The reaction does not slow down at low temperatures as earlier models predicted.
In the earliest chapter of cosmic history, the universe's very first molecule — helium hydride — was quietly doing more work than anyone realized. Researchers in Heidelberg have now recreated that ancient chemistry in the laboratory, discovering that a long-standing mathematical error had caused scientists to underestimate how active these primordial ions remained at near-absolute-zero temperatures. The correction is not a minor footnote: it suggests that the cooling processes which allowed the first stars to ignite were shaped by helium chemistry far more deeply than our models have ever reflected. What we thought we knew about the universe's first light is being gently, but fundamentally, revised.
- A decades-old mathematical error in a widely used energy model has been quietly distorting our picture of how the early universe's chemistry actually worked.
- Experiments at a 35-meter cryogenic storage ring in Heidelberg showed that helium hydride ions remain chemically active at near-absolute-zero temperatures — directly contradicting every theoretical prediction made until now.
- The assumed energy barrier blocking this reaction in extreme cold does not exist, meaning the reaction is barrierless and far more efficient in primordial conditions than scientists had calculated.
- This overturns a cornerstone assumption about how vast gas clouds cooled and collapsed to birth the universe's first stars, implicating helium chemistry as a far larger player in that process.
- With experiment and corrected theory now in agreement, cosmologists must rebuild star formation models from more accurate chemical foundations — a recalibration with sweeping implications for how we understand the cosmos's earliest moments.
Roughly 380,000 years after the Big Bang, as the universe cooled from its fiery origins, helium and hydrogen atoms combined to form the cosmos's first molecule: the helium hydride ion, or HeH⁺. For decades, physicists believed they had a solid grasp of how this molecule behaved and how quickly it reacted. A new set of experiments in Heidelberg has shown that understanding was built on a flawed foundation.
Researchers at the Max-Planck-Institut für Kernphysik used a cryogenic 35-meter storage ring cooled to just above absolute zero to recreate this ancient reaction, colliding helium hydride ions with deuterium atoms as a hydrogen proxy. The results were striking: rather than slowing dramatically at low temperatures as every theoretical model had predicted, the reaction rate remained nearly constant. The assumed energy barrier — a kind of chemical wall thought to block the reaction in frigid primordial conditions — simply does not exist. The reaction is barrierless.
The source of the error was traced to a widely used mathematical model called a potential energy surface, which had been systematically underestimating the reaction's speed under early-universe conditions. Physicist Yohann Scribano's group had already flagged the problem theoretically; the new experiments now confirm it with direct measurement.
The stakes are considerable. Before the first stars could ignite, enormous gas clouds had to shed energy and cool enough to collapse under gravity. Hydrogen atoms could assist at high temperatures, but became ineffective below roughly 10,000 degrees Celsius. Molecules like helium hydride — with its unusually large dipole moment — were among the few tools available to keep the cooling process going. If HeH⁺ was more abundant and longer-lived than previously believed, it played a far greater role in enabling the universe's first stars to form than our models have ever captured.
With experiment and theory now aligned, cosmologists face the task of rebuilding star formation models using corrected reaction rates. The universe's earliest and darkest chapter is slowly yielding its secrets.
Thirteen billion years ago, in the first few hundred thousand years after the Big Bang, the universe was still too hot for chemistry to exist. But around 380,000 years in, as everything cooled, something remarkable happened: helium and hydrogen atoms began to combine, and the cosmos got its first molecule. It was called helium hydride ion, or HeH⁺. It was simple. It was fleeting. And for decades, physicists thought they understood what it did and how quickly it did it. They were wrong.
Researchers at the Max-Planck-Institut für Kernphysik in Heidelberg have now recreated this ancient reaction in the laboratory, and their findings overturn a cornerstone assumption about how the early universe worked. The team used a specialized 35-meter storage ring cooled to just a few degrees above absolute zero, colliding helium hydride ions with deuterium atoms—a stand-in for hydrogen—and measuring how the reaction rate changed as temperature dropped. What they found was unexpected: the reaction did not slow down at low temperatures the way every theoretical model had predicted it would. Instead, it remained almost constant, suggesting that helium hydride was far more chemically active in the primordial universe than anyone had calculated.
Why does this matter? Because in those first few hundred million years after the Big Bang, before any stars existed, the universe was dark and cold. For the first stars to ignite, vast clouds of gas had to collapse under their own gravity. But gravity alone was not enough—the gas had to cool by shedding energy. Hydrogen atoms could help at high temperatures, but below about 10,000 degrees Celsius they became nearly useless. Molecules, however, could keep cooling the gas through their rotational and vibrational motions. Helium hydride, with its unusually large dipole moment, was thought to be one of the most efficient coolants available. If it was more abundant and longer-lived than previously believed, it would have played a much larger role in allowing those first stars to form.
For years, theoretical physicists had assumed there was an energy barrier blocking this reaction at low temperatures—a kind of chemical wall that the particles would struggle to cross in the frigid conditions of the early cosmos. But the new experiments, combined with corrected theoretical calculations, revealed no such barrier exists. The reaction is what physicists call barrierless, meaning it proceeds efficiently even in extreme cold. The error, it turned out, lay in a widely used mathematical model called a potential energy surface that had been used to describe how the reaction's energy changed as it progressed. That model had been wrong for years, systematically underestimating how fast the reaction could occur under primordial conditions.
Dr. Holger Kreckel, who led the experimental work, noted that previous theories had predicted a significant drop in reaction probability at low temperatures, but neither the new experiments nor updated theoretical calculations could verify this. The implication is stark: helium chemistry in the early universe needs to be fundamentally re-evaluated. Because molecules like helium hydride and molecular hydrogen were crucial for cooling the gas clouds that would become the first stars, getting their chemistry right is essential to understanding how the universe's first light sources ever ignited.
The findings also validate recent theoretical work by physicist Yohann Scribano, whose group had identified the error in the potential energy surface model. With experiment and theory now aligned, cosmologists have a clearer picture of the chemical processes that shaped the universe in its infancy. The next step is to build more accurate models of star formation using these corrected reaction rates, potentially reshaping our understanding of how the cosmos transitioned from a dark, chemically simple place to one filled with stars and galaxies. What was hidden in the universe's first moments is slowly coming into view.
Citas Notables
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— Dr. Holger Kreckel, Max-Planck-Institut für Kernphysik
The reactions of HeH⁺ with neutral hydrogen and deuterium appear to have been far more important for chemistry in the early universe than previously assumed— Dr. Holger Kreckel, Max-Planck-Institut für Kernphysik
La Conversación del Hearth Otra perspectiva de la historia
Why does a reaction that happened 13 billion years ago matter to us now?
Because it's the foundation. If we get the chemistry wrong, we get the entire story of star formation wrong. And if we don't understand how the first stars formed, we don't understand how galaxies formed, or how the universe became what it is.
But this is just one molecule, right? Helium hydride. How much difference could it really make?
That's the thing—it's not about the molecule itself. It's about what it tells us about how we've been calculating these reactions. If we've been wrong about this one, we might be wrong about others. And in the early universe, there weren't many molecules to choose from. The ones that existed mattered enormously.
So the old models said the reaction would slow down in cold conditions. The new experiments say it doesn't. What does that mean practically?
It means helium hydride stuck around longer and stayed more chemically active than we thought. That means it could cool gas clouds more efficiently. And cooler gas clouds collapse more easily under gravity. So the first stars could have formed more readily than our old models suggested.
How did they even test this? You can't exactly go back in time.
They used deuterium, a heavier form of hydrogen, and recreated the collision in a storage ring at temperatures near absolute zero. It's not perfect, but it's close enough to the primordial conditions that the results tell us something real about what happened then.
And the error—the mathematical model that was wrong—how long had that been sitting there?
Years. Decades, probably. It's one of those things that becomes accepted, gets cited in paper after paper, and nobody questions it until someone actually tests it carefully. This team did the testing.
What happens now?
Now cosmologists have to rebuild their models of the early universe with the correct reaction rates. It's not a small adjustment. It could change how we understand when and how the first stars formed.