 As a specific instance of this challenge, consider the status of the cosmological principle, the global symmetry assumed in the derivation of the FLRW models. The results above show that all evidence available to us is equally compatible with models in which the cosmological principle does or does not hold. One might take the principle as holding a priori, or as a pre-condition for cosmological theorizing (Beisbart 2009). A recent line of work aims to justify the FLRW models by appealing to a weaker general principle in conjunction with theorems relating homogeneity and isotropy. Global isotropy around every point implies global homogeneity, and it is natural to seek a similar theorem with a weaker antecedent formulated in terms of observable quantities. The Ehlers-Geren-Sachs theorem (Ehlers et al. 1968) shows that if all geodesic fundamental observers in an expanding model find that freely propagating background radiation is exactly isotropic, then their spacetime is an FLRW model. If our causal past is “typical”, observations along our worldline will constrain what other observers should see. This is often called the Copernican principle—namely, no point \(p\) is distinguished from other points \(q\) by any spacetime symmetries or lack thereof (there are no “special locations”). There are indirect ways of testing this principle empirically: the Sunyaev-Zel’dovich effect can be used to indirectly measure the isotropy of the CBR as observed from distant points. Other tests are direct tests with a good enough set of standard candles, and an indirect test based on the time drift of cosmological redshift. This line of work provides an empirical argument that the observed universe is well-approximated by an FLRW model, thus changing that assumption from a philosophically based starting point to an observationally tested foundation.