The James Webb Space Telescope has revealed very young, elongated galaxies that standard cold-dark-matter simulations struggle to reproduce. New simulations show that ultralight "fuzzy" axions or faster-moving warm dark matter (for example, sterile neutrinos) produce smoother filaments that naturally form these stretched galaxies. Continued JWST observations combined with refined models could help identify which dark matter candidates are most plausible.
James Webb May Reveal Dark Matter's True Nature Through Oddly Shaped Early Galaxies
An illustration of the JWST against a "filament" of dark matter stretching many light-years through space. | Credit: Robert Lea (created with Canva)/NASA
Since beginning science operations in 2022, the James Webb Space Telescope (JWST) has revolutionized our view of the early universe. Among its surprising finds are a growing number of very young galaxies with unusually elongated, filament-like shapes. New research suggests those shapes could carry clues about the type of dark matter that seeded cosmic structure.
What JWST Is Seeing
Unlike familiar, roughly spherical galaxies common today, some of the earliest galaxies JWST images show thread-like morphologies. These stretched shapes are challenging to reproduce in standard simulations based on the widely accepted Lambda Cold Dark Matter (LCDM) model, in which slow-moving (“cold”) dark matter forms dense, clumpy scaffolds that gather gas into star-forming regions.
Alternative Dark Matter Explanations
To investigate, Rogier Windhorst (Arizona State University), Álvaro Pozo (Donostia International Physics Center) and colleagues ran simulations that include alternative dark matter candidates. Two promising possibilities emerged:
- Fuzzy (Ultralight Axion) Dark Matter: If dark matter consists of extremely light, axion-like particles, their quantum, wave-like behavior suppresses structure on very small scales. That produces smoother, broader filaments in the early universe and can naturally lead to elongated galaxies streaming along those threads.
- Warm Dark Matter (e.g., Sterile Neutrinos): Faster-moving warm dark matter particles erase the smallest clumps, also creating smoother filaments that favor the formation of stretched galaxy morphologies.
"If ultralight axion particles make up the dark matter, their quantum wave-like behavior would prevent physical scales smaller than a few light-years from forming for a while, contributing to the smooth filamentary behavior that JWST now sees at very large distances," said team leader Álvaro Pozo.
Why This Matters
Dark matter makes up roughly 85% of the universe's matter, yet it does not interact with light in ways we can directly detect. That invisibility means its properties must be inferred from gravity’s effects on visible matter and radiation. If JWST’s observations of elongated, filamentary galaxies are confirmed to be common in the early cosmos, they could help rule out some dark matter candidates and favor others.
Next Steps
JWST will continue to survey the universe’s infancy, while theorists refine simulations that include different dark matter physics. Combining deeper Webb observations with improved models may narrow which dark matter scenarios remain viable. The research was published Dec. 8 in Nature Astronomy.
Source: Pozo et al., Nature Astronomy (Dec. 8, 2024).
