The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB

TL;DR

This paper investigates how inhomogeneous universe models, specifically void models, can explain the CMB spectrum without dark energy, emphasizing the importance of including radiation effects for accurate constraints.

Contribution

It provides a detailed analysis of small-scale CMB in inhomogeneous, spherically symmetric models, highlighting the significance of radiation effects often neglected in prior studies.

Findings

Open at CMB distances models fit the CMB spectrum without fine tuning

Such models also align with supernovae and local Hubble rate data

Asymptotically flat models require additional assumptions

Abstract

The dimming of Type Ia supernovae could be the result of Hubble-scale inhomogeneity in the matter and spatial curvature, rather than signaling the presence of a dark energy component. A key challenge for such models is to fit the detailed spectrum of the cosmic microwave background (CMB). We present a detailed discussion of the small-scale CMB in an inhomogeneous universe, focusing on spherically symmetric `void' models. We allow for the dynamical effects of radiation while analyzing the problem, in contrast to other work which inadvertently fine tunes its spatial profile. This is a surprisingly important effect and we reach substantially different conclusions. Models which are open at CMB distances fit the CMB power spectrum without fine tuning; these models also fit the supernovae and local Hubble rate data which favours a high expansion rate. Asymptotically flat models may fit the CMB, but require some extra assumptions. We argue that a full treatment of the radiation in these models is necessary if we are to understand the correct constraints from the CMB, as well as other observations which rely on it, such as spectral distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect or the Baryon Acoustic Oscillations.