Searching for a cosmological preferred axis: Union2 data analysis and comparison with other probes

Abstract

We review, compare and extend recent studies searching for evidence for a preferred cosmological axis. We start from the Union2 SnIa dataset and use the hemisphere comparison method to search for a preferred axis in the data. We find that the hemisphere of maximum accelerating expansion rate is in the direction (l, b) = (309•+11• −10 • ) (Ω0m = 0.19) while the hemisphere of minimum acceleration is in the opposite direction (l, b) = (129The level of anisotropy is described by the normalized difference of the best fit values of Ω0m between the two hemispheres in the context of ΛCDM fits. We find a maximum anisotropy level in the Union2 data of= 0.43 ± 0.06. Such a level does not necessarily correspond to statistically significant anisotropy because it is reproduced by about 30% of simulated isotropic data mimicking the best fit Union2 dataset. However, when combined with the axes directions of other cosmological observations (bulk velocity flow axis, three axes of CMB low multipole moments and quasar optical polarization alignment axis), the statistical evidence for a cosmological anisotropy increases dramatically. We estimate the probability that the above independent six axes directions would be so close in the sky to be less than 1%. Thus either the relative coincidence of these six axes is a very large statistical fluctuation or there is an underlying physical or systematic reason that leads to their correlation.

Asymmetry and dark energy evolution

“…Again we observe the asymmetry in the distribution, although less severe than in the case of the Union 2 sample. In fact, using the Union 2 set we have verified the value (ΔΩ m ) max /Ω m = 0.43 ± 0.06 from [27]. Using the same procedure with the Loss sample we get (ΔΩ m ) max /Ω m = 0.30 ± 0.06.…”

“…In this section we use the result of [27] using the hemispherical asymmetry technique, were they found a dipolar asymmetry with a maximum acceleration towards (l, b) = (309 o , 18 o ) and a minimum acceleration towards (l,…”

“…In order to find this axis, we follow the procedure of [27], using a flat ΛCDM as a base model, and start a scan of random axes looking for the maximum variation in the best fit of Ω m . Using the Loss sample we found that the direction of maximal acceleration points towards (l, b) = (309 o , 31 o ).…”

“…This issue as been discussed in the past. As we have mentioned before, using the Union 2 dataset, the authors of [27] Table 2 for the best fit values for each case. a preferred axis in the data, but they also checked if such anisotropy can be obtained using simulated isotropic data.…”

“…In this context, it is interesting to cite the case of a preferred direction of expansion, as for example, the large scale bulk-flow [20,22,21,24,23], the low multipoles alignment in the CMB angular power spectrum [25], the large scale alignment of quasar polarization [26], and studies using SNIa [27,32,33,28,34,35,30,31,29]. So the evidence points towards the necessity to a study further the effects on the cosmological parameters of a inhomogeneous local distribution of matter.…”