A river drawn on a map is always a single line. But stand on the bank, and you will see that within the channel there are faster currents, eddies, and branches that merge and split. The cosmic web is like that river, but made of dark matter, and until recently astronomers drew it as a single line.
The universe is structured like a sponge: galaxies are not scattered randomly but gathered into filaments that stretch between nodes, which are galaxy clusters. The filaments themselves are streams of dark matter, invisible but holding ordinary matter together. Imagine a river channel through which water flows: the filament is the channel itself, and galaxies are boats drifting with the current. But what if within that channel there is another, narrower and faster flow?
That is exactly what has been shown. The researchers took five computer simulations, each with a billion dark matter particles moving under gravity, and applied the Skeletor algorithm. It works like a cartographer who first sees only the widest rivers, then, looking more closely, finds tributaries within them. In this way, sub-filaments were discovered inside large filaments: smaller filaments nested within larger ones.
Comparing sub-filaments with their parents, the authors found three differences. First, sub-filaments are denser: at their outskirts, their density is 1.5 times higher than that of parent filaments of the same mass. Second, they spin faster: their angular momentum is twice as large. Third, they attract matter more strongly: the infall speed of gas toward their axis is higher.
Previously, all filaments were considered identical, and a single averaged profile was built for them. Now it is clear: this is like measuring the average flow speed in a river without noticing that the water is still near the banks and rushes along the main current. Sub-filaments are not a random sample but a distinct population, and models describing matter transport along filaments will need to be revised.
The river drawn as a single line turned out to be a river with branches. And to understand where the galaxy-boats are heading, you need to see not only the channel but also all its internal currents.
The filament radius is determined by the position of the minimum radial velocity, the place where matter stops falling onto the filament and starts moving along it.