0.5 eV QCD Axion Cosmology

TL;DR

This paper proposes a cosmological model involving 0.5 eV QCD axions as dark matter, predicting key universe parameters with high precision and aligning well with multiple observational data sets.

Contribution

It introduces a new cosmological framework where 0.5 eV QCD axions account for dark matter, dark energy, and the Big Bang, providing precise predictions matching observations.

Findings

Redshift of matter-radiation equality matches axion crossover.

Hubble constant prediction aligns with late-universe measurements.

Cosmological constant and matter abundance agree with observations.

Abstract

A simple yet compelling physical picture is proposed for the nature of dark matter, dark energy, and the Big Bang. The proposal leads to predictions, from first-principles with high precision, for the values of the Hubble constant, cosmological constant, and matter abundance in the universe. Early-universe observations yield a value for the redshift of matter-radiation equality, within the standard cold dark matter cosmology, that roughly matches the redshift where the equation of state crosses over from radiation-like to matter-like for the quantum chromodynamic (QCD) axion particles, with rest-mass energy per particle around 0.5 eV, and with number density six times that of the photons made in the Big Bang, that form the dark matter and that dominate the early-universe expansion dynamics within the proposed cosmology. Late-universe observations suggest a value for the Hubble constant that agrees, within the percent-level uncertainty of the comparison, with the value predicted by 0.5 eV QCD axion cosmology. Observations near cosmic noon, in the middle-universe, show pleasing agreement with the predicted values for the cosmological constant and matter abundance. Bolstered by this broad range of observational support, we re-visit the conventional astrophysical assumptions that have been used to rule out, constrain, and exclude 0.5 eV QCD axion dark matter for the past half century.