  The question of whether the Universe is spatially homogeneous and isotropic
on the largest scales is of fundamental importance to cosmology, but has not
yet been answered decisively. Surprisingly, neither an isotropic primary CMB
nor combined observations of luminosity distances and galaxy number counts are
sufficient to establish such a result. The inclusion of the Sunyaev-Zel'dovich
effect in CMB observations, however, dramatically improves this situation. We
show that even a solitary observer who sees an isotropic blackbody CMB can
conclude that the universe is homogeneous and isotropic in their causal past
when the Sunyaev-Zel'dovich effect is present. Critically, however, the CMB
must either be viewed for an extended period of time, or CMB photons that have
scattered more than once must be detected. This result provides a theoretical
underpinning for testing the Cosmological Principle with observations of the
CMB alone.
