Cosmology with Peculiar Velocities: Observational Effects

TL;DR

This paper examines how observational biases in peculiar velocity measurements, especially in bulk flow estimates, can lead to overestimations and suggests tailored comparison methods for better cosmological inference.

Contribution

It demonstrates that traditional bulk flow estimation methods can overestimate velocities due to survey geometry and sampling, proposing a new approach for more accurate model comparisons.

Findings

Traditional methods overestimate bulk flow magnitudes.

Survey geometry and sampling affect velocity estimates.

A tailored comparison approach improves cosmological inferences.

Abstract

In this paper we investigate how observational effects could possibly bias cosmological inferences from peculiar velocity measurements. Specifically, we look at how bulk flow measurements are compared with theoretical predictions. Usually bulk flow calculations try to approximate the flow that would occur in a sphere around the observer. Using the Horizon Run 2 simulation we show that the traditional methods for bulk flow estimation can overestimate the magnitude of the bulk flow for two reasons: when the survey geometry is not spherical (the data do not cover the whole sky), and when the observations undersample the velocity distributions. Our results may explain why several bulk flow measurements found bulk flow velocities that seem larger than those expected in standard {\Lambda}CDM cosmologies. We recommend a different approach when comparing bulk flows to cosmological models, in which the theoretical prediction for each bulk flow measurement is calculated specifically for the geometry and sampling rate of that survey. This means that bulk flow values will not be comparable between surveys, but instead they are comparable with cosmological models, which is the more important measure.