One of the first chemical reactions thought to have occurred in the universe has long posed a conundrum, as it seemed the ancient cosmos was far too cold for it to take place. However, new modeling and experimental evidence now show that the reaction between helium hydride (HeH⁺) and hydrogen was indeed feasible in the primordial universe, a discovery that will help scientists better predict the timing and abundance of the first molecules, stars and galaxies.
In the first moments after the Big Bang, the universe was an extremely hot plasma of particles, with temperatures reaching billions of degrees. Over hundreds of thousands of years, it expanded and cooled. Roughly 380,000 years after the Big Bang, temperatures had fallen to around 3000K, allowing electrons and nuclei to combine into the first elements: neutral atomic hydrogen and helium gas. This era is known as the ‘cosmic dark ages,’ since the universe was filled with gas but no visible sources of light.
From these clouds of helium and hydrogen, HeH⁺ is thought to have been the very first molecule to form. ‘The chemistry of the early universe is fascinating because it involves only a few molecular species,’ says corresponding author Holger Kreckel at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. ‘And only a handful of [their reactions] have been measured accurately.’
Understanding how molecules like HeH⁺ formed and interacted with other particles helps explain the chemistry that led to the first stars and galaxies, because these molecules allowed the primordial gas to cool enough to collapse and ignite the first stars.
Past theoretical studies of HeH⁺’s reaction with hydrogen (HeH⁺ + H → H2⁺ + He) suggested that the early universe was too cold – only a few thousand kelvin – for the reaction to take place. ‘All previous theoretical predictions implied that the reaction slows down significantly at low temperature,’ says Kreckel. ‘That is rather unusual for a barrierless ion–neutral reaction.’
No barrier to reaction in the primordial universe
To take a closer look, Kreckel and his colleagues carried out both lab-based experiments and new dedicated calculations on the isotopic variant of that reaction, HeH+ + D → HD+ + He. ‘We studied the HeH+ + D reaction for technical reasons,’ explains Kreckel. ‘Both reactions proceed on the same potential energy surface, so the theoretical findings … applies to both reactions.’
They found that the reaction was actually barrierless and occurred at rates much faster than previously expected in the primordial universe’s relatively frigid temperatures. This finding is important for cosmological simulations that aim to identify detectable signatures of different molecules in the early universe, explains Xavier Urbain at the Catholic University of Louvain in Belgium.
‘Seeing that this reaction is the main destruction path for HeH+, this is a fundamental breakthrough in our understanding of chemical evolution during the first phases of the universe,’ comments Stefano Bovino at the Sapienza University of Rome, who was not involved in the study.
For their lab experiments, the scientists used the Cryogenic Storage Ring in Heidelberg to study cold molecular and cluster ions, as well as their reactions for interstellar chemistry. ‘Since we can store the HeH+ ions in a cryogenic environment … for several seconds before we conduct the measurements, the ions have time to cool to their lowest rotational states,’ says Kreckel.
These low-energy states are the ones that would have existed in the early universe, allowing the scientists to ensure their lab experiment mimics the conditions found in interstellar clouds. ‘This way, we can probe quantum states that are relevant for astrophysical environments,’ adds Kreckel.
Both lab experiments and theoretical calculations pointed to a faster reaction, ultimately suggesting that something was wrong with previous calculations. ‘Calculations of molecular reactions rely on … potential energy surfaces,’ explains study author Yohann Scribano at the University of Montpellier in France. ‘These surfaces describe the “energetic landscape” on which the reaction proceeds. [We found that the] analytical description of the potential surface that was used in all previous studies had an inherent flaw that caused a small barrier to appear for this particular reaction.’
This seemingly small potential energy barrier ended up dominating predictions that HeH⁺ would not be destroyed in the low-energy collisions of the early universe. ‘This highlights how important it is to determine very accurate analytical potential energy surfaces for a correct prediction of the chemical reactivity,’ says Scribano.
The findings suggest less HeH⁺ existed shortly after the Big Bang, but during the era of first star formation its destruction likely shifted away from reaction with hydrogen and was dominated by collisions with free electrons. Kreckel says that to fully assess the impact, all reactions must be considered in future cosmological simulations.
‘It would [also] certainly be interesting to re-run some cosmological simulations of first star formation to see the effect of a decrease in HeH+ abundance on the thermal evolution and final masses of these stars,’ adds Bovino .