In the initial moments after the Big Bang, the universe existed as an extremely hot plasma with temperatures soaring into the billions of degrees. Over countless years, the cosmos expanded and cooled, arriving at approximately 3000K around 380,000 years after the Big Bang. This reduction in temperature permitted electrons and nuclei to combine into the first neutral elements, particularly atomic hydrogen and helium, signaling the beginning of the cosmic dark ages—an era lacking visible light sources despite a wealth of gas.
Within these clouds, helium hydride (HeH⁺) is thought to have formed as the fundamental molecule. Holger Kreckel from the Max Planck Institute for Nuclear Physics emphasizes the simplicity yet significance of early universe chemistry, noting that only a few molecular interactions have been accurately measured. The creation and interactions of molecules like HeH⁺ offer vital insights into the cooling of primordial gas, crucial for the formation of stars and galaxies.
Up until recently, theories suggested that the temperatures of a few thousand kelvin in the primordial universe were too cold for effective reactions between HeH⁺ and hydrogen, as such conditions were believed to slow down barrierless ion-neutral reactions. However, Kreckel’s recent experiments and models uncover that the reaction in question was indeed barrierless and surprisingly swift, even under these cooler circumstances.
The researchers carried out laboratory experiments alongside new calculations on the isotopic variant, HeH⁺ + D, indicating that the previously assumed energy barrier was a result of inaccurate analytical potential energy surface models from earlier studies. This correction exposed the absence of a true barrier, enabling the reaction to take place at considerable rates and influencing the anticipated cosmic molecular evolution.
This finding mainly suggests that HeH⁺ was subjected to considerable destruction through collisions rather than hydrogen reactions, altering the understanding of early universe chemistry. This advancement assists in improving cosmological simulations that track molecular signatures in the early universe, possibly changing predictions regarding star formation and timelines for molecular abundance.
The empirical findings from Kreckel’s team, supported by the Cryogenic Storage Ring, emulate early universe conditions, delivering essential data for forthcoming astrophysical inquiries. As potential energy barriers were reassessed, they provide refined viewpoints on chemical reactivity, indicating a lower presence of HeH⁺ following the Big Bang than previously estimated. Consequently, these revelations might incite a reevaluation of early star formation models, as proposed by related researchers, and encourage more comprehensive cosmological simulations to reevaluate the effects of HeH⁺ abundances on stellar evolution.