标题： 一种开放系统方法的重力 显示英文标题

标题： An Open System Approach to Gravity

摘要： 在宇宙学中，几个主要的未解问题，包括暴胀的本质、暗物质和暗能量，具有共同的结构：它们涉及具有未知微观物理的充满时空的介质，并且到目前为止只能通过它们的引力效应进行探测。 这一观察结果促使我们采用一种系统性的开放系统方法来研究宇宙学，在这种方法中，引力在存在一般不可观测环境的情况下演化。 在本工作中，我们基于广义相对论和施温格-凯尔迪什形式主义，开发了一个开放引力动力学的一般框架，并仔细处理了微分同构不变性所施加的非平凡约束。 在量子层面，我们的路径积分公式在半经典时空周围进行微扰理论计算，以得出引力密度矩阵。 作为说明性应用，我们在环境相互作用不可忽略的经典区域中研究了暴胀和引力波的传播。 在暴胀背景下，我们的框架在解耦极限下再现了已知的暴胀开放有效场论，并将其扩展以包括引力相互作用。 对于引力波，我们推导了传播中最一般的保守和耗散修正。 值得注意的是，我们发现一阶引力双折射是耗散性质的，而保守双折射仅在更高阶导数项中出现，这与电磁情况相反。 我们的结果为建模晚期宇宙中的耗散效应铺平了道路。 显示英文摘要

摘要： Several major open problems in cosmology, including the nature of inflation, dark matter, and dark energy, share a common structure: they involve spacetime-filling media with unknown microphysics, and can be probed so far only through their gravitational effects. This observation motivates a systematic open-system approach to cosmology, in which gravity evolves in the presence of a generic, unobservable environment. In this work, we develop a general framework for open gravitational dynamics based on general relativity and the Schwinger-Keldysh formalism, carefully addressing the nontrivial constraints imposed by diffeomorphism invariance. At the quantum level, our path integral formulation computes the gravitational density matrix in perturbation theory around a semi-classical spacetime. As illustrative applications, we study inflation and the propagation of gravitational waves in classical regimes where environmental interactions are non-negligible. In the inflationary context, our framework reproduces the known Open Effective Field Theory of Inflation in the decoupling limit and extends it to include gravitational interactions. For gravitational waves, we derive the most general conservative and dissipative corrections to propagation. Remarkably, we find that the leading-order gravitational birefringence is dissipative in nature, whereas conservative birefringence appears only at higher derivative order, opposite to the electromagnetic case. Our results pave the way to modeling dissipative effects in the late universe.