New findings on universe's first ever molecules

 Scientists findings on universe’s first molecule reveal bigger role in forming early stars and the results challenge our understanding      

In a first, scientists have recreated the formation of the first ever molecules in the universe to learn more about early star formation. HeH⁺, the universe’s first molecule, didn’t fade quietly. It may have helped trigger the very first stars we see today. New findings on universe’s first molecule reveal bigger role in forming early stars. The universe’s first molecule just surprised us again. In a discovery which could rewrite our understanding of how the first stars formed, researchers at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg have revealed unexpected behaviour in helium hydride (HeH⁺), the earliest known molecule in the cosmos.

For the first time, researchers have recreated the universe's first ever molecules by mimicking the conditions of the early universe. The findings shake up our understanding of the origin of stars in the early universe and "calls for a reassessment of the helium chemistry in the early universe," the researchers wrote in the new study. Contrary to long-standing predictions, HeH⁺ remained chemically reactive even at extremely low temperatures: conditions that mimic the early universe. To test how this ancient molecule behaved initially, researchers recreated early-universe conditions at the Cryogenic Storage Ring (CSR) in Heidelberg. The world’s only facility of its kind, CSR simulates space-like environments just a few degrees above absolute zero. Just after the start 13.8 billion years ago, the universe was subject to extremely high temperatures. A few seconds later, though, temperatures decreased enough for hydrogen and helium to form as the first ever elements. Hundreds of thousands of years after those elements formed, temperatures became cool enough for their atoms to combine with electrons in a variety of different configurations, forging molecules.

By colliding stored HeH⁺ ions with a beam of neutral deuterium atoms, the team was able to observe the molecule’s reaction rates at ultra-cold temperatures for the first time. Formed shortly after the Big Bang, HeH⁺ is a simple molecule made from a helium atom and a proton. It marked the beginning of chemical bonding in the universe and laid the foundation for molecular hydrogen (H₂), the fuel which powers stars. For decades, HeH⁺ has been assumed to play a passive role in the cooling processes which allowed protostars to condense and ignite. But new experimental results challenge this narrative. According to the researchers, a helium hydride ion, or HeH+, became the first ever molecule. The ion is needed to form molecular hydrogen, now the most abundant molecule in the universe. Both helium hydride ions and molecular hydrogen were critical to the development of the first stars hundreds of millions of years later, the researchers said. For a protostar to begin fusion, the process which enables stars to create their own energy, atoms and molecules within it must collide with each other and release heat. This process is largely ineffective at temperatures under 18,000 degrees Fahrenheit (10,000 degrees Celsius). However, helium hydride ions are particularly good at continuing the process, even under cool temperatures, and are considered to be a potentially integral factor of star formation in the early universe. The amount of helium hydride ions in the universe may therefore have had significant bearing on the speed and efficacy of early star formation, the researchers said.

The researchers found that instead of slowing down as the temperature dropped, the reaction between HeH⁺ and deuterium remained surprisingly constant. This contradicts earlier models, which predicted a steep decline in reactivity at low temperatures. “Previous theories predicted a significant decrease in the reaction probability at low temperatures, but we were unable to verify this in either the experiment or new theoretical calculations by our colleagues,” said Dr. Holger Kreckel of MPIK, who studies nuclear physics at the Max Planck Institute for Nuclear Physics in Germany. This matters because in the young universe, during the so-called “cosmic dark ages” before stars began to shine, molecules like HeH⁺ played a key role in cooling the primordial gas. The researchers recreated early helium hydride reactions by storing the ions at minus 449 degrees Fahrenheit (minus 267 degrees Celsius) for up to 60 seconds to cool them down before forcing them to collide with heavy hydrogen. Researchers studied how the collisions, similar to those that kickstart fusion in a star, changed depending on the temperature of the particles. Reaction rates between these particles do not slow down at lower temperatures, which contradicts older assumptions.        

Effective cooling is necessary for gas clouds to collapse under gravity and form stars. Since hydrogen atoms alone can’t release heat efficiently below 10,000°C, molecules with dipole moments like HeH⁺ were critical for shedding energy via radiation. HeH⁺ also degrades through collisions with hydrogen atoms, producing ions that eventually lead to molecular hydrogen formation. This chain of reactions was vital to star formation, and the new findings suggest HeH⁺ was far more active in that chemistry than previously thought. "This new finding of how helium hydride ions function challenges how physicists think stars formed in the early universe. Reactions between the ions and other atoms "appear to have been far more important for chemistry in the early universe than previously assumed," Kreckel said. The MPIK team’s results also exposed flaws in older theoretical models. Collaborating with theoretical physicist Yohann Scribano, researchers found a long-standing error in the potential energy surface used to predict HeH⁺ behaviour. Correcting this surface brought simulations in line with experimental data, sharpening our understanding of early-universe chemistry. These findings reframe HeH⁺ as a central player in star formation rather than a passive bystander.