Precision studies are being made of the subtle differences in the cosmic background radiation (CBR). Given that along any line of sight, there is bound to be multiple galaxies, seen and unseen, which according to general relativity can act as a lens for light, how are researchers able to rule out that the variations (i.e., ripples) are not caused by intervening matter bending the microwave radiation as it makes its journey to earth?
There is a web page which talks extensively about this issue.
Lensing can have a significant effect on very small ripples in the CBR, but a negligible effect on the larger ones. It can certainly change the shape of any preexisting ripples in the CBR, but astronomers are less interested in measuring the exact shape of the ripples than they are in measuring the amount of temperature variation contained in ripples of different sizes, summed up over the entire sky. It turns out that gravitational lensing doesn't affect this measurement too much.
Also, gravitational lensing isn't going to create ripples where there were none before - all it will do is change the shape of preexisting ones. Think about it this way: If you look through a magnifying glass or lens at a blank wall, the wall will still be blank - you aren't going to be able to arrange lenses in such a way that the wall will appear to have spots!
This page was last updated June 27, 2015.