James Webb maps universe's hidden megastructures

On May 18, Live Science highlighted a new James Webb Space Telescope result that is easier to understand if you start with the phrase astronomers use for it: the “cosmic web.” That web is the large-scale structure of the universe — a network of filaments, sheets and dense knots where galaxies cluster, separated by vast, relatively empty voids. Researchers say JWST has now produced the clearest map yet of that structure by using the COSMOS-Web survey, the largest JWST survey so far. Hossein Hatamnia of the University of California, Riverside, the study’s lead author, said the team reconstructed large-scale structure using JWST data to trace galaxy evolution up to about redshift 7, which corresponds to very early cosmic time. The work appears in *The Astrophysical Journal* and draws on deep infrared observations that let researchers detect galaxies earlier telescopes missed. (cnas.ucr.edu) ### So what exactly did Webb map? The map is not a photograph of dark matter itself. The team mapped where galaxies sit across space and time, then used that distribution to reconstruct the web-like large-scale structure that cosmologists expect matter to follow. In the paper abstract, the researchers said they applied a weighted kernel density estimation method to about 160,000 galaxies with robust photometric redshifts. (iopscience.iop.org) UC Riverside said the resulting map traces the network of galaxies back to when the universe was about 1 billion years old. The survey covers a contiguous patch of sky about the size of three full Moons, which is unusually wide for data this deep. ### Why is this being described as “hidden” structure? JWST’s advantage is sensitivity in the infrared. (arxiv.org) UC Riverside said its instruments can pick up faint, distant galaxies that were invisible to earlier observatories and can see through dust more effectively. That matters because the cosmic web is inferred from many dim, faraway galaxies spread over enormous distances, not from a few bright nearby objects. (eurekalert.org) Live Science described the result as a map of the universe’s “hidden megastructures” because earlier surveys could not resolve this combination of depth and area as cleanly. The new dataset gives astronomers a denser and earlier census of galaxies, which makes the underlying web easier to reconstruct. That is an inference from the paper and university release, which both emphasize the survey’s depth, area and redshift reach. (eurekalert.org) ### What did the researchers say this changes? Hatamnia said, “JWST has completely changed our view of the universe,” adding that COSMOS-Web was designed to provide the wide and deep view needed to see the cosmic web. He said the team can now study galaxies in cluster and filament structures “across cosmic time,” from when the universe was about a billion years old to the nearby universe. (cnas.ucr.edu) The paper goes beyond cartography. In the abstract, the authors said stellar mass correlates positively with density at all redshifts they studied, while environmental effects on star formation vary with time and galaxy type. They wrote that the findings show large-scale structure helps drive galaxy evolution, first by enhancing early mass assembly in dense regions and later by suppressing star formation in some low-mass systems. (eurekalert.org) ### Why does the COSMOS-Web survey matter here? COSMOS-Web is the key enabling dataset. UC Riverside called it the largest General Observer program selected for JWST, and the paper describes it as the largest JWST survey used to trace the cosmic web this far back. That combination of breadth and depth is what let the team connect individual galaxies into a large-scale map instead of isolated snapshots. (arxiv.org) The next place to look is the underlying paper, “Large-Scale Structure in COSMOS-Web: Tracing Galaxy Evolution in the Cosmic Web up to z ∼ 7,” by Hatamnia and co-authors in *The Astrophysical Journal*. That study is the primary source behind the May 18 coverage and the broader COSMOS-Web release. (iopscience.iop.org) (eurekalert.org)