Constraints on the Hubble and matter density parameters with and without modelling the CMB anisotropies

TL;DR

This paper combines multiple cosmological constraints to examine the Hubble and matter density parameters, finding that the standard CMB fit aligns with a narrow, consistent region and suggesting late-time modifications to cosmic expansion as a solution to the Hubble tension.

Contribution

It demonstrates that diverse, independent cosmological measurements converge on a consistent parameter region, challenging early-universe solutions to the Hubble tension and proposing late-time expansion modifications.

Findings

Standard CMB fit aligns with constraints from age, power spectrum, and supernovae/BAO data.

Early universe solutions to the Hubble tension are disfavored by combined constraints.

Late-time modifications, such as dark energy evolution, are promising alternatives.

Abstract

We consider constraints on the Hubble parameter $H_0$ and the matter density parameter $\Omega_{\mathrm{M}}$ from: (i) the age of the Universe based on old stars and stellar populations in the Galactic disc and halo (Cimatti & Moresco 2023); (ii) the turnover scale in the matter power spectrum, which tells us the cosmological horizon at the epoch of matter-radiation equality (Philcox et al. 2022); and (iii) the shape of the expansion history from supernovae (SNe) and baryon acoustic oscillations (BAOs) with no absolute calibration of either, a technique known as uncalibrated cosmic standards (UCS; Lin, Chen, & Mack 2021). A narrow region is consistent with all three constraints just outside their $1\sigma$ uncertainties. Although this region is defined by techniques unrelated to the physics of recombination and the sound horizon then, the standard $Planck$ fit to the CMB anisotropies falls precisely in this region. This concordance argues against early-time explanations for the anomalously high local estimate of $H_0$ (the 'Hubble tension'), which can only be reconciled with the age constraint at an implausibly low $\Omega_{\mathrm{M}}$. We suggest instead that outflow from the local KBC supervoid (Keenan, Barger, & Cowie 2013) inflates redshifts in the nearby universe and thus the apparent local $H_0$. Given the difficulties with solutions in the early universe, we argue that the most promising alternative to a local void is a modification to the expansion history at late times, perhaps due to a changing dark energy density.