摇动，而不是搅动：引力标量张量理论中的动力学混合
Shaken, not stirred: kinetic mixing in scalar-tensor theories of gravity
摘要：
度规自由度和标量自由度之间的动力学混合是当代标量-张量理论的一个重要组成部分。这通常使得很难理解它们的物理内容，特别是当存在导数混合时，就像Horndeski作用的情况一样。在这项工作中，我们开发了一种方法，允许写一个Ricci曲率自由标量场方程，并讨论了一些这样的改写的稳定性问题的研究中存在的物质，爱因斯坦框架的存在和推广的非正式屏蔽机制的优点。对于四次Horndeski理论，这样的过程通常会留下一个剩余的曲率耦合，由Weyl张量给出。这就产生了一个二元分类的标量张量理论到搅拌理论，其中曲率可以取代，和动摇的理论，其中剩余耦合曲率仍然存在。值得注意的是，我们发现广义DBI Galileons属于第一类。最后，我们讨论动力学混合五次理论的非线性混合项出现，并在最近提出的理论超越Horndeski显示一种新的形式的动力学混合，其中的场方程是来源于衍生物的能量动量张量。
英文摘要：
Kinetic mixing between the metric and scalar degrees of freedom is an essential ingredient in contemporary scalar-tensor theories. This often makes hard to understand their physical content, especially when derivative mixing is present, as it is the case for Horndeski action. In this work we develop a method that allows to write a Ricci curvature-free scalar field equation and discuss some of the advantages of such rephrasing in the study of stability issues in the presence of matter, the existence of an Einstein frame and the generalization of the disformal screening mechanism. For quartic Horndeski theories, such procedure leaves, in general, a residual coupling to curvature, given by the Weyl tensor. This gives rise to a binary classification of scalar-tensor theories into stirred theories, for which the curvature can be substituted for, and shaken theories for which a residual coupling to curvature remains. Quite remarkably, we have found that generalized DBI Galileons belong to the first class. Finally, we discuss kinetic mixing in quintic theories for which non-linear mixing terms appears and in the recently proposed theories beyond Horndeski which display a novel form of kinetic mixing, in which the field equation is sourced by derivatives of the energy-momentum tensor.
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