The isotropic blackbody CMB as evidence for a homogeneous universe

TL;DR

This paper demonstrates that the isotropic blackbody CMB, combined with the Sunyaev-Zel'dovich effect, can be used to confirm the universe's homogeneity and isotropy from a single observer's perspective, providing a theoretical basis for testing the Cosmological Principle.

Contribution

It introduces a new method showing that the Sunyaev-Zel'dovich effect enables an observer to verify cosmic homogeneity and isotropy using only CMB observations, even from a single vantage point.

Findings

Isotropic blackbody CMB implies universe's homogeneity and isotropy with SZ effect.

Extended observation or multiple scattering detection is necessary.

Provides a theoretical foundation for testing the Cosmological Principle.

Abstract

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.