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[1007.4347] Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes

Authors:  I. Antoniou, L. Perivolaropoulos (U. of Ioannina) 
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)=(306^\circ, 15^\circ) (\Omega_m=0.19) while the hemisphere of
minimum acceleration is in the opposite direction (l,b)=(126^\circ, 15^\circ)
(\Omega_m=0.30). The level of anisotropy is described by the normalized
difference of the best fit values of \Omega_m between the two hemispheres in
the context of \lcdm fits. We find a maximum anisotropy level in the Union2
data of \frac{\Delta \Omega_m_max}{\Omega_m}=0.42. This 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. 

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Syksy Rasanen
Joined: 02 Mar 2005 Posts: 127 Affiliation: University of Helsinki

Posted: August 12 2010 


The authors look at preferred directions in two ways. First, they analyse the Union2 supernova dataset for the axis of maximum asymmetry. Second, they compare various directions determined from different cosmological datasets and determine the probability that they are all so close to each other by accident.
The asymmetry direction is determined by fitting the spatially flat ΛCDM model separately to two hemispheres and finding the direction which maximises the difference in Ω_{m0}. By comparison with simulations, the authors conclude that there is no statistical significance to the asymmetry. The directions determined from the supernova data, the CMB dipole, quadrupole and octopole, galaxy flows, and quasar polarization naively look rather close to each other (see table 1 on page 6). The authors get a probability of 1% for the axes to be so close to each other by accident. (This goes up to 7% if one drops the CMB axes  for example, the CMB dipole is not independent of local galaxy flows.)
It is not clear how much the result would change if the directions were to have error bars, but the significance could drop a lot. For example, since the the amplitude of the maximum hemispheric asymmetry of the Union2 dataset is not statistically significant, the direction should not matter at all. (I am not familiar with the direction determined from polarization of quasars  perhaps someone can comment on that?)
Nevertheless, it is interesting to have a quantitative analysis of the coincidence of directions. 

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