In this manuscript, we show that there are three fundamental building blocks 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 ‘ΛCDM-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.

中文翻译：

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将宇宙学原理分解：本质上均匀且各向同性时空的前奏

在这份手稿中，我们展示了支持宇宙学原理的三个基本组成部分。第一个观点指出，宇宙中有一个特殊的框架，其中空间几何本质上是均匀的和各向同性的。第二个要求存在一个基准观察者，其哈勃参数是各向同性的。最后一篇文章指出物质和辐射表现为完美的流体。我们证明，这三个假设为我们提供了弗里德曼-勒梅特-罗伯逊-沃克（FLRW）时空，这是宇宙学标准模型的中心支柱。我们继续其中的第一个，并开始研究所谓的本质均匀和各向同性时空。它们是在 CMB 与几何参考系脱钩后出现的。此外，在这些背景下自然出现了“类似ΛCDM”的有效理论，以及一些与局部不均匀性、内部能量密度以及哈勃各向异性的局部和全局幅度相关的新密度参数。所有这些特性使得这类非齐次模型，粗略地说，保留了宇宙学原理的“1/3”，值得在宇宙学应用中进行研究，因为它可以容纳标准模型的相同成分，作为几何框架和自由落体各向同性宇宙背景辐射，当一些可观测参数消失时减少到后者。