Cosmic Microwave Background (CMB) radiation is the imprint radiation after the BigBang. It is the earliest electromagnetic wave that we can observe. By studying CMB we can learn variously about the Universe e.g. the geometry of the Universe, the matter species of the Universe, the early stat of the Universe, etc. The CMB projects include
According to General Relativity (GR), light can be deflected by the gravitational field when it pass through the matter field. We call this effect Gravitational Lensing. The large scale structure such as voids and cluster can distorts the temperature field around the foreground masses. The lensing itself does not directly change the temperature of CMB photons but it changes the appearance of CMB filed.
We can use CMB weak lensing to constrain to mass distribution of large scale structure and cosmological parameters.