Mitigating the binary viewing angle bias for standard sirens

TL;DR

This paper introduces a new method to correct for viewing angle biases in gravitational-wave standard siren measurements of the Hubble constant, reducing systematic errors to improve cosmological inferences.

Contribution

The authors develop a formalism to infer and mitigate viewing angle biases, ensuring unbiased Hubble constant measurements from gravitational-wave observations.

Findings

Systematic uncertainty can be reduced below statistical uncertainty with 5-20 binary neutron star observations.

The formalism effectively mitigates biases even if the viewing angle distribution shape is not precisely known.

Unbiased Hubble constant measurements are achievable when using electromagnetic observations for viewing angles.

Abstract

The inconsistency between experiments in the measurements of the local Universe expansion rate, the Hubble constant, suggests unknown systematics in the existing experiments or new physics. Gravitational-wave standard sirens, a method to independently provide direct measurements of the Hubble constant, have the potential to address this tension. Before that, it is critical to ensure there is no substantial systematics in the standard siren method. A significant systematic has been identified when the viewing angle of the gravitational-wave sources, the compact binary coalescences, is inferred inaccurately from electromagnetic observations of the sources. Such systematic has led to more than 10% discrepancy in the standard siren Hubble constant measurements with the observations of binary neutron star merger, GW170817. In this Letter, we develop a new formalism to infer and mitigate this systematic. We demonstrate that the systematic uncertainty of the Hubble constant measurements can be reduced to smaller than their statistical uncertainty with 5, 10, and 20 binary neutron star merger observations. We show that our formalism successfully reduces the systematics even if the shape of the biased viewing angle distribution does not follow precisely the model we choose. Our formalism ensures unbiased standard siren Hubble constant measurements when the binary viewing angles are inferred from electromagnetic observations.