The Polarization Signature of Local Bulk Flows

TL;DR

This paper predicts a detectable polarization anisotropy in the cosmic microwave background caused by large-scale bulk flows of the intergalactic medium, which could be observed with future experiments.

Contribution

It introduces a new polarization signature of local bulk flows in the CMB, providing a potential method to measure large-scale cosmic motions.

Findings

Polarization signal is a quadrupole pattern caused by bulk flows.

Signal is small but within reach of future observational capabilities.

Detectability depends on the magnitude of the bulk flow.

Abstract

A large peculiar velocity of the intergalactic medium produces a Doppler shift of the cosmic microwave background with a frequency-dependent quadrupole term. This quadrupole will act as a source for polarization of the cosmic microwave background, creating a large-scale polarization anisotropy if the bulk flow is local and coherent on large scales. In the case where we are near the center of the moving region, the polarization signal is a pure quadrupole. We show that the signal is small, but detectable with future experiments for bulk flows as large as some recent reports.