 The remarkable success of perturbed FLRW models in describing the observed universe has led many cosmologists to focus almost exclusively on them, yet there are drawbacks to such a myopic approach. For example, the observations at best establish that the observed universe can be well-approximated by an almost FLRW model within some (large) domain. But they are not the only models that fit the data: there are other cosmological models that mimic FLRW models in the relevant domain, yet differ dramatically elsewhere (and elsewhen). Specifically, on the one hand there are a class of spatially homogeneous and anisotropic models (Bianchi models) that exhibit “intermediate isotropization”: namely, they have physical properties that are arbitrarily close to (isotropic) FLRW models over some time scale  \(T\).[11]  Agreement over the time interval \(T\) does not imply global agreement, however, as these models have large anisotropies at other times. Relying on the FLRW models in making extrapolations to the early or late universe requires some justification for ignoring models, such as these Bianchi models, that mimic their behavior for a finite time interval. On the other hand there are inhomogeneous spherically symmetric models that can reproduce exactly the background model observations (number counts versus redshifts and angular diameter distance versus redshift, for example) with or without a cosmological constant (Mustapha et al. 1997). These can be excluded by direct observations with good enough standard candles (Clarkson et al. 2008) or by observations of structure formation features in such universes (Clarkson &amp; Maartens 2010); but that exclusion cannot take place unless one indeed examines such models and their observational consequences.