Testing the cosmic opacity at higher redshifts: implication from quasars with available UV and X-ray observations

TL;DR

This study tests the cosmic opacity at high redshifts using quasars, Hubble parameter data, and simulated gravitational wave events, finding support for a transparent universe with implications for high-redshift cosmology.

Contribution

It introduces a model-independent method combining quasar emissions, H(z), and GW data to constrain cosmic opacity at z~5, highlighting the importance of parametrization and calibration.

Findings

Current data supports a transparent universe at 2σ confidence.

Cosmic opacity parameter is sensitive to the chosen parametrization.

Combining quasar and GW data provides a novel high-redshift opacity constraint.

Abstract

In this paper, we present a cosmological model-independent test for the cosmic opacity at high redshifts ($z\sim5$). We achieve this with the opacity-dependent luminosity distances derived from nonlinear relation between X-ray and UV emissions of quasars, combined with two types of opacity-independent luminosity distances derived from the Hubble parameter measurements and simulated gravitational wave (GW) events achievable with the Einstein Telescope (ET). In the framework of two phenomenological parameterizations adopted to describe cosmic opacity at high redshifts, our main results show that a transparent universe is supported by the current observational data at 2$\sigma$ confidence level. However, the derived value of the cosmic opacity is slightly sensitive to the parametrization of $\tau(z)$, which highlights the importance of choosing a reliable parametrization to describe the optical depth $\tau(z)$ in the early universe. Compared with the previous works, the combination of the quasar data and the $H(z)$/GW observations in similar redshift ranges provides a novel way to confirm a transparent universe ($\epsilon=0$ at higher redshifts $z\sim 5$), with an accuracy of $\Delta \epsilon\sim 10^{-2}$. More importantly, our findings indicate that a strong degeneracy between the cosmic opacity parameter and the parameters characterizing the $L_{UV}-L_X$ relation of quasars, which reinforces the necessity of proper calibration for such new type of high-redshift standard candle (in a cosmological model-independent way).