Peculiar velocity effects on the Hubble constant from time-delay cosmography

TL;DR

This paper analyzes how peculiar velocities of lenses, sources, and observers bias Hubble constant measurements from time-delay cosmography, highlighting the importance of accounting for these effects in high-precision cosmology.

Contribution

It provides a detailed linear-order calculation of peculiar velocity effects on H0 inference from seven lenses, quantifying biases and uncertainties relevant for future precision measurements.

Findings

Observer's peculiar velocity can bias H0 by up to 1.15%.

Peculiar velocities introduce about 0.24% additional uncertainty for the full sample.

Systematic biases are significant for percent-level H0 measurements, especially in aligned sky regions.

Abstract

Two major challenges of contemporary cosmology are the Hubble tension and the cosmic dipole tension. At the crossroad of these, we investigate the impact of peculiar velocities on estimations of the Hubble constant from time-delay cosmography. We quantify the bias on the inference of the Hubble constant due to peculiar velocities of the lens, the source and of the observer. The former two, which may cancel from one system to another, affect the determination of the angular diameter distances in the time-delay formula, and reconstructed quantities like the angle to the source, via a lens model. On the other hand, the peculiar velocity of the observer, which is a debated quantity in the context of the cosmic dipole tension, systematically affects observed angles through aberration, redshifts, angular diameter distance and reconstructed quantities. We compute in detail the effect of these peculiar velocities on the inference of the Hubble constant to linear order in the peculiar velocities for the seven lenses of the H0LiCOW/TDCOSMO collaboration. The bias generated by the observer's peculiar velocity alone can reach $1.15\%$ for the lenses which are well aligned with it. This results in a $0.25 \%$ bias for the seven combined lenses. Assuming a typical peculiar velocity of $300$ km s$^{-1}$ for the lens and the source galaxies, these add an additional random uncertainty, which can reach $1\%$ for an individual lens, but reduces to $0.24\%$ for the full TDCOSMO sample. The picture may change if peculiar velocities turn out to be larger than expected. Any time-delay cosmography program which aims for percent precision on the Hubble constant may need to take this source of systematic bias into account. This is especially so for future ground-based surveys which cover a fraction of the celestial sphere that is well aligned with the observer's peculiar velocity.