The universe has thrown astronomers a curveball. An international team led by Canadian researchers has uncovered a galaxy cluster so hot, so early, that it challenges the very foundations of how scientists think clusters form.
In today’s galaxy clusters, most normal matter (baryons) is found as very hot gas called the intracluster medium (ICM). Computer models suggest that in the early universe, this gas should have been cooler and smaller, only heating up as clusters grew older. But scientists have now discovered extremely hot ICM just 1.4 billion years after the Big Bang.
The discovery centers on a “baby” galaxy cluster called SPT2349-56, located nearly 12 billion light-years away. Using ALMA, a powerful group of radio telescopes that includes parts built by Canada’s National Research Council, the scientists looked deep into the past and discovered a galaxy cluster much hotter than current theories say it should be.
“We didn’t expect to see such a hot cluster atmosphere so early in cosmic history,” said lead author Dazhi Zhou, a PhD candidate in the University of British Columbia’s department of physics and astronomy. “In fact, at first I was skeptical about the signal as it was too strong to be real. But after months of verification, we’ve confirmed this gas is at least five times hotter than predicted, and even hotter and more energetic than what we find in many present-day clusters.”
What happened before Big Bang?
The team believes the culprit may be three recently discovered supermassive black holes lurking within the cluster.
“This tells us that something in the early universe, likely three recently discovered supermassive black holes in the cluster, were already pumping huge amounts of energy into the surroundings and shaping the young cluster, much earlier and more strongly than we thought,” explained co-author Dr. Scott Chapman, professor at Dalhousie University and UBC affiliate, who researched while at the NRC.
SPT2349-56 is not a normal young cluster. Its center is huge; about 500,000 light-years wide, similar to the Milky Way’s halo. Inside, there are over 30 active galaxies. Stars are forming at an incredible rate, more than 5,000 times faster than in our own galaxy, all squeezed into a small area.
To measure the thermal energy of the intracluster medium, the team relied on the Sunyaev-Zeldovich effect, a cosmological tool that reveals how hot gas distorts the cosmic microwave background.
“Understanding galaxy clusters is the key to understanding the biggest galaxies in the universe,” said Dr. Chapman. “These massive galaxies mostly reside in clusters, and their evolution is heavily shaped by the extreme environment of the clusters as they form, including the intracluster medium.”
Current models suggest that intracluster gas is gradually heated by gravitational collapse as clusters stabilize. But this finding points to a more explosive, accelerated process.
The researchers now face a new puzzle: how do intense star formation, active black holes, and an overheated atmosphere coexist in such a young, compact system?
“We want to figure out how the intense star formation, the active black holes, and this overheated atmosphere interact, and what it tells us about how present galaxy clusters were built,” said Zhou. “How can all of this be happening at once in such a young, compact system?”
This finding changes what we know about how galaxy clusters grow. It suggests that hidden forces were shaping the universe when it was very young. If young clusters can already burn with hot gas, scientists may need to rethink how galaxies, black holes, and their surroundings develop together.
Journal Reference:
- Zhou, D., Chapman, S.C., Aravena, M. et al. Sunyaev–Zeldovich detection of hot intracluster gas at redshift 4.3. Nature (2026). DOI: 10.1038/s41586-025-09901-3
