Anisotropic universes with isotropic cosmic microwave background radiation

TL;DR

This paper demonstrates that certain anisotropic cosmological models can produce an isotropic cosmic microwave background at a specific moment, challenging the assumption that isotropy implies a Friedmann-Lemaitre universe.

Contribution

It introduces models where the universe appears isotropic in the CMB at a moment despite underlying anisotropic expansion, highlighting limitations of using CMB isotropy alone.

Findings

Existence of anisotropic models with isotropic CMB at one instant

Isotropic CMB does not necessarily imply a Friedmann-Lemaitre universe

Anisotropic expansion can occur alongside isotropic CMB

Abstract

We show the existence of spatially homogeneous but anisotropic cosmological models whose cosmic microwave background temperature is exactly isotropic at one instant of time but whose rate of expansion is highly anisotropic. The existence of these models shows that the observation of a highly isotropic cosmic microwave background temperature cannot alone be used to infer that the universe is close to a Friedmann-Lemaitre model.