The Matter of the First Matter

Scientists who are curious about the beginnings of the Universe – how the very first matter formed, for example – can only dream of getting into time machines and traveling back some 13 billion years. The next best thing is to try to recreate the conditions that may have existed in the Universe’s infancy, soon after the Big Bang. That is precisely what Prof. Daniel Zajfman does in his lab in the Particle Physics and Astrophysics Department at the Weizmann Institute of Science.

Some fifteen years ago, Prof. Zajfman had proposed the idea of a “cryogenic storage ring” (CSR) to the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. This facility, called “the ring” for short, was planned to be 35 meters long, and the budget allotted to build it was some ten million dollars. The ring’s ambitious design was a giant device for trapping molecular ions using electrostatic fields. Small molecular ions of the types that would have come together soon after being created in the Big Bang would be trapped in the ring and cooled to extremely cold temperatures – that is, they would be taken to the lowest energy level in which molecules can exist, and in which the movements of such molecular ions, which normally vibrate and rotate, come to all but a complete halt. When molecular ions frozen in this way slow down, it makes them much easier to study. And from the behavior of a few ions in a cold storage ring, one can learn about the processes that led to the first star and galaxy formation.

Soon after construction of the ring began, Prof. Zajfman was elected president of the Weizmann Institute of Science, a job that took him away from the group in Heidelberg. Still, in his lab in Rehovot, he already had a smaller device – just 50 centimeters long and costing just several thousand dollars – that he had built together with Dr. Oded Heber and other members of his group. That little device works with mirrors – two of them that “message” molecular ions back and forth. This little device, does, indeed trap ions, but the planned large ring in Heidelberg held much greater promise when it came to understanding the behavior of cold molecular ions.

From the behavior of a few ions in a cold storage ring, one can learn about the processes that led to the first star and galaxy formation

The plan for the CSR turned out to be as challenging as it was ambitious, and the builders faced a long series of technological and scientific hurdles that, at times, seemed insurmountable. Although Prof. Zajfman was not involved in the daily overseeing of the construction, he did continue to keep track of the progress and to advise on ways to overcome various difficulties. As Zajfman took on a second, and then another half term at the Weizmann Institute, the scientists and engineers on the CSR team persevered, solving problems one at a time as they arose.

So the cry of “Eureka” was particularly joyful this past year when the group announced that the CSR had begun operation, and had even produced its first results. One of the feats they managed to perform was to cool the entire ring down to around four degrees above absolute zero, and the first experiments performed in the ring have already yielded some surprising discoveries. These experiments looked at molecular ions formed of a hydrogen and a helium atom (HeH⁺), these being the first two elements created in the young Universe. The CSR enabled the scientists to focus on the way that this molecule grabs on to an electron from the surround plasma. The electron neutralizes the positive charge on the ion, but it also breaks up the molecule. That is, the process of attracting electrons is ultimately a destructive one; as it splits molecules apart into atoms, it hinders the creation of the more complex matter from which stars are made.

Prof. Zajfman says that one of the most impressive features of the large CSR is that it gives researchers the ability to isolate a single molecule and track it “personally.” That is, the results can follow what happens to one molecule as it comes together and falls apart, in contrast with previous experiments of this kind, which give an average for a group of molecules, and certainly not at the relevant cold temperature.

One of the surprising results calls into question a very basic assumption – the idea that the amount of HeH⁺ existing in the Universe is the outcome of a sort of stable ratio between the rates of molecule formation and their subsequent break down. Since the ion is rare, scientists had concluded that the breakdown is faster and more common that the opposite process – that of molecular creation. But the experiments – some of the most controlled and precise ever conducted – showed more production and slower breakdown that the theory posited. Could this have been the case in the earliest Universe? The findings suggest a rethink is needed for theories and models explaining how the first molecular matter – and thus the first stars and galaxies -- formed.

Prof. Daniel  Zajfman's research is supported by the Benoziyo Endowment Fund for the Advancement of Science; the Veronika A. Rabl Physics Discretionary Fund; and the Comisaroff Family Trust. Prof. Zajfman is the incumbent of the Simon Weinstock Professorial Chair of Astrophysics.