ACW模型的不稳定性，以及暴胀过程中大量向量的问题
Instability of the ACW model, and problems with massive vectors during inflation
摘要：
我们证明了由固定范数向量场刻画的Ackerman-Carroll-Wise模型的各向异性暴胀背景是不稳定的。我们发现的不稳定性显式求解线性方程组的最一般的一组扰动在这个背景下，并注意到，解决方案发散接近地平线交叉。这是因为一个扰动在那一刻变成了幽灵。一个简化的计算，只包括向量场的扰动，显示出同样的不稳定性，澄清了问题的根源。然后，我们讨论了其他几个模型，特别强调的情况下，非最小耦合的曲率，其中矢量场被用来支持各向异性膨胀，或产生宇宙学扰动的各向同性背景。在许多情况下，矢量的质量项需要有“错误”的符号;我们表明，作为结果，纵向矢量模式是一个鬼（一个领域与负动力学项，负能量;不简单的快子）。我们评论在量子水平上出现的问题。特别是，鬼的存在可能是一个严重的困难，紫外线完成，这种模式需要在地平线下的制度。
英文摘要：
We prove that the anisotropic inflationary background of the Ackerman-Carroll-Wise model, characterized by a fixed-norm vector field, is unstable. We found the instability by explicitly solving the linearized equations for the most general set of perturbations around this background, and by noticing that the solutions diverge close to horizon crossing. This happens because one perturbation becomes a ghost at that moment. A simplified computation, with only the perturbations of the vector field included, shows the same instability, clarifying the origin of the problem. We then discuss several other models, with a particular emphasis on the case of a nonminimal coupling to the curvature, in which vector fields are used either to support an anisotropic expansion, or to generate cosmological perturbations on an isotropic background. In many cases, the mass term of the vector needs to have the ``wrong'' sign; we show that, as a consequence, the longitudinal vector mode is a ghost (a field with negative kinetic term, and negative energy; not simply a tachyon). We comment on problems that arise at the quantum level. In particular, the presence of a ghost can be a serious difficulty for the UV completion that such models require in the sub-horizon regime.
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