0.5 eV QCD Axion Cosmology

TL;DR

The paper proposes a simple, first-principles cosmology in which dark matter consists of 0.5 eV QCD axions and dark energy arises from the gravitational self-energy of massless particle-antiparticle pairs. This framework yields a high-precision prediction for the present expansion rate $H_0 \approx 71.94$ km s$^{-1}$ Mpc$^{-1}$ and compatible values for $\Omega_A h^2$, $\Omega_B h^2$, and $\Omega_\Lambda h^2$, addressing the Hubble tension while remaining concordant with early- and middle-universe observations. It also claims alignment with the timing of matter–radiation equality, primordial deuterium, and helium abundances, and provides a precise axion rest-mass energy $m_A c^2 \approx 0.504$ eV together with a predicted axion-photon coupling $g_{A\gamma\gamma} \approx 0.68 \times 10^{-10}$ GeV$^{-1}$. The work further argues for reinterpreting many astrophysical constraints on axions, situates the proposal within a conformal cross-over picture, and outlines concrete next steps in Big Bang nucleosynthesis and large-scale structure calculations to test the framework.

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.

Figures (4)

• Figure 1: Distance modulus, $\mu(z)=5~{\rm mag}\times\log_{10}(D(z)/{\rm Mpc})+25~{\rm mag}$, grows with redshift, $z$, in 0.5 eV QCD axion cosmology (teal line, —) in a way that is predicted from first-principles with high-precision by Eq. (\ref{['eq:luminosity_distance']}) and that lands on top of observations of 1635 high-$z$ SNe Ia from the Dark Energy Survey (DES) and 194 low-$z$ SNe Ia from the CfA/CSP + Foundation sample (violet pluses, +) vincenzi2024davis2024.
• Figure 2: Cooling of 0.5 eV QCD axion dark matter in the early universe shows a cross-over in the adiabatic compressibility, $dp_A/du_A$ (violet pluses, +), from the radiation-like value, $dp_A/du_A\approx1/3$, at large redshift, to the matter-like value, $dp_A/du_A\approx0$, at small redshift, starting right around the redshift of "matter-radiation equality," $z_{\rm eq}=3~376~(19)$ (vertical orange arrow), predicted by the standard cold dark matter cosmology with cosmological constant ($\Lambda{\rm CDM}$) using observations of the early universe planck_baselineplanck2020.
• Figure 3: Cooling of the universe in the first three minutes after the Big Bang for a mix with six 0.5 eV QCD axions for each photon (violet line, —) and for the standard mix of photons and light leptons (filled and unfilled boxes) both reach the freeze-out of the neutron-to-proton ratio (red horizontal line with dots) around the same time, but the standard mix peebles1966steigman2007 takes roughly twice as long to start forming helium nuclei (green horizontal line with boxes).
• Figure 4: Axion-photon coupling grows with initial core helium mass fraction in simulations of stellar evolution for horizontal branch stars constrained by observations of globular clusters (curved blue lines with dots) ayala2014. The predicted value for the axion-photon coupling strength (horizontal green lines with boxes), given in Eq. (\ref{['eq:ga']}), combines with this empirical-computational relation to give a prediction for the helium mass fraction (red box with whiskers) that differs by $5.0\sigma$ from the primordial helium abundance expected within the standard cold dark matter cosmology with cosmological constant ($\Lambda {\rm CDM}$) based on simulations of Big Bang nucleosynthesis (thick vertical orange line) parthenope2021primat2018, but that agrees rather well with the best present estimates for the initial core helium mass fraction in the Sun (pair of thin vertical red lines) piersanti2007serenelli2010.