M. Murgia

In the Coma galaxy cluster, about 320 million light-years away, an invisible cloud stretches out. It shines in the radio band: this is a radio halo, a giant collection of charged particles accelerated to nearly the speed of light.

The cosmic microwave background is faint light left over from the Big Bang. It fills the sky uniformly, but it encounters obstacles along the way. One such obstacle is galaxy clusters, which contain very hot gas. When CMB photons pass through this gas, some of them get an extra kick and change energy.