引力熵与平坦性、均匀性和各向同性之谜
Gravitational entropy and the flatness, homogeneity and isotropy puzzles
摘要：
我们对观测到的宇宙的大尺度平坦性、均匀性和各向同性提出了一种新的解释。基本的成分是基本的和众所周知的，即爱因斯坦的引力理论和霍金计算引力熵的方法。新的扭曲提供了边界条件，我们最近提出的"大爆炸"型奇点占主导地位的共形物质，强制$CPT$对称性和解析性。在这里，我们表明，除了允许我们描述大爆炸，这些边界条件允许新的引力瞬子，使我们能够计算宇宙学的引力熵，其中包括辐射，暗能量和空间曲率的任何一个符号。我们找到了这些宇宙的引力熵，$S_g \sim S_Λ^{1/ 4} S_r$，其中$S_Λ$是著名的德西特熵，$S_r$是辐射的总熵。在$S_g$超过$S_Λ$的范围内，最可能的宇宙是平坦的。通过分析我们的新瞬子的扰动，我们认为它在大尺度上也是均匀和各向同性的。
英文摘要：
We suggest a new explanation for the observed large scale flatness, homogeneity and isotropy of the universe. The basic ingredients are elementary and well-known, namely Einstein's theory of gravity and Hawking's method of computing gravitational entropy. The new twist is provided by the boundary conditions we recently proposed for "big bang" type singularities dominated by conformal matter, enforcing $CPT$ symmetry and analyticity. Here, we show that, besides allowing us to describe the big bang, these boundary conditions allow new gravitational instantons, enabling us to calculate the gravitational entropy of cosmologies which include radiation, dark energy and space curvature of either sign. We find the gravitational entropy of these universes, $S_g \sim S_Λ^{1/ 4} S_r$, where $S_Λ$ is the famous de Sitter entropy and $S_r$ is the total entropy in radiation. To the extent that $S_g$ exceeds $S_Λ$, the most probable universe is flat. By analysing the perturbations about our new instantons, we argue it is also homogeneous and isotropic on large scales.
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