CMB Anomalies from Imperfect Dark Energy: Confrontation with the Data
  We test anisotropic dark energy models with the 7-year WMAP temperature
observations data. In the presence of imperfect sources, due to large-scale
gradients or anisotropies in the dark energy field, the CMB sky will be
distorted anisotropically on its way to us by the ISW effect. The signal
covariance matrix then becomes nondiagonal for small multipoles, but at $\ell
\gtrsim 20$ the anisotropy is negligible. We parametrize possible violations of
rotational invariance in the late universe by the magnitude of a
post-Friedmannian deviation from isotropy and its scale dependence. This allows
to obtain hints on possible imperfect nature of dark energy and the large-angle
anomalous features in the CMB. A robust statistical analysis, subjected to
various tests and consistency checks, is performed to compare the predicted
correlations with those obtained from the satellite-measured CMB full sky maps.
The preferred axis point towards $(l,b) = (168^\circ, -31^\circ)$ and the
amplitude of the anisotropy is $\varpi_0 = (0.51\pm 0.94)$ (1$\sigma$ deviation
quoted). The best-fit model has a steep blue anisotropic spectrum
($n_{\mathrm{de}} = 3.1\pm1.5$).
