Cosmic Microwave Background Anomalies Research Articles

Large‐Scale Vector Modes and the First CMB Temperature Multipoles

Recent observations have pointed out various anomalies in some multipoles (small l ) of the cosmic microwave background (CMB). In this paper, it is proved that some of these anomalies could be explained in the framework of modified concordance model, in which there is an appropriate distribution of vector perturbations with very large spatial scales. Vector modes are associated with divergenceless (vortical) velocity fields. Here, the generation of these modes is not studied in detail (it can be done a posteriori); on the contrary, we directly look for the distributions of these vector modes which lead to both alignments of the second and third multipoles and planar octopole. A general three-dimensional (3D) superimposition of vector perturbations does not produce any alignment, but we have found rather general 2D superimpositions leading to anomalies similar to the observed ones; in these 2D cases, the angular velocity has the same direction at any point of an extended region, and moreover, this velocity has the same distribution in all the planes orthogonal to it. Differential rotations can be seen as particular cases, in which the angular velocity only depends on the distance to rotation axis. Our results strongly suggest that appropriate mixtures of scalar and vector modes with very large spatial scales could explain the observed CMB anomalies.

Bianchi model CMB polarization and its implications for CMB anomalies

We derive the cosmic microwave background (CMB) radiative transfer equation in the form of a multipole hierarchy in the nearly Friedmann–Robertson–Walker limit of homogeneous, but anisotropic, universes classified via their Bianchi type. Compared with previous calculations, this allows a more sophisticated treatment of recombination, produces predictions for the polarization of the radiation and allows for reionization. Our derivation is independent of any assumptions about the dynamical behaviour of the field equations, except that it requires anisotropies to be small back to recombination; this is already demanded by observations. We calculate the polarization signal in the Bianchi VIIh case, with the parameters recently advocated to mimic the several large-angle anomalous features observed in the CMB. We find that the peak polarization signal is ∼1.2 μK for the best-fitting model to the temperature anisotropies, and is mostly confined to multipoles l < 10. Remarkably, the predicted large-angle EE and TE power spectra in the Bianchi model are consistent with Wilkinson Microwave Anisotropy Probe (WMAP) observations that are usually interpreted as evidence of early reionization. However, the power in B-mode polarization is predicted to be similar to the E-mode power and parity-violating correlations are also predicted by the model; the WMAP non-detection of either of these signals casts further strong doubts on the veracity of attempts to explain the large-angle anomalies with global anisotropy. On the other hand, given that there exist further dynamical degrees of freedom in the VIIh universes that are yet to be compared with CMB observations, we cannot at this time definitively reject the anisotropy explanation.

Local Voids as the Origin of Large‐Angle Cosmic Microwave Background Anomalies: The Effect of a Cosmological Constant

We explore the large angular scale temperature anisotropies in the cosmic microwave background (CMB) due to homogeneous local dust-filled voids in a flat Friedmann-Robertson-Walker universe with a cosmological constant. In comparison with the equivalent dust-filled void model in the Einstein-de Sitter background, we find that the anisotropy for compensated asymptotically expanding local voids can be larger, because second-order effects enhance the linear integrated Sachs-Wolfe (ISW) effect. However, for local voids that expand sufficiently faster than the asymptotic velocity of the wall, the second-order effect can suppress the fluctuation due to the linear ISW effect. A pair of quasi-linear compensated asymptotic local voids with radius (2-3) × 102 h-1 Mpc and a matter density contrast δm ~ -0.3 can be observed as cold spots with a temperature anisotropy ΔT/T ~ O(10-5) that might help explain the observed large-angle CMB anomalies. We predict that the associated anisotropy in the local Hubble constant in the direction of the voids could be as large as a few percent.

Local Voids as the Origin of Large‐Angle Cosmic Microwave Background Anomalies. I.

We explore the large angular scale temperature anisotropies in the cosmic microwave background due to expanding homogeneous local voids at redshift z ≲ 1. A compensated spherically symmetric homogeneous dust-filled void with radius ~3 × 102 h-1 Mpc and density contrast δ ~ -0.3 can be observed as a cold spot with a temperature anisotropy ΔT/T ~ -1 × 10-5 surrounded by a slightly hotter ring. We find that a pair of these circular cold spots separated by ~50° can account both for the planarity of the octopole and for the alignment between the quadrupole and the octopole in the cosmic microwave background (CMB) anisotropy. The cold spot in the Galactic southern hemisphere, which is anomalous at the ~3 σ level, can be explained by such a large void at z ~ 1. The observed north-south asymmetry in the large-angle CMB power can be attributed to the asymmetric distribution of these local voids between the two hemispheres. The statistical significance of the low quadrupole is further reduced in this interpretation of the large angular scale CMB anomalies.