The many faces of Strange Matter in the Universe

(A summary of the project supported by the John Templeton Foundation, “Beyond the Horizons 2011”)

What is the origin of mass? What is the nature of ultra-high energy cosmic rays and the puzzling Centauro events? What is the real state of matter at extraordinarily high densities? All these challenges could be related to the single fundamental question of whether strange matter truly exists or not in the Universe. The solution of that deep question would help mankind to understand one of the seven Millennium Prize Problems named by the Clay Mathematical Institute.

Strange matter is also known as strange quark matter. We have known that the nuclei within atoms are made of nucleons (protons and neutrons) since the discovery of neutrons by Chadwick in 1932, but it was not until 1964 that Murray Gell-Mann and George Zweig proposed that those nucleons are themselves made of quarks. Hypothetical matter consisting of a huge number of nucleons is known as nuclear matter, while that of quarks is quark matter. The standard model of particle physics now contains six flavors of quark; normal matter is dominated by combinations of two of these, called the “up” and “down” quarks. Strange quark matter also contains a third type of quark, which was whimsically called the “strange” quark since the first particles inferred to contain them were unusual. Strange quark matter is expected to be present in the Universe at high densities, and a further fascinating possibility is that strange quark matter is the most natural stable state of matter. However, there is still no clear evidence for such matter.

Quarks all carry a property called color and, in a similar way to how electrically charged particles experience electromagnetic forces, quarks interact via this color charge. The dynamical theory of the latter interaction is believed to be QCD (quantum chromo-dynamics). QCD is one kind of Yang-Mills theory, in which beautiful underlying symmetries govern the nature of the fundamental interactions. Quantum electro-dynamics (QED) is fundamentally different from QCD since the strength of the coupling in QED is weak at all energies, and so the theory can be solved using perturbative methods. By contrast, at low energies the QCD coupling is problematically strong; solving non-perturbative QCD is a massively significant challenge for today’s particle physicists. The state of strange matter is in the low-energy regime, hence a fearsomely difficult theoretical problem.

In astrophysics, the nature of strange matter has interesting and deeply profound implications for understanding a wide variety of objects and events. Strange matter could be most commonly present in pulsar-like compact stars, but it could also be closely related to such events as supernovae, gamma-ray bursts, the production of cosmic rays and the cosmic QCD phase transition in the early Universe, for example.

Thanks to the advanced facilities now available to both physics and astronomy, the intricate relationships between microphysics and cosmophysics are becoming clearer and clearer. On one hand, many new astrophysical observations challenge some conventional scenarios, which were generally formed before the standard model of particle physics was established. On the other hand, it is still very optimistic for physicists to try to understand the broad sweep of the complex physical Universe by starting from the level of the standard model. Nevertheless, we would like to do fundamental physics via astronomical observations. Based on his previous investigations, Prof. Renxin Xu of Peking University will study various astrophysical implications and manifestations of strange matter, especially those of compact stars after supernovae, of cosmic rays beyond the GZK cutoff and of quark nuggets created in the early Universe, in order to understand the interaction between quarks.