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In this manuscript, we show that three fundamental building blocks aresupporting the Cosmological Principle. The first of them states that there is aspecial frame in the universe where the spatial geometry is intrinsicallyhomogeneous and isotropic. The second demands the existence of a fiducialobserver to whom the Hubble parameter is isotropic. The last piece states thatmatter and radiation behave as a perfect fluid. We show that these threehypotheses give us the Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetimes,the central pillar of the standard model of Cosmology. We keep with the firstof them and start to investigate the so-called intrinsically homogeneous andisotropic spacetimes. They emerge after the decoupling of the CMB with thegeometric frame of reference. Furthermore, a ``$\Lambda$CDM-like" effectivetheory arises naturally in those backgrounds, together with some new densityparameters relating to the local inhomogeneities, the internal energy density,and the local and global magnitudes of the Hubble anisotropy. All thoseproperties make this class of inhomogeneous models, which roughly speaking,keeps "1/3" of the Cosmological Principle, worth investigating in applicationsto Cosmology, for it can accommodate the same ingredients of the standardmodel, as a geometric frame and a free-falling isotropic cosmic backgroundradiation, and reduce to the latter when some observable parameters vanish.

Breaking the Cosmological Principle into pieces: a prelude to the intrinsically homogeneous and isotropic spacetimes