heavier, stabler elements like helium and lithium. But some survived to the present day. “You have a few grams of deuterium in your body, which comes all the way from the Big Bang,” said Brian Fields, an astrophysicist at the University of Illinois, Urbana-Champaign.

The precise amount of deuterium that remains reveals key details about those first minutes, including the density of protons and neutrons and how quickly they became separated by cosmic expansion. Deuterium is “a special super-witness of that epoch,” said Carlo Gustavino, a nuclear astrophysicist at Italy’s National Institute for Nuclear Physics.

But physicists can only deduce those pieces of information if they know the rate at which deuterium fuses with a proton to form the isotope helium-3. It’s this rate that the new measurement by the Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration has pinned down.

The Earliest Probe of the Universe

Deuterium’s creation was the first step in Big Bang nucleosynthesis, a sequence of nuclear reactions that occurred when the cosmos was a super hot but rapidly cooling soup of protons and neutrons.

Starting in the 1940s, nuclear physicists developed a series of interlocking equations describing how various isotopes of hydrogen, helium, and lithium assembled as nuclei merged and absorbed protons and neutrons. (Heavier elements were forged much later inside stars.) Researchers have since tested most aspects of the equations by replicating the primordial nuclear reactions in laboratories.

In doing so, they made radical discoveries. The calculations offered some of the first evidence of dark matter in the 1970s. Big Bang nucleosynthesis also enabled physicists to predict the number of different types of neutrinos, which helped drive cosmic expansion.

But for almost a decade now, uncertainty about deuterium’s likelihood of absorbing a proton and turning into helium-3 has fogged up the picture of the universe’s first minutes. Most importantly, 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. This process released radiation called the cosmic microwave background that provides a snapshot of the universe at the time.

Cosmologists want to check whether the density of the cosmos changed from one period to the other as expected based on their models of cosmic evolution. If the two pictures disagree, “that would be a really, really important thing to understand,” Cooke said. Solutions to stubbornly persistent cosmological problems—like the nature of dark matter—could be found in this gap, as could the first signs of exotic new particles. “A lot can happen between a minute or two after the Big Bang and several hundred thousand years after the Big Bang,” Cooke said.

But the all-important deuterium reaction rate that would allow researchers to make these kinds of comparisons is very difficult to measure. “You’re simulating the Big Bang in the lab in a controlled way,” said Fields.

Physicists last attempted a measurement in 1997. Since then, observations of the cosmic microwave background have become increasingly precise, putting pressure on physicists who study Big Bang nucleosynthesis to match that precision—and so allow a comparison of the two epochs.

In 2014, Cooke and coauthors precisely measured the abundance of deuterium in the universe through observations of faraway gas clouds. But to translate this abundance into a precise prediction of the primordial matter density, they needed a much better measure of the deuterium reaction rate.