Chemistry that shaped the cosmos revealed in helium hydride reaction study

Chemistry that shaped the cosmos revealed in helium hydride reaction study
by Robert Schreiber
Berlin, Germany (SPX) Jul 30, 2025

Immediately following the Big Bang some 13.8 billion years ago, the universe was a searing cauldron of energy and particles. Within seconds, temperatures dropped enough for the first elements - mostly hydrogen and helium - to form, though they remained fully ionized. It wasn't until about 380,000 years later that conditions allowed for recombination, giving rise to neutral atoms and initiating the first chemical reactions.

The earliest known molecule, the helium hydride ion (HeH+), formed when a neutral helium atom bonded with an ionized hydrogen nucleus. This triggered a sequence of reactions that eventually led to the creation of molecular hydrogen (H2), the universe's most prevalent molecule.

Although the universe became transparent after recombination, it remained devoid of stars during what is known as the cosmic 'dark age'. The emergence of stars took several hundred million more years. During this period, simple molecules like HeH+ and H2 played vital roles in cooling gas clouds, a prerequisite for gravitational collapse and the onset of nuclear fusion in nascent stars.

At temperatures below 10,000 degrees Celsius, atomic hydrogen becomes inefficient at shedding heat. Molecules such as HeH+ - which possesses a strong dipole moment - can emit radiation through rotational and vibrational transitions, facilitating further cooling. This makes HeH+ a prime candidate in early star formation models. However, HeH+ is also highly reactive, particularly with hydrogen atoms, leading to its conversion into H2+ and ultimately to stable molecular hydrogen.

Researchers at the Max-Planck-Institut fur Kernphysik (MPIK) have now experimentally replicated one of these key reactions under conditions that mimic the early universe. Instead of hydrogen, they used deuterium, an isotope of hydrogen with one proton and one neutron. In collisions between HeH+ and deuterium, a neutral helium atom and an HD+ ion were formed.

The study was conducted at MPIK's Cryogenic Storage Ring (CSR), a one-of-a-kind 35-meter-diameter facility designed to simulate interstellar conditions. HeH+ ions were confined at a few kelvins for up to 60 seconds and collided with a beam of neutral deuterium atoms. By varying the velocity of the beams, researchers measured how reaction rates depended on temperature.

Surprisingly, the reaction rate remained nearly constant as temperatures decreased, contradicting earlier theoretical predictions. "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. He noted that this suggests HeH+ reactions with hydrogen and deuterium were more influential in early cosmic chemistry than previously thought.

This conclusion is supported by theoretical work led by Yohann Scribano, who identified a flaw in earlier computational models of the reaction's potential energy surface. Updated calculations now closely match experimental results from the CSR.

Given that molecules like HeH+ and H2 were critical in early star formation, this breakthrough offers a more refined understanding of how the first stars emerged from the primordial cosmos.

Research Report:Experimental confirmation of barrierless reactions between HeH+ and deuterium atoms suggests a lower abundance of the first molecules at very high redshifts