What if everything scientists ever thought about the universe’s earliest chemical reactions was wrong? Recent tests have turned decades of theory on its head, showing that the first molecular processes after the Big Bang were stronger than anyone realized, and perhaps took a massive shortcut through the formation of the first stars.
After the Big Bang, the universe was a hot plasma of basic particles, too hot to have atoms. Not until 380,000 years later did temperatures drop to the point where electrons and nuclei would come together to form the first neutral atoms mostly hydrogen and helium, with a little lithium. But the journey from these early elements to the breathtaking intricacy of stars and galaxies depended on a quiet but powerful chemical step: the creation of the helium hydride ion (HeH⁺), the first molecule in the universe.
HeH⁺, formed when a neutral helium atom captures an ionized hydrogen nucleus, is the start of a chain of chemical reactions that eventually produce molecular hydrogen (H₂) the most populous molecule in the universe and the primary coolant of star-forming clouds. Theoretical frameworks long speculated that HeH⁺ was instrumental in cooling primordial gas clouds, allowing them to collapse under gravity and ignite nuclear fusion, the process that gives rise to stars. This efficiency in cooling is due to HeH⁺’s large dipole moment, with which it can shed energy through rotational and vibrational transitions at low temperatures, particularly below 10,000 degrees Celsius, where atomic hydrogen cooling becomes inefficient. Since, as one paper points out, “Due to its pronounced dipole moment, the HeH⁺ ion is particularly effective at these low temperatures and has long been considered a potentially important candidate for cooling in the formation of the first stars.“
But for decades, the subject was hampered by a foundational assumption: that reaction rates with HeH⁺ would dive at the extremely low temperatures of the early universe, restricting its role as the universe cooled. This was based on theoretical calculations estimating a large energy barrier at low temperatures, essentially benching HeH⁺ from the central act of cosmic chemistry as the universe grew old.
That story has now been reversed. At the Max Planck Institute for Nuclear Physics, scientists, led by Florian Grussie and Holger Kreckel, conducted a revolutionary series of experiments using the Cryogenic Storage Ring (CSR), a 35-meter-diameter storage ring that is able to keep ions at a few kelvins approximating the near-absolute-zero temperatures of deep space. The CSR’s ultra-high vacuum and electrostatic ion optics design allows molecular ions to be stored in their lowest quantum states for as much as one minute, a technical achievement that makes possible the accurate exploration of reactions in the real conditions of truly interstellar space.
The researchers stored HeH⁺ ions and combined them with a beam of neutral deuterium atoms, hydrogen’s isotope, with the relative velocities changed to mimic various collision energies proxies for temperature. They saw the creation of HD⁺ ions and neutral helium, a reaction similar to the early-universe mechanism that creates molecular hydrogen. The key result:the reaction rate remained nearly constant across the entire temperature range, contrary to what earlier theories had predicted. According to Kreckel, “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.”
This outcome was no experimental anomaly. Theoretical research under Yohann Scribano uncovered a flaw in the former applied potential energy surfaces for these reactions. With fixed models, the new calculations now correlate with the experimental data, establishing that the HeH⁺-driven chemistry was much more efficient and long-lasting than thought. Consequently, HeH⁺ likely played a much more significant role in cooling primordial gas clouds and facilitating early star formation than models had permitted.
The consequences cascade through astrochemistry and cosmology. A greater abundance and more persistent existence of HeH⁺ in the cosmic “dark ages” would have allowed molecular hydrogen to condense earlier and more quickly, hastening gas cloud collapse and the onset of the first stars. This, in turn, will influence the timescale for heavy element formation, for galaxy evolution, and even for the interpretation of the cosmic microwave background, with the presence of molecules such as HeH⁺ being able to subtly affect it.
The CSR’s ability goes beyond this one response. Its ultra-low-density, ultra-cold atmosphere is perfect for preparing and examining ions in quantum states that closely resemble the states found in interstellar clouds, ushering in an entirely new period of laboratory astrochemistry. As the discipline incorporates these updated reaction rates into cosmological models, scientists look forward to a cascade of new knowledge about the formative periods of the universe.
As the authors of the study conclude, “The reactions of HeH⁺ with neutral hydrogen and deuterium therefore appear to have been far more important for chemistry in the early Universe than previously assumed.” The universe’s very first molecule, previously assumed to be a momentary aside, now appears as a key builder of cosmic evolution.
