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 > 3 sigma 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 A(L) that scales the lensing potential such that A(L)=0 corresponds to unlensed while A(L)=1 is the expected lensed result in the standard Lambda-CDM model. We find from WMAP5+ACBAR a 2.5 sigma indication for a lensing contribution larger than expected, with A(L)=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.