将宇宙学原理拆解成碎片：内禀均匀且各向同性时空的前奏
Breaking the Cosmological Principle into pieces: a prelude to the intrinsically homogeneous and isotropic spacetimes
AI总结：
本文将宇宙学原理拆解为三个基本假设，指出保留其中关于内禀均匀且各向同性时空的假设，可自然导出包含新密度参数的“$Λ$CDM-like”有效理论，为宇宙学应用提供容纳标准模型成分的非均匀模型。
摘要：
在本文中，我们展示了支撑宇宙学原理的三个基本构建块。第一个指出宇宙中存在一个特殊参考系，其空间几何是内禀均匀且各向同性的。第二个要求存在一个基准观测者，对其而言Hubble参数是各向同性的。最后一块指出物质和辐射表现为完美流体。我们证明这三个假设给出了Friedmann-Lemaître-Robertson-Walker (FLRW)时空，即宇宙学标准模型的中心支柱。我们保留第一个假设，并开始研究所谓的内禀均匀且各向同性时空。它们是在CMB与几何参考系退耦后出现的。此外，在这些背景中自然产生了一个“$Λ$CDM-like”有效理论，以及一些与局部不均匀性、内能密度和Hubble各向异性的局部及全局量级相关的新密度参数。所有这些性质使得这类非均匀模型（粗略地说，保留了宇宙学原理的“1/3”）值得在宇宙学应用中研究，因为它能容纳标准模型的相同成分，作为几何参考系和自由下落的各向同性宇宙背景辐射，并在某些可观测参数消失时还原为后者。
英文摘要：
In this manuscript, we show that three fundamental building blocks are supporting the Cosmological Principle. The first of them states that there is a special frame in the universe where the spatial geometry is intrinsically homogeneous and isotropic. The second demands the existence of a fiducial observer to whom the Hubble parameter is isotropic. The last piece states that matter and radiation behave as a perfect fluid. We show that these three hypotheses give us the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetimes, the central pillar of the standard model of Cosmology. We keep with the first of them and start to investigate the so-called intrinsically homogeneous and isotropic spacetimes. They emerge after the decoupling of the CMB with the geometric frame of reference. Furthermore, a ``$Λ$CDM-like" effective theory arises naturally in those backgrounds, together with some new density parameters relating to the local inhomogeneities, the internal energy density, and the local and global magnitudes of the Hubble anisotropy. All those properties make this class of inhomogeneous models, which roughly speaking, keeps "1/3" of the Cosmological Principle, worth investigating in applications to Cosmology, for it can accommodate the same ingredients of the standard model, as a geometric frame and a free-falling isotropic cosmic background radiation, and reduce to the latter when some observable parameters vanish.
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