Development History of the Universe is Being Rewritten Based on a Bachelor’s Project from NBI

A small group of young researchers at the Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, have, through observations of the early stages of an extremely large galaxy cluster’s evolution, shown that the largest structures we know have a different history than previously thought.

The researchers began by observing a very high density of cold, neutral hydrogen gas—much greater than expected—which is still actively forming stars and galaxies within the cluster.

“This is not something we’ve seen before in these systems, nor this far back in the universe’s history. The structure of the galaxy cluster we observed is unusual—it’s massive; there is an enormous amount of material in it, and we expect that it would evolve into one of the largest galaxy clusters we’ve ever seen if we continued its development to the present day,” explains Kasper Heintz, Assistant Professor at the Cosmic Dawn Center, Niels Bohr Institute and first author of the study.

Kasper continues: “In itself it was a bit mysterious that the galaxy cluster  was so large, but that might make sense given that we found this huge amount of cold, neutral gas falling into the structure and ‘feeding’ the formation of galaxies.”

The Amount of Cold Neutral Gas Didn’t Match Previous Assumptions

One thing that puzzled the researchers was that the observations of this large amount of neutral hydrogen gas didn’t really fit with the existing models for the universe’s evolution.

It was previously assumed that about one billion years after the Big Bang, galaxy clusters would shine so brightly that the gas would have been ionized by the light or radiation—meaning the gas had transformed from its original primordial state and was no longer neutral.

But the amount of cold, neutral hydrogen gas contradicted this assumption. What researchers believed was the driving effect of the last phase transition in the universe’s evolution—namely the ionization of primordial matter in the process called “large-scale ionization of the universe”—was challenged by the large amounts of non-ionized gas.

Previous assumptions held that ionization was driven by “pockets” of luminous galaxy clusters. But there is a much larger proportion of cold, neutral hydrogen gas remaining than models predicted at this point in the universe’s history.

Many More Similar Galaxy Structures Have Been Found

Researchers will now investigate this question by observing other galaxy clusters. Master’s student Chamilla Terp is using her thesis project to observe several different types of galaxy clusters with the James Webb Space Telescope (JWST).

Chamilla discovered the first overdensity of cold, neutral gas through the studies she conducted in her bachelor project, so the natural progression is for her to continue these investigations.

Not only that—she also succeeded in developing a method that made it possible to separate observations of the gas belonging to the galaxies under study from the gas lying “in front” of them—in the enormous space along the line of sight from the James Webb telescope. This allowed for much more precise observation of the evolution of individual galaxy clusters—a crucial methodological breakthrough.

And already, more of these types of structures have appeared than researchers expected, even when looking “deep” into the universe, as JWST does—in a small field of view. Looking deep and far into the universe also means looking far back in time, so we can observe early developmental stages—such as galaxy clusters.

This raises yet another question: Why do we see the early birth of numerous very large structures in the universe, but today we don’t actually see those structures? So where did they go? Why did they disappear along the way in the universe’s developmental history?

Link to the scientific article: https://www.nature.com/articles/s41550-025-02745-x