[Paper Review] Planck intermediate results. LVI. Detection of the CMB dipole through modulation of the thermal Sunyaev-Zeldovich effect: Eppur si muove II

Planck Collaboration, Y. Akrami+127 more|arXiv (Cornell University)|Mar 27, 2020

Cosmology and Gravitation Theories46 references45 citations

TL;DR

This paper presents the first significant detection of the cosmic microwave background (CMB) dipole through its modulation of the thermal Sunyaev-Zeldovich (tSZ) effect in Planck data. Using both harmonic-space and map-space methods, the study measures the dipole's signature in tSZ maps, finding a 5–6σ detection consistent with direct CMB dipole measurements, and confirms that this signal contributes negligible noise to the tSZ bispectrum.

ABSTRACT

The largest temperature anisotropy in the cosmic microwave background (CMB) is the dipole, which has been measured with increasing accuracy for more than three decades, particularly with the Planck satellite. The simplest interpretation of the dipole is that it is due to our motion with respect to the rest frame of the CMB. Since current CMB experiments infer temperature anisotropies from angular intensity variations, the dipole modulates the temperature anisotropies with the same frequency dependence as the thermal Sunyaev-Zeldovich (tSZ) effect. We present the first, and significant, detection of this signal in the tSZ maps and find that it is consistent with direct measurements of the CMB dipole, as expected. The signal contributes power in the tSZ maps, which is modulated in a quadrupolar pattern, and we estimate its contribution to the tSZ bispectrum, noting that it contributes negligible noise to the bispectrum at relevant scales.

Motivation & Objective

• To detect the CMB dipole through its frequency-dependent modulation of the tSZ effect, independent of direct CMB temperature measurements.
• To validate that the dipole-induced modulation in tSZ maps is consistent with the known CMB dipole velocity vector.
• To assess whether this signal introduces significant bias or noise in the tSZ bispectrum, a key probe of non-Gaussianity in cluster physics.
• To demonstrate that the tSZ maps inherently contain a contamination from the CMB dipole due to the way intensity anisotropies are converted to temperature.

Proposed method

• Applied a Lorentz boost transformation to model the dipole modulation of CMB anisotropies in the tSZ effect's frequency response.
• Used harmonic-space analysis to cross-correlate CMB anisotropy maps with tSZ maps, isolating the dipole-modulated signal via its unique frequency dependence.
• Performed a map-space-based analysis using cross-correlations between CMB templates and tSZ maps to detect the dipole modulation directly in pixel space.
• Employed multiple CMB and tSZ map products (SMICA, 2D-ILC, MILCA, 2D-ILC SZ) and tested various masks to ensure robustness against foregrounds.
• Calculated the contribution of the dipole modulation to the tSZ bispectrum, finding it negligible at relevant scales.
• Used Bayesian inference with posterior distributions to estimate the dipole amplitude and direction, comparing results to direct Planck CMB dipole measurements.

Experimental results

Research questions

• RQ1 Can the CMB dipole be detected through its modulation of the tSZ effect in Planck data?
• RQ2 Is the frequency-dependent dipole modulation signal in the tSZ maps consistent with the known CMB dipole velocity vector?
• RQ3 Does the dipole-induced modulation in the tSZ maps introduce a significant bias or noise in the tSZ bispectrum?
• RQ4 How robust is the detection across different tSZ map products and analysis methods?

Key findings

• The dipole modulation signal in the tSZ maps is detected with a significance of 5–6σ, depending on the data set and analysis method.
• The measured dipole direction and amplitude are consistent with the direct CMB dipole measurement: (l, b) = (264.021° ± 0.011°, 48.253° ± 0.005°) and v = 369.82 ± 0.11 km s⁻¹.
• The dipole modulation adds power to the tSZ maps in a quadrupolar pattern aligned with the CMB dipole direction, with a frequency response matching the tSZ effect.
• The contamination from the dipole modulation contributes negligible noise to the tSZ bispectrum at scales relevant for cluster physics and non-Gaussianity studies.
• The detection is robust across multiple tSZ map products (MILCA, 2D-ILC, 2D-ILC SZ) and analysis methods (harmonic and map space).
• Foregrounds and mask choices have minimal impact on the detection, as confirmed by testing multiple mask configurations and signal-to-noise optimization.