Cosmic microwave weak lensing data as a test for the dark universe

Combined analyses of WMAP 3-year and ACBAR cosmic microwave anisotropies angular power spectra have presented evidence for gravitational lensing at >3σ level. This signal could provide a relevant test for cosmology. After evaluating and confirming the statistical significance of the detection in light of the new WMAP 5-year data, we constrain a new parameter AL that scales the lensing potential such that AL=0 corresponds to unlensed while AL=1 is the expected lensed result in the standard Λ-CDM model. We find from WMAP5+ACBAR a 2.5σ indication for a lensing contribution larger than expected, with AL=3.1_-1.5+1.8 at 95% C.L. The result is stable under the assumption of different templates for an additional Sunyaev-Zel'dovich foreground component or the inclusion of an extra background of cosmic strings. We find negligible correlation with other cosmological parameters as, for example, the energy density in massive neutrinos. While unknown systematics may be present, dark energy or modified gravity models could be responsible for the over-smoothness of the power spectrum. Near-future data, most notably from the Planck satellite mission, will scrutinize this interesting possibility.