The Sunyaev-Zel'dovich Effect


Massive clusters of galaxies are filled with very hot gas:
Heated by falling into deep gravitational potential.
Electron temperature approx. 100 million degrees.

CMB photons scatter on average to higher energy:
Spectrum is distorted relative to thermal.
Order of 1% of photons scattered.

Strength of effect is nearly independent of red-shift (with adequate angular resolution).