Cosmic microwave background anomalies from imperfect dark energy

Confrontation with the data

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
e-mail: magnusax@astro.uio.no
² Institute for Theoretical Physics and the Spinoza Institute, Utrecht University, Leuvenlaan 4, Postbus 80.195, 3508 TD Utrecht, The Netherlands

Received: 10 June 2013
Accepted: 3 February 2014

Abstract

We test anisotropic dark energy models with the 7-year WMAP temperature observation data. In the presence of imperfect sources, large-scale gradients or anisotropies in the dark energy mean that the CMB sky will be distorted anisotropically on its way to us by the ISW effect. The signal covariance matrix then becomes non-diagonal for small multipoles, but at ℓ ≳ 20 the anisotropy is negligible for any reasonably probable values of the already constrained dark energy fluid parameters. As a consequence, only possible large-scale anisotropies are studied in this paper. 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, where the deviation from isotropy is modeled through a mismatch between the φ and ψ potentials that arise due to anisotropic stresses caused by some (unknown) mechanism. In this sense, our model is general. In this paper we explore the possibility that the stresses are caused by an imperfect dark energy component in the form of a vector field aligned with some axis. This way we may obtain hints of the 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 points toward (l,b) = (168°, −31°) and the amplitude of the anisotropy is ϖ₀ = (0.51 ± 0.94) (1σ deviation quoted). The best fit model has a steep blue anisotropic spectrum (n_de = 3.1 ± 1.5). In light of recent studies, the model provides an interesting extension of the standard model of cosmology, since it is able to account for the apparent deficit in large-scale power in the spectrum through a physically motivated late time ISW effect. Further studies of this class of models are justified by the results of the analysis, which suggest that it cannot be ruled out at present.