The Cosmic Microwave Background (CMB) in the afterglow of the Big Bang. Why would this cluster of galaxies punch a hole in it? The CMB flows right through most of the gas and dust in the universe. It is all around us. However, large clusters of galaxies have enough gravity to contain gas hot enough to up-scatter the CMB photons into light of significantly higher energy, creating “holes” in the CMB. This is known as the Sunyaev–Zel’dovich (SZ) effect, and it’s used for decades to study the hot gas in clusters. This picture combines CMB data from ESO’s ALMA with imagery from the Hubble Space Telescope to measure the galaxies in the massive galaxy cluster RX J1347.5-1145. False-color blue shows light from the CMB; almost every yellow object is a galaxy. The shape of the SZ hole indicates not only that hot gas is present in this galaxy cluster, but also that it is distributed in a surprisingly uneven manner.
Image Credit: ESO / ESA / NASA