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Pear-shaped nuclei may be a key to why our universe survived the big bang

The difference between our universe existing and not existing hinges upon some astonishingly small things—among them, atomic nuclei that are shaped like pears. In an article published in Nature, University of Liverpool scientists who observed some atomic nuclei form atypical pear-like shapes wrote that the atomic forces that created these shapes might have been critical to the universe not self-destructing in a fiery conflagration of matter and antimatter right after its birth.

Physicists have been able to explain myriad atomic and subatomic interactions by way of a sweeping Standard Model of Particle Physics, which was developed by generations of contributing scientists. But the model has left a lingering question of why the universe has more matter than antimatter—i.e., why there is a universe at all. According to the model, every atom of matter should be accompanied by an atom of antimatter. And if that is so, then our universe should have obliterated itself almost as soon as it began, since antimatter and matter blow each other up whenever they meet. That the universe is still here implies that the quantity of matter was higher.


To understand why there was initially more matter, scientists took a look into physics phenomena outside the Standard Model. One vital testing ground for exploring these phenomena, according to researchers, would be “electric dipole moments,” in which an atom’s center of positive charge and its center of negative charge lie at differing, asymmetrical points.

Electric dipole moments may or may not exist within the 136 different atoms known to molecular chemistry, all of whose nuclei have a fairly recognizable shape: somewhat like a rugby ball, symmetrical while not quite spherical. Even if one of these atoms’ nuclei has an electric dipole moment, however, it is too faint for even our highest-resolution electron microscopes to study it.

But the pear-shaped atoms that the Liverpool scientists were describing might have much larger electric dipole moments, in which case they could be optimal places to look. These atoms formed at the CERN laboratory in Switzerland, during experiments in which volleys of protons were shot at a mass of uranium carbide. Isotopes formed, out of which several exhibited distinctive pear shapes, more mass concentrated on one end of the nuclei than the other. The same forces that created these nucleic imbalances might have been at work in the early universe and led to the imbalance of matter over antimatter. More testing of pear-shaped nuclei is going to ensue and may yield further answers.