How do the billions of galaxies in the Universe organize themselves? While the night sky gives the impression of a random scattering, an immense invisible framework actually orchestrates their distribution. Predicted by theoretical models, this structure connects galaxy clusters via long filaments, evoking the strands of a spider's web on a cosmic scale.
In the region of the Ursa Major supergroup, astronomers have identified a linear alignment of galaxies extending for nearly four million light-years. This discovery, made possible by the sensitivity of the Chinese FAST radio telescope, was shared in a preprint on
arXiv. It corresponds to a tenuous filament, a kind of cosmic path where matter gathers under the dominant influence of dark matter, that invisible component of the Universe of which we only perceive the gravitational effects.
In this image, the diffuse gas (yellow to purple) contained in the cosmic filament connecting two galaxies, extending over a vast distance of 3 million light-years.
Credit: Davide Tornotti/University of Milan-Bicocca These filaments are not mere visual alignments. They act as cosmic highways, channeling interstellar gas which serves as fuel for the formation of stars and galaxies. Dark matter, through its gravitational force, acts like a giant magnet within these structures, attracting ordinary matter and initiating the birth of galaxies. This observation thus shows how the Universe actively directs its own development on a large scale.
The ability to detect such tenuous filaments marks a significant advance in observational astronomy. Instruments like the FAST radio telescope now allow us to probe regions of the cosmos where light is very faint. By studying the radio emission from neutral hydrogen, researchers can map the distribution and movements of gaseous matter, revealing the hidden geometry of these filamentary structures.
This discovery lifts the veil on galactic formation processes. Galaxies located along a filament seem to share a common history, influenced by the same gravitational environment. They can thus evolve in a synchronized manner, grow, or even merge over time. Understanding these dynamics helps to retrace the scenario that shaped the Universe from the Big Bang to its current cosmic web structure.
Research continues to identify other similar filaments and measure their physical properties with more precision.
A simulation of a vast region of the cosmos performed with a supercomputer and based on the standard model of cosmology.
Credit: Alejandro Benitez-Llambay/MPA/University of Milan-Bicocca Radio observation: listening to the Universe's hydrogen
The discovery of tenuous filaments often relies on radio astronomy, a technique that captures radio waves emitted by celestial objects. Unlike visible light, these waves pass more easily through dust clouds and can reveal cold, diffuse regions, such as the vast reservoirs of neutral hydrogen gas.
The hydrogen atom, the most abundant element in the Universe, emits a very specific radio wave at a wavelength of 8.3 inches (21 centimeters). By pointing a radio telescope like FAST towards a region of the sky, astronomers can detect this signal. Its intensity and its redshift indicate the amount of hydrogen present and the distance at which it is located.
By mapping this emission over large areas of the sky, it becomes possible to reconstruct the spatial distribution of the gas. Alignments and gaseous concentrations then reveal the presence of filamentary structures, even if the associated galaxies are few in number or not very luminous. This method thus allows us to 'see' the gaseous skeleton of the cosmic web.
The advantage of this approach is its sensitivity to very low-density environments, precisely where the thinnest filaments are found. It complements observations in visible light or X-rays, which are more effective for studying dense galaxy clusters. Together, these techniques offer a more complete vision of the architecture of the Universe.