[Paper Review] First constraints on the intrinsic CMB dipole and our velocity with Doppler and aberration

Pedro da Silveira Ferreira, Miguel Quartin|arXiv (Cornell University)|Nov 17, 2020

Cosmology and Gravitation TheoriesPhysics and Astronomy67 references42 citations

TL;DR

This paper presents the first model-independent constraints on the intrinsic Cosmic Microwave Background (CMB) dipole by disentangling Doppler and aberration effects from Planck 2018 component-separated maps (SMICA and NILC). It measures our solar system's peculiar velocity independently of the dipole, finding v = 300+111−93 km/s (SMICA) with a 95% credible upper limit of 3.7 mK on the intrinsic CMB dipole amplitude.

ABSTRACT

We test the usual hypothesis that the Cosmic Microwave Background (CMB) dipole, its largest anisotropy, is due to our peculiar velocity with respect to the Hubble flow by measuring independently the Doppler and aberration effects on the CMB using Planck 2018 data. We remove the spurious contributions from the conversion of intensity into temperature and arrive at measurements which are independent from the CMB dipole itself for both temperature and polarization maps and both SMICA and NILC component-separation methods. Combining these new measurements with the dipole one we get the first constraints on the intrinsic CMB dipole. Assuming a standard dipolar lensing contribution we can put an upper limit on the intrinsic amplitude: 3.7 mK (95% CI). We estimate the peculiar velocity of the solar system without assuming a negligible intrinsic dipole contribution: $v = (300^{+111}_{-93})$ km/s with $(l,b) = (276 \pm 33, 51 \pm 19)^\circ$ [SMICA], and $v = (296^{+111}_{-88})$ km/s with $(l,b) = (280 \pm 33, 50 \pm 20)^\circ$ [NILC] with negligible systematic contributions. These values are consistent with the peculiar velocity hypothesis of the dipole.

Motivation & Objective

• To test the hypothesis that the CMB dipole is entirely due to our peculiar velocity relative to the CMB rest frame.
• To isolate and measure the Doppler and aberration effects on the CMB independently of the dipole itself, avoiding degeneracy.
• To estimate the solar system's peculiar velocity without assuming a negligible intrinsic dipole contribution.
• To place the first upper bound on the intrinsic CMB dipole amplitude using a model-independent approach.
• To reduce systematic errors by simulating 482 sky orientations and removing dipole distortion (DD) artifacts in component-separated maps.

Proposed method

• Removal of Dipole Distortions (DD) in SMICA and NILC component-separated maps using multipole-binned estimation, as DD arises from intensity-to-temperature conversion and is degenerate with the dipole.
• Application of idealized estimators for Doppler (βD) and aberration (βA) couplings that are independent of the dipole and each other, using Healpix-Boost simulations with 482 sky directions.
• Use of both temperature (TT) and E-mode polarization (EE) two-point correlation estimators to enhance signal-to-noise and cross-validate results.
• Calibration of estimators via χ² fitting to 2 nuisance parameters (mask and noise) and 2 additional parameters to de-correlate βD and βA, minimizing leakage and bias.
• Cross-checking with a standard boost assumption (βA ≡ βD ≡ βB) to confirm consistency, though the primary analysis avoids this assumption.
• Utilization of final Planck 2018 component-separated maps (SMICA and NILC) and 143/217 GHz data, improving over prior work by reducing systematics and increasing precision.

Experimental results

Research questions

• RQ1 Can the intrinsic CMB dipole be constrained independently of the observed dipole using Doppler and aberration effects?
• RQ2 What is the precise value of the solar system’s peculiar velocity with respect to the Hubble flow, without assuming the dipole is purely kinematic?
• RQ3 How significant are systematic errors from mapmaking artifacts (e.g., DD) and noise anisotropy in measuring Doppler and aberration signals?
• RQ4 To what extent do the Doppler and aberration signals deviate from the standard boost model, indicating a non-kinematic origin of the dipole?
• RQ5 What is the upper limit on the intrinsic CMB dipole amplitude under a standard dipolar lensing model?

Key findings

• The first upper limit on the intrinsic CMB dipole amplitude is established at 3.7 mK at 95% credible interval, assuming a standard dipolar lensing contribution.
• The solar system’s peculiar velocity is measured as v = (300+111−93) km/s with direction (l, b) = (276 ± 33, 51 ± 19)° using the SMICA map, with negligible systematic error.
• Using the NILC map, the velocity is v = (296+111−88) km/s at (l, b) = (280 ± 33, 50 ± 20)°, consistent with the SMICA result and the standard kinematic hypothesis.
• The Doppler and aberration estimators are successfully decoupled from the dipole and from each other, confirming the method’s robustness and independence.
• The measured peculiar velocity is consistent with the standard kinematic dipole value of ~370 km/s, supporting the conventional interpretation.
• The use of component-separated maps (SMICA/NILC) and full-sky simulations with realistic noise and beam effects significantly reduces systematic errors compared to prior work.