Applicable Scale of FLRW Metric: The Friedman Lemaitre Robertson Walker metric applies on large scales where the universe appears homogeneous and isotropic, typically on the order of billions of light-years. • Gravitational Binding and Structure Formation: Objects like planets, stars, galaxies, and galaxy clusters are gravitationally bound and resist the expansion of spacetime. • Cosmic Density Homogeneity: On the largest scales, the average density of the universe is remarkably uniform, with variations of less than one part in 10,000, unlike the vast density differences observed on smaller scales. • Universe Homogeneity Measurement: Two main ways to measure the homogeneity of the universe: measuring galaxy distribution and measuring the cosmic microwave background. • Early Universe Density Fluctuation: The average density fluctuation in the early universe was roughly the same on all cosmic size scales, about one part in 30,000,000. • Universe Homogeneity and Isotropy: The universe is roughly homogeneous and isotropic everywhere with only small quantifiable imperfections, justifying the use of a metric that assumes perfect homogeneity and isotropy.