The universe's oldest light just showed gas swirling inside galaxy clusters

The cosmic microwave background is the faint afterglow of the Big Bang, arriving from every direction. Researchers at Columbia University have now used it like a backlight: as that ancient light passes through galaxy clusters — cities of hundreds of galaxies — the clusters' hot gas leaves a subtle hot-and-cold signature if the gas is rotating. Stacking many clusters, the team found the signal at 3.6-sigma significance, reported in Physical Review Letters: the first clear evidence of this long-predicted effect. It is a new way to watch the largest structures in the universe move, using the oldest light there is.