Galaxies Wind Around a Cosmic Filament
Editor’s Note: This story was one of two winning selections for the IYQ Quantum Pitch Competition.
The Universe is threaded with cosmic filaments—strands of galaxies and gas that stretch for tens of millions of light-years. These large-scale structures are typically regarded as rigid, “skeletal” features. But researchers have now identified, for the first time in a single filament, coherent rotation on a system-wide scale [1]. Madalina Tudorache from the University of Cambridge in the UK and colleagues used hydrogen-emission data from South Africa’s MeerKAT radio telescope to locate 14 hydrogen-rich galaxies embedded in a filament roughly 50 million light-years long. The sharp features of the emission—which comes from a quantum “spin-flip” transition in the hydrogen atom—allowed the team to precisely measure the galactic velocities and uncover a rotational pattern around the filament’s spine. “Seeing rotation in an individual filament is quite surprising,” Tudorache says. If similar rotation is detected in other structures, it could offer a test of cosmological models.
Cosmologists today picture the Universe as a “cosmic web”: galaxies collected into dense clusters, connected by sprawling filaments, and separated by vast, almost empty voids. In simulations, this web emerges as gravity slowly amplifies tiny early-Universe ripples into a rich network of structures. “You have this field of galaxies, and what we want to do is to quantify it, to characterize it,” says cosmologist Ofer Lahav of University College London, who was not involved with the study. Quantifying that web, he explains, is “very useful to actually learn about what the Universe is made of,” because different mixtures of dark matter and dark energy imprint different patterns of filaments, walls, clusters, and voids.
Cosmological models not only predict where filaments and other structures sit within the cosmic web but also how they move. A handful of simulations have suggested that, under the right conditions, filaments could acquire angular momentum from surrounding matter and begin to rotate, subtly twisting the trajectories and spins of the galaxies embedded in them [2]. Observationally, however, the evidence has been indirect [3], relying on statistical averages of millions of “stacked” filaments to tease out weak signatures of coherent motion, explains team member Lyla Jung from the University of Oxford in the UK. She and her colleagues go beyond those stacked hints by focusing on a single, well-resolved system in which the dynamics can be mapped object by object.
The team selected 14 bright galaxies from a MeerKAT survey of the sky at the 21-cm wavelength, which corresponds to an emission line of neutral hydrogen. This emission has a sharply peaked spectrum owing to a discrete (quantized) jump in the energy of a hydrogen atom as its sole electron flips the direction of its spin. When the researchers combined MeerKAT’s 21-cm observations with optical redshifts from large surveys, they realized that the 14 hydrogen-rich systems sit inside a much larger structure—a filament containing roughly 280 galaxies stretched across about 50 million light-years.
Using the radio data, the researchers built a three-dimensional velocity map of the filament structure. For each hydrogen-rich galaxy, they inferred the line-of-sight speed—relative to the receding motion of the filament—by measuring tiny Doppler shifts in the 21-cm line. Plotting those velocities against distance from the filament’s spine revealed a clear gradient: Galaxies on one side of the spine systematically move toward us, while those on the other side move away. The researchers also found that the galaxies’ own rotation axes tend to line up with the filament. “Both individual galaxies and their host filament are spinning,” Jung says. Together, these signatures point to a single, coherent structure resembling a cosmic amusement-park ride, in which the galaxies themselves spin like teacups while the entire filament slowly turns around a central axis.
This discovery was made possible by MeerKAT’s exceptional sensitivity to the tiny Doppler shifts in individual galaxy emissions. “MeerKAT has been the gift that keeps on giving,” Tudorache says. But the telescope owes its scientific productivity not only to its hardware but also to more than a decade of strategic capacity building. “[The MeerKAT project] has given science in South Africa and Africa an enormous boost,” says Renee Kraan-Korteweg of the University of Cape Town in South Africa. She notes that the instrument arrived alongside a substantial human-capacity development program that supported postgraduate researchers, postdoctoral fellows, and international research chairs in radio astronomy and engineering.
The growth of local expertise has allowed much of the raw MeerKAT data to be processed and reduced on-site before being transferred to the international radio community. “It would be physically impossible for us to transfer everything,” Tudorache says. Lahav described this development as a broader shift in the locus of radio astronomy from the Northern Hemisphere toward southern infrastructures, such as MeerKAT and the Square Kilometer Array (SKA), a next-generation radio observatory that will link thousands of antennae across southern Africa and Australia.
The detection of a rotating filament may have implications for cosmological models, but astronomers are cautious about what conclusions to draw from a single system. If coherent filament rotation turns out to be common, it could offer a new way to test how the properties of dark matter can shape structure on the largest scales. Lahav notes that varying the cosmological ingredients—by, for example, including interactions between dark matter or assuming greater mass for neutrinos—can subtly alter the dynamics within the cosmic web.
Lahav stresses, however, that the filament-rotation result still demands careful scrutiny. “Extraordinary claims require extraordinary proofs,” he says, noting that while simulations can generate rotating structures, they do not yet agree on how frequently such systems should occur or how strongly galaxies’ spins should align with their host filaments. Establishing whether the observed rotation is a rare curiosity or a generic feature of the cosmic web, he adds, will require independent detections across a wider range of environments and epochs.
New instruments and surveys should accelerate those tests. MeerKAT will continue to map neutral hydrogen in individual galaxies with enough sensitivity to trace their dynamics, while SKA will survey the hydrogen Universe at unprecedented depth and scale. “With SKA we should be able to find more of these individual rotating filaments,” Tudorache says. Furthermore, optical surveys such as DESI, Euclid, and the Vera C. Rubin Observatory should help tie gas dynamics to galaxy assembly, possibly explaining observed alignment between galaxy spins and large-scale structure. Jung expects that the real gains will come from moving beyond single detections toward larger samples of filaments that can be compared systematically rather than anecdotally.
–Paul Adepoju
Paul Adepoju is a freelance science journalist based in Ibadan, Nigeria.
References
- M. N. Tudorache et al., “A 15 Mpc rotating galaxy filament at redshift z = 0.032,” Mon. Not. R. Astron. Soc. 544, 4306 (2025).
- S. Codis et al., “Spin alignments within the cosmic web: A theory of constrained tidal torques near filaments,” Mon. Not. R. Astron. Soc. 452, 3369 (2015).
- P. Wang et al., “Possible observational evidence for cosmic filament spin,” Nat. Astron. 5, 839 (2021).