Systematic uncertainty of standard sirens from the viewing angle of binary neutron star inspirals

TL;DR

This paper investigates how viewing angle uncertainties in binary neutron star mergers can introduce significant systematic errors in measuring the Hubble constant with gravitational-wave standard sirens, potentially impacting cosmological tension resolution.

Contribution

It identifies and quantifies the systematic uncertainties from viewing angle effects, highlighting their importance over gravitational-wave calibration errors in standard siren measurements.

Findings

Viewing angle selection bias can cause >2% bias in Hubble constant.

Uncontrolled viewing angle constraints can lead to >3% systematic uncertainty.

Viewing angle effects are more significant than calibration uncertainties.

Abstract

The independent measurement of Hubble constant with gravitational-wave standard sirens will potentially shed light on the tension between the local distance ladders and Planck experiments. Therefore, thorough understanding of the sources of systematic uncertainty for the standard siren method is crucial. In this paper, we focus on two scenarios that will potentially dominate the systematic uncertainty of standard sirens. First, simulations of electromagnetic counterparts of binary neutron star mergers suggest aspherical emissions, so the binaries available for the standard siren method can be selected by their viewing angles. This selection effect can lead to $\gtrsim 2\%$ bias in Hubble constant measurement even with mild selection. Second, if the binary viewing angles are constrained by the electromagnetic counterpart observations but the bias of the constraints is not controlled under $\sim 10^{\circ}$, the resulting systematic uncertainty in Hubble constant will be $>3\%$. In addition, we find that both of the systematics cannot be properly removed by the viewing angle measurement from gravitational-wave observations. Comparing to the known dominant systematic uncertainty for standard sirens, the $\leq 2\%$ gravitational-wave calibration uncertainty, the effects from viewing angle appear to be more significant. Therefore, the systematic uncertainty from viewing angle might be a major challenge before the standard sirens can resolve the tension in Hubble constant, which is currently $\sim$9%.