# A hint of matter underdensity at low $z$?

## TL;DR

This paper investigates potential matter underdensity at low redshift by analyzing supernova data, revealing a mild discrepancy with Planck measurements that may suggest new physics or modifications to the standard cosmological model.

## Contribution

It introduces a two-parameter fit of supernova data with a variable redshift cutoff, highlighting a possible matter underdensity at low redshift and implications for the Swampland conjecture.

## Key findings

- Discrepancy between supernova data and Planck at z~0.1-0.16
- Evidence supporting matter underdensity at low redshift
- Implications for the de Sitter Swampland conjecture

## 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.