Observational Scan Induced Artificial CMB Anisotropy

TL;DR

This paper discusses how observational scan strategies can induce artificial anisotropies in CMB maps, potentially contaminating measurements crucial for understanding the universe's origins, and emphasizes the need for correction methods.

Contribution

It models and predicts scan-induced artificial anisotropies in CMB maps, demonstrating their impact on large-scale features like quadrupoles in WMAP and Planck data.

Findings

Scan-induced anisotropies correlate with observation patterns.

Predicted quadrupole patterns match WMAP observations.

Artificial anisotropies must be removed for reliable CMB analysis.

Abstract

To reliably detect the cosmic microwave background (CMB) anisotropy is of great importance in understanding the birth and evolution of the Universe. One of the difficulties in CMB experiments is the domination of measured CMB anisotropy maps by the Doppler dipole moment from the motion of the antenna relative to the CMB. For each measured temperature the expected dipole component has to be calculated separately and then subtracted from the data. A small error in dipole direction, antenna pointing direction, sidelobe pickup contamination, and/or timing synchronism, can raise significant deviation in the dipole cleaned CMB temperature. After a full-sky observational scan, the accumulated deviations will be structured with a pattern closely correlated to the observation pattern with artificial anisotropies on large scales, including artificial quadrupole, octopole etc in the final CMB map. Such scan-induced anisotropies on large scales can be predicted by the true dipole moment and observational scan scheme. Indeed, the expected scan-induced quadrupole pattern of the WMAP mission is perfectly in agreement with the published WMAP quadrupole. With the scan strategy of the Planck mission, we predict that scan-induced anisotropies will also produce an artificially aligned quadrupole. The scan-induced anisotropy is a common problem for all sweep missions and, like the foreground emissions, has to be removed from observed maps. Without doing so, CMB maps from COBE, WMAP, and Planck as well, are not reliable for studying the CMB anisotropy.