Dense Matter in Compact Stars

紧凑型恒星是宇宙中密度仅次于黑洞的物体。它们和太阳一样重，但半径只有10公里左右。它们的极高密度使致密恒星成为基础物理学的完美“实验室”。原因是，当物质被压缩得越来越大时，在某一点上，相关的自由度不再是原子，而是中子和质子，或者夸克——这可能与致密恒星的中心有关。换句话说，我们可以通过将天体物理观测与微观理论的预测联系起来，来了解一些关于自然的基本理论。而且，我们可以把这个论点反过来，了解一些关于恒星的知识（“致密恒星是由什么组成的？”）通过计算基本理论中的可观测量。天体物理学和粒子物理学之间的相互作用是拟议研究的核心。我研究的一条主线将是研究“恒星超流体”及其流体力学性质。超流动性（和超导性）的潜在机制非常普遍：就像氦-3原子或电子一样，致密恒星中的费米子可能形成库珀对。因此，在致密恒星的内部，核物质和夸克物质很可能变成超流体（它们也可以是超导的，例如在质子形成库珀对凝聚的阶段）。与普通实验室中的超流体不同，恒星超流体具有相对论性质，至少在恒星内部深处是如此。因此，我研究的一个主要目标是将相对论超流体的微观场论描述与天体物理观测联系起来，这些观测对物质是否是超流体很敏感。这样的一个可观测值是旋转频率：一些恒星每秒旋转1000次；这是值得注意的，因为我们知道存在某些不稳定性，这些不稳定性往往会减慢恒星的旋转速度（通过同时发射引力波）。为了了解这些不稳定性的必要阻尼，必须彻底了解致密物质的水动力特性。特别地，超流体中的粘性效应与正常流体中的粘性效应有很大的不同。紧凑型恒星不仅密度大、自转快，而且磁场也非常大。在这种情况下，它们被称为磁星。再一次，从基本观点来看，这是非常有趣的：磁场，如果足够大，可能会影响甚至戏剧性地改变基本物质的性质。例如，人们可能会问，在没有磁场的情况下，一颗完全由普通核物质构成的恒星，在有磁场的情况下，是否包含一个由夸克物质组成的核心？从理论上理解这种相结构的变化是当前研究的一个持续努力。一个长期的理论目标是“绘制”量子色动力学的相位，不仅在温度和重子密度的平面上，而且在包含磁场的三维相位图中。致密恒星位于这个三维空间的某个地方。

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.