On June 15, 2026 at 10:10am local time in Pasadena, California, Science Synergy Science Chair Dr. Noah Bray-Ali presents Cooling Dark Matter Cosmology in the Cosmological Models and Survey Tests session of the 248th meeting of the American Astronomical Society. The standard cold dark matter cosmology with cosmological constant emerged a quarter century ago as a way of reconciling the low baryon abundance favored by cold dark matter cosmological simulations of Big Bang nucleosynthesis with the spatial flatness of the universe suggested by early-universe observations. However, over the past ten years, the prediction within this cold dark matter cosmology for the present expansion rate of the universe has been systematically ruled out by progressively more precise and robust direct late-universe observations of the expansion rate.
Cooling dark matter cosmology keeps the spatial flatness but roughly doubles the baryon abundance compared to the cold dark matter case to give a prediction for the present expansion rate that agrees with late-universe observations. Nevertheless, simulations of Big Bang nucleosynthesis within cooling dark matter cosmology, shown in the figure, are consistent with astrophysical observations of primordial helium abundance once the effect of cooling dark matter on stellar evolution is taken into account. By contrast, the standard cold dark matter cosmology is ruled out when this effect is included.
In the cores of the horizontal branch stars in globular clusters — some of the oldest stars in the galaxy — a trio of helium nuclei, known as alpha particles, fuse together to form the stable carbon-12 nucleus that is critical to the existence of all known life. Each gamma ray released by this so-called triple-alpha fusion reaction has a small chance of converting into cooling dark matter — in the form of particles known as axions (See Science Synergy posts for more on the Axion) — using a virtual photon from the electrostatic field of an alpha particle in the hot dense core of the star. State-of-the-art stellar evolution simulations that include this core-cooling effect — at the value of the axion-photon coupling strength predicted by Science Synergy in August 2021 (See 0.5 eV QCD Axion Cosmology) — can be compared with observations of the relative abundance of horizontal branch and red giant stars in more than three dozen globular clusters, and the comparison yields the value for the core helium abundance at star formation — which happened in the first billion years after the Bang for horizontal branch stars in globular clusters — shown in the figure as an observational estimate of the primordial helium abundance.