The convergence fluctuation of light by galaxy groups - implications on Type 1a supernovae and cosmic microwave background observations

The magnification of distant sources by galaxy groups, assumed to be isothermal spheres with properties inferred from an extensive survey, are calculated analytically. It is found that in the weak lensing limit the average effect is exactly counteracted by the Dyer-Roeder beam. When applied to Type 1a supernovae brightness, the results were in broad agreement with the numerical computations of earlier authors they all point to a small perturbation which is difficult to measure. When applied to the cosmic microwave background, however, a rather large lensing induced dispersion in the angular size of the primary acoustic peaks of the TT power spectrum is inconsistent with WMAP observations. Two possible ways of resolving the conflict is proposed: either the groups are in reality much more numerous in their population (with a smaller size for each) than what was inferred from the survey, or the fraction of properly virialized groups possessing a significant intra-group gravitational potential is small. The overall conclusion is that at the present epoch only a very small fraction of matter has been virialized into Mpc scale isothermal spheres. Our finding therefore supports the ‘bottom-up’ scenario of large scale structure formation.