Although the universe is a big place, and all the things in it look like it’s just higgledy-piggledy, there’s more structure than we can see.
According to our models of the universe and increasing evidence, filaments of dark matter connect massive objects such as galaxies and galaxies into a large, cosmic web.
Along these filaments, hydrogen flows in the galaxies, but it is not so easy to see – among all the bright glowing stars and galaxies and galactic nuclei, the faint emission of diffuse hydrogen in the intergalactic space is difficult to see, never mind map.
However, we only got one step closer. At the height of years of work, an international team of astronomers led by Roland Bacon of the Lyon Astrophysical Research Center in France has directly imaged several filaments of the cosmic web in the early universe, about 12 billion light-years away.
Hydrogen filaments (in blue). (Roland Bacon / David Mary / ESO / NASA)
Their results are not only the strongest evidence for the cosmic web; they also found evidence that a large population of dwarf systems drive the hydrogen glow within the filaments. This discovery could dramatically change our understanding of galaxy formation in the infancy of the universe.
Because the cosmic web is so difficult to see, much of our evidence so far has been indirect. Some scientists have used the way in which mass bends space-time – gravitational lens – to look for skewed numbers in the path of distant light, suggesting that a wire of the cosmic web is between the source and us.
Other researchers use the light of quasars, extremely bright distant galaxies, to search for light absorbed by hydrogen along the filaments.
(Jeremy Blaizot / SPHINX project)
Above: Cosmological simulation of the distant universe, with light emitting hydrogen atoms in the cosmic web in a region of about 15 million light-years.
Bacon and his team followed a different approach – they really looked at a tiny, tiny piece of sky for a very long time with an amazing telescope. Using the MUSE instrument on ESO’s Very Large Telescope in Chile, the team took an incredible 140 hours of observation of a section of air that also appeared in the Hubble Space Telescope’s Ultra-Deep Field.
Similar research was done while astronomers were looking for light strings in a group of galaxies – gas wires ionized by the galaxies themselves. Here, too, the work of Bacon’s team differs from previous attempts: the previous research examined an extreme environment, while the new research deliberately looked indescribable.
After the planning phase, the team’s observations took months to obtain, from August 2018 to January 2019. It had to be taken in blocks during the new moon to limit interference.
(Thibault Garel / Roland Bacon)
Above: Cosmological simulation of a filament consisting of hundreds of thousands of small galaxies (as seen in situ on the left, as seen by MUSE on the right).
The team had to process and analyze the data, which took another year. But it was worth it – 40 percent of the galaxies in their data were not only undetectable in the Ultra Deep Field, but the researchers imaged glowing hydrogen in filaments of the cosmic web that stretched millions of light years.
Fascinatingly, the team’s analysis shows that the bulk of the hydrogen emission could be responsible for a large population of star-forming dwarf systems, scattered along the filament. Of course, we can not see them – they are far too far to solve – but future work can help confirm this discovery, with great implications for our understanding of the universe.
If dwarf systems are also channeled along cosmic web filaments, such as droplets of water in a piece of string, it may help to explain how galaxies formed and grew – and in the early universe grew to amazing sizes, a question that confused cosmologists.
In addition, the search for the emission of star-forming dwarf systems can help us to find more filaments of the cosmic web and a deeper understanding of how everything in the universe is interconnected.
The research was published in Astronomy & Astrophysics.