Since the 1920s, Edwin Hubble Ever since it was discovered that the universe is expanding, astrophysicists have been asking themselves the question, “Where does matter come from?” In the Big Bang theory, a possible explanation, not a TV show, astrophysicists propose that the universe began with an explosion, a single hot, dense point expanding, then cooling down to transform from pure energy into solid matter. But that origin story ends with the two smallest elements: hydrogen and helium. Not everything in the universe is made of these two elements, leaving scientists with a new question: “Where does other matter come from?”

The emergence of nuclear physics in the early 20th century gave astronomers their first big clue. Researchers studying stars noted that stars are very bright and require a large source of energy to produce that much light. Nuclear physicists, including Albert Einstein and his famous E = mc² The equations showed that one of the most powerful sources of energy in the universe is the smashing of smaller atoms together to create larger ones – nuclear fusion. And that's exactly what stars do in the hot, dense regions at their centers, called “nuclear fusion.” coreBut there's a limit to this process in stars — specifically, iron, which is the 26th of the 92 naturally occurring elements. Stars create energy by colliding elements with each other, but elements bigger than iron need to generate more energy than they can give off, which is why elements heavier than iron, like gold and uranium, remain unexplained.

Researchers have discovered the next clue in a massive, bright stellar explosion in the night sky. SupernovaIt turns out that massive stars, more than 10 times the size of the Sun, burn up their accumulated elements to fuse rapidly. These stars not only shine, but also run out of energy to hold themselves together, exploding and scattering their outer layers of elements in all directions. This is a supernova explosion. For decades, astrophysicists thought that heavy elements were created from a chaotic mixture of light elements and free energy. However, careful observation of supernovae has shown that the amount of heavy elements produced in the explosion is less than what is needed to explain the abundance of heavy elements in the universe.

Astrophysicists got the final clue in 2017 when the Laser Interferometer Gravitational-Wave Observatory detected the first binary neutron star (BNS) merger. RaigoThe final stage in the life cycle of a massive star, between 10 and 25 times the mass of the Sun, is Neutron StarDuring this stage, the star's core collapses, and the electrons and protons in atoms get so close together that they fuse into neutrons. Two neutron stars orbiting each other collide, scattering debris into the surrounding galaxy. Researchers propose that this phenomenon could provide the energy and matter needed to fuse heavy elements into the heaviest naturally occurring elements.

Researchers from Peking University and Guangxi University wanted to test whether BNS mergers could produce elements heavier than iron. Because the event is extremely rare, occurring only a few dozen times per year across our galaxy, they couldn't just point their telescopes into space and hope for luck. Instead, they used advanced nuclear physics software to simulate a BNS merger.

The researchers gave their simulations specific initial conditions, such as what atoms were present in the stars when the collision began, the rates of nuclear reactions and decay, the number of electrons mixing, and the sizes of the colliding neutron stars. They then mathematically described how temperature, volume, and pressure relate to matter. Equation of stateIt simulates the effects of the collision and calculates what elements would be formed and released into space.

The team found that these BNS mergers could produce huge amounts of very heavy elements, between 300 and 30,000 times the mass of the Sun, which is 10 to 1,000 times the amount produced by supernovae. The team believes that this result could explain the abundance of heavy elements observed in the Galaxy in relation to other cosmic effects, e.g. Galactic WindHowever, the researchers acknowledged that their findings cannot explain the abundance of all heavy elements, especially those at the lower end of the atomic mass range they studied. They explained that these elements are probably still being created in the cores of collapsing stars, but suggested that future researchers should further test this hypothesis.

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