Cosmological CMBR dipole in open universes ?

TL;DR

This paper investigates whether the cosmic microwave background radiation dipole can be explained by large-scale isocurvature perturbations in open universes, concluding that such an explanation is only viable in universes extremely close to flat.

Contribution

It critically examines the isocurvature interpretation of the CMBR dipole within open cosmological models, establishing its limitations.

Findings

Isocurvature explanation is invalid for open universes unless very close to flat.

The standard Doppler interpretation remains the most consistent explanation.

Open universes with significant curvature cannot account for the dipole via isocurvature perturbations.

Abstract

The observed CMBR dipole is generally interpreted as a Doppler effect arising from the motion of the Earth relative to the CMBR frame. An alternative interpretation, proposed in the last years, is that the dipole results from ultra-large scale isocurvature perturbations. We examine this idea in the context of open cosmologies and show that the isocurvature interpretation is not valid in an open universe, unless it is extremely close to a flat universe, $|\Omega_0 -1|< 10^{-4}$.