In a secluded place laboratory buried under a mountain in Italy, physicists have recreated a nuclear reaction that occurred between two and three minutes after the Big Bang.
Their measure of reaction speed, published on November 11 in Nature, nails the most uncertain factor in a sequence of steps known as the Big Bang nucleosynthesis that forged the first atomic nuclei in the universe.
The researchers are “delighted” with the result, according to Ryan Cooke, an astrophysicist at Durham University in the United Kingdom who was not involved in the work. “There will be a lot of people interested in particle physics, nuclear physics, cosmology and astronomy,” he said.
The reaction involves deuterium, a form of hydrogen made up of a proton and a neutron that have fused together within the first three minutes of the cosmos. Most of the deuterium quickly fused into heavier, more stable elements like helium and lithium. But some have survived to the present day. “You have a few grams of deuterium in your body, which comes from the Big Bang,” said Brian Fields, an astrophysicist at the University of Illinois at Urbana-Champaign.
The precise amount of deuterium that remains reveals key details about those first few minutes, including the density of protons and neutrons and how quickly they separated by cosmic expansion. Deuterium is “a special super-witness of this era,” said Carlo Gustavino, nuclear astrophysicist at the Italian National Institute of Nuclear Physics.
But physicists can only infer this information if they know the rate at which deuterium fuses with a proton to form the isotope helium-3. It is at this rate that the new measurement of the Laboratory of Underground Nuclear Astrophysics (LUNA) the collaboration has ended.
The first probe in the universe
The creation of Deuterium was the first step in the nucleosynthesis of the Big Bang, a sequence of nuclear reactions this happened when the cosmos was a very hot but rapidly cooling soup of protons and neutrons.
Departure in the 40s, nuclear physicists developed a series of nested equations describing how various isotopes of hydrogen, helium, and lithium came together as nuclei coalesced and absorbed protons and neutrons. (Heavier elements were forged much later inside stars.) Researchers have since tested most aspects of the equations by replicating primordial nuclear reactions in the laboratory.
In doing so, they made some radical discoveries. The calculations offered some of the earliest evidence for the presence of dark matter in the 1970s. The nucleosynthesis of the Big Bang also allowed physicists to predict the number of different types of neutrinos, which contributed to cosmic expansion.
But for almost a decade now, the uncertainty on the probability that the deuterium absorbs a proton and turns into helium-3 has blurred the picture of the first minutes of the universe. More importantly, uncertainty prevented physicists from comparing this image to what the cosmos looked like 380,000 years later, when the universe cooled enough for electrons to begin to orbit atomic nuclei. This process released a radiation called the microwave cosmic background which provides a snapshot of the universe at the time.
Cosmologists want to see if the density of the cosmos has changed from period to period as expected based on their models of cosmic evolution. If the two images don’t agree, “that would be a really, really important thing to figure out,” Cooke said. Solutions to stubbornly persistent cosmological problems – like the nature of dark matter – could be found in this loophole, as could the first signs of new alien 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 speed of reaction of the very important deuterium which would allow researchers to make these kinds of comparisons is very difficult to measure. “You are simulating the Big Bang in the lab in a controlled manner,” Fields said.
Physicists last attempted a measure in 1997. Since then, observations of the microwave cosmic background have become increasingly precise, putting pressure on physicists studying Big Bang nucleosynthesis to match that precision – and thus allow a comparison of the two eras.
In 2014, Cooke and his co-authors accurately measured the abundance of deuterium in the universe thanks to the observation of distant gas clouds. But to translate this abundance into an accurate prediction of the density of primordial matter, they needed a much better measure of the reaction rate of deuterium.