Taking the cosmic neutrino mass apart: geometry VS growth, and robustness to the late Universe

Recent cosmological data hint at a neutrino mass sum below the minimum value physically allowed by neutrino oscillations, with a preference for the opposite of massive neutrinos. Within ΛCDM, Planck CMB, ACT+Planck+SPT CMB lensing and DESI DR2 BAO give Σm_ν less than 0.06 eV (95% CL). At the same time, these data have also been interpreted as evidence for evolving dark energy. What should we make of these cosmic neutrino mass constraints, and how can we make progress?

In this seminar, I will show a new way of splitting Σm_ν into a geometry-only and a growth-only parameters. This provides two independent channels and a consistency test. With Simons Observatory and DESI, the pair will become a diagnostic: a misspecified dark energy shows up in the geometry channel, while an underestimated optical depth shows up in the growth channel.

Second, I explore whether neutrino masses can be measured independently of late-Universe physics. Marginalization over two dark energy evolution parameters is robust to more flexible dark energy models. A new late-Universe free data combination removes all dark energy dependence by construction, but at a much larger cost in statistical power.