The ISW-tSZ cross correlation: ISW extraction out of pure CMB data

Understand

If Dark Energy introduces an acceleration in the universal expansion then large scale gravitational potential wells should be shrinking, causing a blueshift in the CMB photons that cross such structures (Integrated Sachs-Wolfe effect, [ISW]).

• Galaxy clusters are known to probe those potential wells.
• In these objects, CMB photons also experience inverse Compton scattering off the hot electrons of the intra-cluster medium, and this results in a distortion with a characteristic spectral signature of the CMB spectrum (the so-called thermal Sunyaev-Zel'dovich effect, [tSZ]).
• Since both the ISW and the tSZ effects take place in the same potential wells, they must be spatially correlated.