Matter in the Universe is distributed in a highly clustered fashion; stars congregate in galaxies and galaxies clump together, forming enormous clusters surrounded by vast, empty spaces. Galaxy clusters can host up to a thousand galaxies and they are permeated by hot gas that shines brightly in X-rays; furthermore, most of their mass consists of dark matter. On an even grander scale are the superclusters, large assemblies of galaxy groups and clusters, located at the intersections of sheets and filaments in the wispy cosmic web. As clusters and superclusters trace the distribution of both luminous and dark matter throughout the Universe, their observation is crucial to probe how cosmic structures formed and evolved.

Planck's primary goal is to capture the most ancient light of the cosmos, the Cosmic Microwave Background (CMB), and for this purpose it boasts a superb set of nine frequency channels, spanning the spectral range from 30 to 857 GHz. Such a broad spectral coverage is not only instrumental in removing all sources of contamination from the CMB, in order to deliver what will be the sharpest image of the early Universe ever achieved - it also makes Planck an excellent hunter of galaxy clusters.

In fact, the nine frequency channels were carefully chosen by the Planck team with a particular phenomenon, known as the Sunyaev-Zel'dovich Effect (SZE), very much in mind. This effect describes the change of energy experienced by CMB photons when they encounter a galaxy cluster as they travel towards us, in the process imprinting a distinctive signature on the CMB itself. Hence, the SZE represents a unique tool to detect galaxy clusters, even at high redshift.

"As the fossil photons from the Big Bang cross the Universe, they interact with the matter that they encounter: when travelling through a galaxy cluster, for example, the CMB photons scatter off free electrons present in the hot gas that fills the cluster," explains Nabila Aghanim of the Institut d'Astrophysique Spatiale in Orsay, France, a leading member of the group of Planck scientists investigating SZE clusters and secondary anisotropies. "These collisions redistribute the frequencies of photons in a particular way that enables us to isolate the intervening cluster from the CMB signal."

Since the hot electrons in the cluster are much more energetic than the CMB photons, interactions between the two species typically result in the photons being scattered to higher energies. This means that, when looking at the CMB in the direction of a galaxy cluster, one observes a deficit, with respect to the average CMB signal, of low-energy photons and a surplus of more energetic ones. The threshold frequency, separating deficit and surplus, corresponds to 217 GHz. Planck's channels probe the spectrum both below and above this threshold, with one of them centred exactly on 217 GHz.

Matter in the Universe is distributed in a highly clustered fashion; stars congregate in galaxies and galaxies clump together, forming enormous clusters surrounded by vast, empty spaces. Galaxy clusters can host up to a thousand galaxies and they are permeated by hot gas that shines brightly in X-rays; furthermore, most of their mass consists of dark matter. On an even grander scale are the superclusters, large assemblies of galaxy groups and clusters, located at the intersections of sheets and filaments in the wispy cosmic web. As clusters and superclusters trace the distribution of both luminous and dark matter throughout the Universe, their observation is crucial to probe how cosmic structures formed and evolved.

Planck's primary goal is to capture the most ancient light of the cosmos, the Cosmic Microwave Background (CMB), and for this purpose it boasts a superb set of nine frequency channels, spanning the spectral range from 30 to 857 GHz. Such a broad spectral coverage is not only instrumental in removing all sources of contamination from the CMB, in order to deliver what will be the sharpest image of the early Universe ever achieved - it also makes Planck an excellent hunter of galaxy clusters.

In fact, the nine frequency channels were carefully chosen by the Planck team with a particular phenomenon, known as the Sunyaev-Zel'dovich Effect (SZE), very much in mind. This effect describes the change of energy experienced by CMB photons when they encounter a galaxy cluster as they travel towards us, in the process imprinting a distinctive signature on the CMB itself. Hence, the SZE represents a unique tool to detect galaxy clusters, even at high redshift.

"As the fossil photons from the Big Bang cross the Universe, they interact with the matter that they encounter: when travelling through a galaxy cluster, for example, the CMB photons scatter off free electrons present in the hot gas that fills the cluster," explains Nabila Aghanim of the Institut d'Astrophysique Spatiale in Orsay, France, a leading member of the group of Planck scientists investigating SZE clusters and secondary anisotropies. "These collisions redistribute the frequencies of photons in a particular way that enables us to isolate the intervening cluster from the CMB signal."

Since the hot electrons in the cluster are much more energetic than the CMB photons, interactions between the two species typically result in the photons being scattered to higher energies. This means that, when looking at the CMB in the direction of a galaxy cluster, one observes a deficit, with respect to the average CMB signal, of low-energy photons and a surplus of more energetic ones. The threshold frequency, separating deficit and surplus, corresponds to 217 GHz. Planck's channels probe the spectrum both below and above this threshold, with one of them centred exactly on 217 GHz.