Scientists have produced the clearest view ever of part of the cosmic web, the enormous hidden network of matter that stretches across the Universe and connects galaxies together. After spending hundreds of hours collecting observations, an international team captured a detailed image of a massive cosmic filament linking two actively forming galaxies from a time when the Universe was only about 2 billion years old.
The discovery offers a rare direct look at one of the largest structures in existence and could help researchers better understand how galaxies grow and evolve over cosmic time.
The Universe's Hidden Structure
Modern cosmology suggests that dark matter makes up roughly 85% of all matter in the Universe. Although invisible, dark matter is believed to shape a gigantic web-like framework made of long filaments. At the points where these filaments intersect, galaxies form and shine brightly.
Scientists think these filaments also act as intergalactic highways, channeling gas into galaxies and fueling the birth of new stars. Learning how this gas moves through the cosmic web is considered essential for understanding how galaxies develop.
But detecting that gas has been extremely difficult. Most intergalactic gas has only been observed indirectly by measuring how it absorbs light from bright objects behind it. Hydrogen, the most abundant element in the cosmos, emits only a very faint glow, making direct observations nearly impossible for older instruments.
The new observations were carried out by researchers from the University of Milano-Bicocca together with scientists from the Max Planck Institute for Astrophysics (MPA). The team used MUSE (Multi-Unit Spectroscopic Explorer), a powerful instrument mounted on the European Southern Observatory's Very Large Telescope in Chile.
Even with such advanced technology, the project required one of the most ambitious MUSE observing campaigns ever conducted in a single region of the sky. Researchers gathered data over hundreds of hours to detect the faint filament clearly enough for detailed analysis.
The study, led by Davide Tornotti, PhD student at the University of Milano-Bicocca, produced the sharpest image ever captured of a cosmic filament stretching roughly 3 million light-years. The structure connects two galaxies that each contain an active supermassive black hole.
The findings were published in Nature Astronomy and provide a new way to study the physical properties of gas inside intergalactic filaments.
"By capturing the faint light emitted by this filament, which traveled for just under 12 billion years to reach Earth, we were able to precisely characterize its shape," explains Davide Tornotti. "For the first time, we could trace the boundary between the gas residing in galaxies and the material contained within the cosmic web through direct measurements."
To better interpret the observations, the researchers compared the data with supercomputer simulations of the Universe created at MPA. These simulations predicted what such filamentary structures should look like under current cosmological models.
"When comparing to the novel high-definition image of the cosmic web, we find substantial agreement between current theory and observations," Tornotti adds.
The successful match between observations and simulations gives scientists greater confidence in their understanding of how gas is distributed around galaxies and how galaxies receive the material needed to continue forming stars.
Researchers now hope to identify many more of these faint structures in order to build a broader picture of how matter flows through the cosmic web.
Fabrizio Arrigoni Battaia, MPA staff scientist involved in the study, concludes: "We are thrilled by this direct, high-definition observation of a cosmic filament. But as people say in Bavaria: 'Eine ist keine' -- one doesn't count. So we are gathering further data to uncover more such structures, with the ultimate goal to have a comprehensive vision of how gas is distributed and flows in the cosmic web."
