Dense Matter in Compact Stars

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Compact stars are, after black holes, the densest objects in the universe. They are as heavy as the sun, but their radius is only about 10 km. Their extreme density makes compact stars a perfect "laboratory" for fundamental physics. The reason is that when matter is squeezed more and more, at some point the relevant degrees of freedom are no longer atoms, but rather neutrons and protons or - possibly relevant for the center of a compact star - quarks. In other words, we can learn something about our fundamental theories of nature by relating astrophysical observations to predictions from the microscopic theory. And, we can turn the argument around and learn something about the star ("What is a compact star made of?") by computing observable quantities from fundamental theories. This interplay between astrophysics and particle physics is at the core of the proposed research. One main line of my research will be the study of "stellar superfluids" and their hydrodynamic properties. The underlying mechanism for superfluidity (and superconductivity) is very general: just like helium-3 atoms or electrons, the fermions in a compact star may form Cooper pairs. Therefore, it is very likely that in the interior of a compact star, nuclear matter and quark matter become superfluid (they can also be superconducting, for instance in a phase where protons form a Cooper pair condensate). In contrast to the superfluids in an ordinary laboratory, the stellar superfluids are of relativistic nature, at least deep inside the star. It is thus one main objective of my research to connect a microscopic, field-theoretical description of relativistic superfluids with astrophysical observables that are sensitive to whether matter is superfluid or not. Such an observable is for instance the rotation frequency: some stars rotate about 1000 times per second; this is remarkable because we know that there are certain instabilities which tend to slow down the star's rotation (by emitting gravitational waves at the same time). In order to understand the necessary damping of these instabilities, a thorough understanding of the hydrodynamic properties of dense matter is mandatory. In particular, viscous effects in a superfluid are very different from viscous effects in a normal fluid. Compact stars are not only very dense and rotate very fast, but can also have enormously large magnetic fields. In this case, they are called magnetars. Again, this is very interesting from the fundamental point of view: magnetic fields, if sufficiently large, may influence or even dramatically change the properties of fundamental matter. For instance, one may ask whether a star that would be entirely made of ordinary nuclear matter in the absence of a magnetic field contains a core of quark matter in the presence of a magnetic field. It is an ongoing effort in current research to understand such changes in the phase structure theoretically. One long-term theoretical goal is to "map out" the phases of Quantum Chromodynamics not only in the plane of temperature and baryon density, but in a three-dimensional phase diagram that also contains the magnetic field. Compact stars sit somewhere in this three-dimensional space.