A novel model-marginalized cosmological bound on the QCD axion mass

Understand

We present model-marginalized limits on mixed hot dark matter scenarios, which consider both thermal neutrinos and thermal QCD axions.

• A novel aspect of our analyses is the inclusion of small-scale Cosmic Microwave Background (CMB) observations from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT), together with those from the Planck satellite and Baryon Acoustic Oscillation (BAO) data.
• After marginalizing over a number of well-motivated non-minimal background cosmologies, the tightest $95\%$ CL upper bound we obtain is $0.21$ eV, both for $\sum m_\nu$ and $m_{\rm a}$, from the combination of ACT, Planck and BAO measurements.
• Restricting the analyses to the standard $\Lambda$CDM picture, we find $\sum m_\nu<0.16$ eV and $m_{\rm a}<0.18$ eV, both at $95\%$ CL.