A study published in Nature Astronomy in June 2021 reported evidence consistent with rotation in cosmic filaments, among the largest structures in the universe. These elongated strands of galaxies, gas and dark matter connect dense regions of the cosmic web. They are not isolated objects like planets: many galaxies trace their extent, and large galaxy clusters lie near the network’s junctions. The study asked whether the motions of those galaxies reveal an organised component around a filament’s long axis.Nature Astronomy: filament-spin study
Astrophysicist, Peng Wang of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany, said of the findings:
"By mapping the motion of galaxies in these huge cosmic superhighways using the Sloan Digital Sky survey - a survey of hundreds of thousands of galaxies - we found a remarkable property of these filaments: they spin."
While being only a few million light-years in diameter, the filaments are hundreds of millions of light-years in length.
By carefully observing the light emitted from the galaxies on cosmic filament and comparing them to each other, astronomers found that galaxies on the side of the filament were redshifted compared to the other side. This is expected to happen when the galaxies are in vortical motion perpendicular to the filament's spine. This is what scientists call, Doppler shifting.
Cosmographer, Noam Libeskind of the AIP, explained:
"On these scales the galaxies within them are themselves just specks of dust. They move on helices or corkscrew-like orbits, circling around the middle of the filament while travelling along it. Such a spin has never been seen before on such enormous scales, and the implication is that there must be an as yet unknown physical mechanism responsible for torquing these objects."
The question behind the finding is how angular momentum develops as cosmic structure grows. Gravity draws matter into an increasingly complex network, and gravitational interactions can exert torques as that structure evolves. Tidal torque theory is one established way of studying the origin of spin; the open question here concerns how such processes operate at the scale of filaments. The observed pattern provides something for those explanations to account for, rather than demonstrating that cosmological models have no way to include torque.
As galaxies are connected by cosmic filaments, such structures play a crucial role in the formation and evolution of galaxies, including their rotation. The team compared the observational result with the possibility of filament rotation explored in simulations. Such a comparison links an inferred pattern in galaxy motion with models of how structure forms. It does not require every filament to rotate in the same way or at the same rate. The strength of the signal depends on how a filament is oriented relative to the observer and on its dynamical state.
Libeskind noted:
"It's fantastic to see this confirmation that intergalactic filaments rotate in the real Universe, as well as in computer simulation."
[h/t: science alert]
The scale is millions of light-years
The AIP account describes filaments hundreds of millions of light-years long and a few million light-years across. The dimensions put these structures far beyond the scale of a single galaxy. A few million light-years across and hundreds of millions along their length are distances that describe a vast network, not a narrow bridge that a telescope could watch turning in real time. Researchers used galaxies as tracers, comparing the average redshift on opposite sides of a filament’s projected axis. A systematic difference is consistent with one side moving toward the observer relative to the other. The team interpreted the pattern as evidence of rotation on exceptionally large scales. AIP’s cosmic-filament study explanation.
A cosmic filament is not a rigid spinning rod
The image of a rotating bridge is helpful only up to a point. A filament contains many objects and matter spread through space; the study concerns an organized statistical pattern of motion rather than a solid structure completing a visible turn. The inference also depends on how a filament is identified and viewed. This is why follow-up work can examine different samples and geometries instead of simply waiting to watch one revolution. The finding links an observable pattern in galaxy motions with the broader question of how angular momentum emerges as cosmic structure forms.
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