Galaxy clusters formed as 'fireworks'.

• Position: Cosmology

Galaxies such as our Milky Way, with its 100,000,000,000 stars, usually are not found in isolation. In the universe today, 13,800,000,000 years after the Big Bang, many are in dense clusters of tens or even hundreds of galaxies. However, these clusters have not always existed, and a key question in modern cosmology is how such massive structures assembled in the early universe.

Pinpointing when and how they formed should provide insight into the process of galaxy cluster evolution, including the role played by dark matter in shaping these cosmic metropolises. Now, using the combined strengths of the Herschel Space Observatory and the Planck Satellite, astronomers have found objects in the distant universe, seen at a time when it was only 3,000,000,000 years old, which could be precursors of the clusters seen around us today.

"Because we are looking so far back in time, and because the universe is assumed to be homogenous in all directions, we think it's very similar to looking at the equivalent of what a baby cluster might look like," says Brenda L. Frye, an assistant astronomer at the University of Arizona's Steward Observatory, Tucson, who was involved in the research.

The main goal of Planck is to provide the most precise map of the relic radiation of the Big Bang, the cosmic microwave background. To do so, it surveyed the entire sky in nine different wavelengths from the far-infrared to radio, in order to eliminate foreground emission from our galaxy and others in the universe. However, those foreground sources can be important in other fields of astronomy, and it was in Planck's short wavelength data that scientists were able to identify 234 bright sources with characteristics that suggest they were located in the distant, early universe.

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