A hint of matter underdensity at low $z$?

Abstract

The $\Lambda$CDM cosmological model provides to first approximation a good description of the universe, but various tensions with data, most notably Hubble tension, persist. In this work we confront $\Lambda$CDM with the Pantheon Type Ia supernovae dataset and perform a two-parameter fit of the distance modulus for a running cut-off $z_{\textrm{max}}$. We observe that in a window between $z_{\textrm{max}} \approx 0.1$ and $z_{\textrm{max}} \approx 0.16$ there is a 1 - 2 $\sigma$ discrepancy with the Planck value $\omega_m = 0.315 \pm 0.007$, which points to a potential matter underdensity. For high-energy theorists, the analysis appears to support the de Sitter Swampland conjecture.