SECONDARY GRAVITATIONAL ANISOTROPIES IN OPEN UNIVERSES

The applicability of the potential approximation in the case of open u niverses is tested. Great Attractor-like structures are considered in the test. Previous estimates of the cosmic microwave background anisot ropies produced by these structures are analysed and interpreted. The anisotropies corresponding to inhomogeneous ellipsoidal models are als o computed. It is proved that, whatever the spatial symmetry may be, G reat Attractor-like objects with extended cores (radii similar to 10 h (-1)), located at redshift z=5.9 in an open universe with density para meter Omega(0)=0.2, produce secondary gravitational anisotropies of th e order of 10(-5) on angular scales of a few degrees. The amplitudes a nd angular scales of the estimated anisotropy decrease as the Great At tractor size decreases. For comparable normalizations and compensation s, the anisotropy produced by spherical realizations is found to be sm aller than that of ellipsoidal models. This anisotropy appears to be a n integrated effect along the photon geodesics. Its angular scale is m uch greater than that subtended by the Great Attractor itself. This is easily understood by taking into account the fact that the integrated effect is produced by the variations of the gravitational potential, which seem to be important in large regions subtending angular scales of several degrees. As a result of the large size df these regions, th e spatial curvature of the universe becomes important and, consequentl y, significant errors (similar to 30 per cent) arise in estimates base d on the potential approximation. As is emphasized in this paper, two facts should be taken into account carefully in some numerical estimat es of secondary gravitational anisotropies in open universes: (1) the importance of scales much greater than those subtended by the cosmolog ical structures themselves, and (2) the compatibility of the potential approximation with the largest scales.