Basic structure of the cosmos pictured for the first time

Stars group together to form galaxies. Galaxies form galaxy clusters. These form superclusters, between which vast, largely empty regions extend, the voids. All superclusters are connected by a honeycomb-like basic structure, the “cosmic web”, which consists of filamentary gas structures of hydrogen. That these filaments must exist has been known for some time. On the one hand, they are known from simulations based on theories of the structure of the universe, which predict such a basic structure. On the other hand, they become visible when energetic quasars illuminate them like car headlights illuminating the nebula.

However, the regions thus detected are poorly representative of the entire network of filaments in which most galaxies, including our own, were born. Direct observation of the faint light emitted by the gas that forms the filaments has been considered the holy grail among astronomers. It has now been found by an international team led by Roland Bacon, CNRS researcher at the Centre de Recherche Astrophysique de Lyon.

The team realized this step with the help of ESO’s Very Large Telescope (VLT). It is equipped with the MUSE instrument, which is coupled to the telescope’s adaptive optics. Together, the two instruments form one of the most powerful optical systems in the world. For more than 140 hours, researchers focused it on a single region of the sky. It is part of Hubble’s Ultra-Deep Field, which was previously the most comprehensive image of the cosmos ever taken. However, the new image has now surpassed Hubble, as 40% of the galaxies discovered by MUSE have no counterpart in Hubble images.

After meticulous planning, it took eight months to complete this extraordinary observing campaign. A year of data analysis followed, revealing light from the hydrogen filaments for the first time. The images show several filaments as they appeared one to two billion years after the Big Bang, a key period for understanding how galaxies formed from the gas in the cosmic web. The biggest surprise for the team, however, was when the simulations showed that the light emitted by the waterfall came from a previously invisible population of billions of dwarf galaxies. Although these galaxies are too faint to be detected individually with current instruments, their existence will have major consequences for galaxy formation models, with implications that scientists are just beginning to explore.

One of the hydrogen filaments (in blue) discovered by MUSE in the Hubble Ultra-Deep Field. It is located in the constellation Fornax at a distance of 11.5 billion light-years and spans 15 million light-years. The image in the background is from Hubble. (Image: Roland Bacon, David Mary, ESO and NASA).
Cosmological simulation of a filament consisting of hundreds of thousands of small galaxies. The image on the left shows the emissions produced by all the galaxies as they would be observed in situ. The image on the right shows the filament as it would be seen by MUSE. Even with a very long exposure time, the vast majority of galaxies cannot be detected individually. However, the light from all these small galaxies is detected as a diffuse background, much like the Milky Way when seen with the naked eye. (Image: Thibault Garel and Roland Bacon)

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  • BrandonQMorris
  • Brandon Q. Morris is a physicist and space specialist. He has long been concerned with space issues, both professionally and privately and while he wanted to become an astronaut, he had to stay on Earth for a variety of reasons. He is particularly fascinated by the “what if” and through his books he aims to share compelling hard science fiction stories that could actually happen, and someday may happen. Morris is the author of several best-selling science fiction novels, including The Enceladus Series.

    Brandon is a proud member of the Science Fiction and Fantasy Writers of America and of the Mars Society.