| Abstract: | The two fundamental assumptions of the standard cosmological model - that the
initial fluctuations are statistically isotropic and Gaussian - are rigorously
tested using maps of the CMB anisotropy from the Planck satellite. The
detailed results are based on studies of four independent estimates of the CMB
that are compared to simulations using a fiducial $Lambda$CDM model and
incorporating essential aspects of the Planck measurement process. Deviations
from isotropy have been found and demonstrated to be robust against component
separation algorithm, mask and frequency dependence. Many of these anomalies
were previously observed in the extit{WMAP} data, and are now confirmed at
similar levels of significance (around $3sigma$). However, we find little
evidence for non-Gaussianity with the exception of a few statistical signatures
that seem to be associated with specific anomalies. In particular, we find that
the quadrupole-octopole alignment is also connected to a low observed variance
of the CMB signal. The dipolar power asymmetry is now found to persist to much
smaller angular scales, and can be described in the low-$ell$ regime by a
phenomenological dipole modulation model. Finally, it is plausible that some of
these features may be reflected in the angular power spectrum of the data which
shows a deficit of power on the same scales. Indeed, when the power spectra of
two hemispheres defined by a preferred direction are considered separately, one
shows evidence for a deficit in power, whilst its opposite contains
oscillations between odd and even modes that may be related to the parity
violation and phase correlations also detected in the data. Whilst these
analyses represent a step forward in building an understanding of the
anomalies, a satisfactory explanation based on physically motivated models is
still lacking. |
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