Immediately after the Big Bang, which occurred around 13.8 billion years ago, the universe was dominated by unimaginably high temperatures and densities. Nonetheless, after just just a few seconds, it had cooled down enough for the primary elements to form, primarily hydrogen and helium. These were still completely ionized at this point, because it took almost 380,000 years for the temperature within the universe to drop enough for neutral atoms to form through recombination with free electrons. This paved the best way for the primary chemical reactions.
The oldest molecule in existence is the helium hydride ion (HeH+), formed from a neutral helium atom and an ionized hydrogen nucleus. This marks the start of a sequence response that results in the formation of molecular hydrogen (H2), which is by far essentially the most common molecule within the universe.
Recombination was followed by the ‘dark age’ of cosmology: although the universe was now transparent attributable to the binding of free electrons, there have been still no light-emitting objects, reminiscent of stars. Several hundred million years passed before the primary stars formed.
During this early phase of the universe, nevertheless, easy molecules reminiscent of HeH⁺ and H2 were essential to the formation of the primary stars. To ensure that the contracting gas cloud of a protostar to collapse to the purpose where nuclear fusion can begin, heat have to be dissipated. This happens through collisions that excite atoms and molecules, which then emit this energy in the shape of photons. Below roughly 10,000 degrees Celsius, nevertheless, this process becomes ineffective for the dominant hydrogen atoms. Further cooling can only happen via molecules that may emit additional energy through rotation and vibration. As a consequence of its pronounced dipole moment, the HeH⁺ ion is especially effective at these low temperatures and has long been considered a potentially vital candidate for cooling within the formation of the primary stars. Consequently, the concentration of helium hydride ions within the universe may significantly impact the effectiveness of early star formation.
During this era, collisions with free hydrogen atoms were a significant degradation pathway for HeH⁺, forming a neutral helium atom and an H2⁺ ion. These subsequently reacted with one other H atom to form a neutral H2 molecule and a proton, resulting in the formation of molecular hydrogen.
Researchers on the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg have now successfully recreated this response under conditions just like those within the early universe for the primary time. They investigated the response of HeH⁺ with deuterium, an isotope of hydrogen containing a further neutron within the atomic nucleus alongside a proton. When HeH⁺ reacts with deuterium, an HD⁺ ion is formed as a substitute of H2⁺, alongside the neutral helium atom.
The experiment was carried out on the Cryogenic Storage Ring (CSR) on the MPIK in Heidelberg — a globally unique instrument for investigating molecular and atomic reactions under space-like conditions. For this purpose, HeH⁺ ions were stored within the 35-metre-diameter ion storage ring for as much as 60 seconds at just a few kelvins (-267 °C), and were superimposed with a beam of neutral deuterium atoms. By adjusting the relative speeds of the 2 particle beams, the scientists were in a position to study how the collision rate varies with collision energy, which is directly related to temperature.
They found that, contrary to earlier predictions, the speed at which this response proceeds doesn’t decelerate with decreasing temperature, but stays almost constant. “Previous theories predicted a big decrease within the response probability at low temperatures, but we were unable to confirm this in either the experiment or latest theoretical calculations by our colleagues,” explains Dr Holger Kreckel from the MPIK. ‘The reactions of HeH⁺ with neutral hydrogen and deuterium due to this fact appear to have been way more vital for chemistry within the early universe than previously assumed,’ he continues. This commentary is consistent with the findings of a gaggle of theoretical physicists led by Yohann Scribano, who identified an error within the calculation of the potential surface utilized in all previous calculations for this response. The brand new calculations using the improved potential surface now align closely with the CSR experiment.
For the reason that concentrations of molecules reminiscent of HeH⁺ and molecular hydrogen (H2 or HD) played a crucial role within the formation of the primary stars, this result brings us closer to solving the mystery of their formation.