  Gravitational interaction of cosmic microwave background (CMB) photons with
matter perturbations present along the line-of-sight to the surface of last
scattering modifies the shape of the CMB anisotropy power spectrum. Here I
focus on (linear) integrated Sachs-Wolfe and (non-linear) gravitational lensing
effects and discuss the detectability of the resulting distortions and their
possible consequences for the CMB-based estimation of cosmological parameters.
Specifically, I discuss if any of those effects may allow us to use CMB
experiments to put independent constraints on the curvature of the universe and
the cosmological constant, i.e. breaking the so-called geometrical degeneracy
in CMB parameter estimation discussed by Bond, Efstathiou and Tegmark (1997)
and Zaldarriaga, Spergel and Seljak (1997).
  I address that issue using the Fisher matrix approach and show that
gravitational lensing of the CMB temperature and polarisation patterns might be
detectable by the Planck Surveyor satellite, leading to useful independent and
precise constraints on the cosmological constant and spatial curvature. The
integrated Sachs-Wolfe effect though bound to restrict those parameters only
very weakly still may set constraints more stringent than those currently
available.
