Cosmic filaments pull in two directions express
On large scales, the universe resembles a giant spiderweb: bright galaxy clusters are connected by long filaments of galaxies, gas, and dark matter. Previously, it was thought that matter flows along these filaments like water through pipes, heading toward the clusters. But new research shows that a filament between two clusters is not just a conduit: it's a tug-of-war arena.
Using data on thousands of galaxies from the Sloan Digital Sky Survey, scientists identified filaments connecting pairs of clusters. They noticed that galaxies within a filament move in opposite directions: those closer to one cluster fly toward it, while those closer to the other fly toward that one. In the middle of the filament, the velocity changes sign, as if a stretched rubber band were cut in half, with each half contracting toward its own end.
This result matters not only for understanding the cosmic web. By observing how galaxies in a filament fly apart, we can measure cluster masses more precisely, without relying on other, more complex methods. It also offers a new way to test whether our understanding of gravity on enormous scales is correct — because if filaments behave differently than theory predicts, it could point to new physics.
Original abstract
as written by the authors arXiv:2601.18434
Velocity fields in the cosmic web are fundamental to structure formation but remain difficult to observe directly beyond the linear regime. Here we present observational evidence that galaxy filaments connecting pairs of galaxy clusters undergo a split infall, with opposite velocity flows toward the two clusters. Using spectroscopic galaxies from the Sloan Digital Sky Survey, we isolate the internal filament velocity field by subtracting its rigid-body background motion and Hubble flow, and detect this effect at greater than $$5\sigma$$ significance across a wide range of cluster and filament selections. The measured velocity profile exhibits a sign reversal near the filament midpoint and a maximum infall amplitude of $$\sim30$$ km/s ($$\sim20$$ km/s projected onto the line-of-sight) for clusters of mass $$\sim10^{14.3}M_\odot$$, substantially lower than expected for infall from an average cosmic environment. Multiple results on density-velocity correlation, mass-dependency, and validation with simulation indicate that filaments dynamically respond to competing gravitational potentials rather than acting as passive mass transport channels. Our results establish a new observational window on quasi-linear velocity fields in the cosmic web and provide a promising probe of mass measurement, testing gravity and velocity reconstruction with upcoming wide-field spectroscopic surveys.
Galaxies in a filament move at about 30 kilometers per second — roughly 100 times faster than a bullet, but thousands of times slower than light.