# Cosmic opacity: cosmological-model-independent tests from gravitational waves and Type Ia Supernova

## TL;DR

This study investigates the universe's opacity using gravitational wave and supernova data, finding it to be nearly transparent at high redshifts and highlighting the potential of future GW measurements for more precise constraints.

## Contribution

It introduces a cosmological-model-independent method combining GW and supernova data to test cosmic opacity, emphasizing the role of future GW observations.

## Key findings

- Universe is nearly transparent at redshift ~2.26
- Strong degeneracy between opacity parameter and supernova magnitude
- Future GW measurements will improve constraints on cosmic opacity

## Abstract

In this paper, we present a scheme to investigate the opacity of the Universe in a cosmological-model-independent way, with the combination of current and future available data in gravitational wave (GW) and electromagnetic (EM) domain. In the FLRW metric, GWs propagate freely through a perfect fluid without any absorption and dissipation, which provides a distance measurement unaffected by the cosmic opacity. Focusing on the simulated data of gravitational waves from the third-generation gravitational wave detector (the Einstein Telescope, ET), as well as the newly-compiled SNe Ia data (JLA and Pantheon sample), we find an almost transparent universe is strongly favored at much higher redshifts ($z\sim 2.26$). Our results suggest that, although the tests of cosmic opacity are not significantly sensitive to its parametrization, a strong degeneracy between the cosmic opacity parameter and the absolute \textit{B}-band magnitude of SNe Ia is revealed in this analysis. More importantly, we obtain that future measurements of the luminosity distances of gravitational waves sources will be much more competitive than the current analyses, which makes it expectable more vigorous and convincing constraints on the cosmic opacity (and consequently on background physical mechanisms) and a deeper understanding of the intrinsic properties of type Ia supernovae in a cosmological-model-independent way.