SEVEN-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP ) OBSERVATIONS: ARE THERE COSMIC MICROWAVE BACKGROUND ANOMALIES?

By C. Bennett, R. Hill, Gary F. Hinshaw, D. Larson, K. Smith, J. Dunkley, B. Gold, Mark Halpern, N. Jarosik, Alan Kogut and 11 more. 2010

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A simple six-parameter ΛCDM model provides a successful fit to WMAP data. This holds both when the WMAP data are analyzed alone or in combination with other cosmological data. Even so, it is appropriate to examine the data carefully to search for hints of deviations from the now standard model of cosmology, which includes inflation, dark energy, dark matter, baryons, and neutrinos. The cosmological community has subjected the WMAP data to extensive and varied analyses. While there is widespread agreement as to the overall success of the six-parameter ΛCDM model, various “anomalies” have been reported relative to that model. In this paper we examine potential anomalies and present analyses and assessments of their significance. In most cases we find that claimed anomalies depend on posterior selection of some aspect or subset of the data. Compared with sky simulations based on the best-fit model, one can select for low probability features of the WMAP data. Low probability features are expected, but it is not usually straightforward to determine whether any particular low probability feature is the result of the a posteriori selection or non-standard cosmology. Hypothesis testing could, of course, always reveal an alternative model that is statistically favored, but there is currently no model that is more compelling. We find that two cold spots in the map are statistically consistent with random cosmic microwave background (CMB) fluctuations. We also find that the amplitude of the quadrupole is well within the expected 95% confidence range and therefore is not anomalously low. We find no significant anomaly with a lack of large angular scale CMB power for the best-fit ΛCDM model. We examine in detail the properties of the power spectrum data with respect to the ΛCDM model and find no significant anomalies. The quadrupole and octupole components of the CMB sky are remarkably aligned, but we find that this is not due to any single map feature; it results from the statistical combination of the full-sky anisotropy fluctuations. It may be due, in part, to chance alignments between the primary and secondary anisotropy, but this only shifts the coincidence from within the last scattering surface to between it and the local matter density distribution. While this alignment appears to be remarkable, there was no model that predicted it, nor has there been a model that provides a compelling retrodiction. We examine claims of a hemispherical or dipole power asymmetry across the sky and find that the evidence for these claims is not statistically significant. We confirm the claim of a strong quadrupolar power asymmetry effect, but there is considerable evidence that the effect is not cosmological. The likely explanation is an insufficient handling of beam asymmetries. We conclude that there is no compelling evidence for deviations from the ΛCDM model, which is generally an acceptable statistical fit to WMAP and other cosmological data.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/19c4929b032c0acb3b02649c497af7002a735687

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Citations in the corpus

NINE-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP) OBSERVATIONS: FINAL MAPS AND RESULTS (2012, methodology): “Hanson et al. (2010) and Bennett et al. (2011) suggested that this was likely due largely to the spatially varying effective beam shape and in this paper we confirm that hypothesis.”
Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX–T scaling relation (2020, background): “Another anisotropic feature present in the CMB is the dipole power asymmetry detected in both WMAP and Planck with a significance of ∼ 2−3.5σ toward (l, b) ∼ (230◦,−20◦) (Eriksen et al. 2004; Hanson & Lewis 2009; Bennett et al. 2011; Akrami et al. 2014; Planck Collaboration XXIII 2014; Planck Collab”
The Atacama Cosmology Telescope: CMB polarization at 200 < ℓ < 9000 (2014, unknown): “While the model is in generally good agreement with all cosmological probes, there are some mild tensions at the 2 σ to 3 σ level (e.g., Planck Collaboration 2013c) and possible hints of “anomalies” (e.g., Planck Collaboration 2013d; Bennett et al. 2011; Copi et al. 2010).”
Cosmology and Fundamental Physics with the Euclid Satellite (2012, background): “Foregrounds and noise are disfavored as possible systematic effects [160, 543].”
PROBING THE INFLATON: SMALL-SCALE POWER SPECTRUM CONSTRAINTS FROM MEASUREMENTS OF THE COSMIC MICROWAVE BACKGROUND ENERGY SPECTRUM (2012, background): “There is no evidence of features in the primordial power spectrum on these scales (Kinney et al. 2008; Mortonson et al. 2009; Barnaby & Huang 2009; Hamann et al. 2010; Peiris & Verde 2010; Dvorkin & Hu 2010; Bennett et al. 2011; Benetti et al. 2011; Dvorkin & Hu 2011).”
POWER ASYMMETRY IN WMAP AND PLANCK TEMPERATURE SKY MAPS AS MEASURED BY A LOCAL VARIANCE ESTIMATOR (2014, background): “Therefore the look-elsewhere effect must be taken care of by taking into account the probability of observing a similar excess anywhere in the range (see, e.g., Gross & Vitells 2010). simply be the product of so-called a posteriori statistical inference10 (see, e.g., Bennett et al. 2011).”
Small scale problems of the $\Lambda$CDM model: a short review (2016, background): “ Gaussian random ﬁeld, which implies a statistical independence in the CMB multipoles. What is not clear is whether these anomalies are related to unknown systematics, if they are statistical effects [30], or a ﬁngerprints of new physics. The LCDM model also encounters problems in describing structu”
Planck2018 results (2018, background): “…power asymmetry has long been observed at the largest scales in the CMB (Eriksen et al. 2004), although its statistical significance is not high and is subject to a posteriori (look-elsewhere) corrections (Bennett et al. 2011; Planck Collaboration XXIII 2014; Planck Collaboration XVI 2016).”