Probing the Universe's Tilt with the Cosmic Infrared Background Dipole

TL;DR

This paper investigates the cosmic infrared background (CIB) dipole to test if it aligns with the CMB dipole, exploring whether the dipole is primordial or due to local motions, and discusses future measurement strategies.

Contribution

It analyzes existing COBE data to assess the CIB dipole and proposes future instrument designs to accurately measure its alignment with the CMB dipole.

Findings

FIRAS data is close to detecting the CIB dipole but is hindered by Galactic foregrounds.

Planck and Herschel datasets are insufficient for a robust CIB dipole measurement.

A future FIRAS-like instrument could significantly improve CIB dipole detection and alignment analysis.

Abstract

Conventional interpretation of the observed cosmic microwave background (CMB) dipole is that all of it is produced by local peculiar motions. Alternative explanations requiring part of the dipole to be primordial have received support from measurements of large-scale bulk flows. A test of the two hypothesis is whether other cosmic dipoles produced by collapsed structures later than last scattering coincide with the CMB dipole. One background is the cosmic infrared background (CIB) whose absolute spectrum was measured to ~30% by the COBE satellite. Over the 100 to 500 {\mu}m wavelength range its spectral energy distribution can provide a probe of its alignment with CMB. This is tested with the COBE FIRAS dataset which is available for such a measurement because of its low noise and frequency resolution important for Galaxy subtraction. Although the FIRAS instrument noise is in principle low enough to determine the CIB dipole, the Galactic foreground is sufficiently close spectrally to keep the CIB dipole hidden. A similar analysis is performed with DIRBE, which - because of the limited frequency coverage - provides a poorer a dataset. We discuss strategies for measuring the CIB dipole with future instruments to probe the tilt and apply it to the Planck, Herschel and the proposed Pixie missions. We demonstrate that a future FIRAS-like instrument with instrument noise a factor of ~10 lower than FIRAS would make a statistically significant measurement of the CIB dipole. We find that the Planck and Herschel data sets will not allow a robust CIB dipole measurement. The Pixie instrument promises a determination of the CIB dipole and its alignment with either the CMB dipole or the dipole galaxy acceleration vector.