Figure 1
APS/Alan Stonebraker: (Top) ESO; (Bottom) ESA/Hubble/NASA

Figure 1: The Atacama Cosmology Telescope observes the motion of a large mass, such as a galaxy cluster, by detecting frequency shifts in the cosmic microwave background, which scatters inelastically from the moving mass. Light that scatters from a galaxy cluster moving away from the observer is shifted to longer wavelengths (cooler, or “redshifted” light). Galaxy clusters moving toward the observer scatter light to shorter wavelengths (warmer, or “blueshifted” light).