Universe is cooling: new Euclid + Herschel analysis shows star formation is in decline
A major new study combining observations from the European Space Agency’s Euclid and Herschel space telescopes finds that the average temperature of galaxies has fallen slightly over the past 10 billion years — a signal that cosmic star formation has already peaked and is steadily winding down.
The international team of 175 researchers used Euclid’s first public data release together with archival far-infrared measurements from Herschel to measure thermal emission from stardust across a wide wavelength range. By analyzing dust emission in a sample of about 2.6 million galaxies drawn from Euclid’s March data release (which cataloged roughly 26 million galaxies out to ~10.5 billion light-years), the authors produced the most statistically robust, large-scale measurement yet of how galactic temperatures have evolved.
Key result: the mean dust temperature of galaxies has declined by roughly 10 kelvins over the past ~10 billion years. Because dust temperature traces recent star formation — hotter dust indicates more massive, short-lived stars and therefore higher star-formation rates — this modest cooling corresponds to a long-term decrease in the global rate of star production.
“The Universe will just get colder and deader from now on,” said Douglas Scott, a co-author and cosmologist at the University of British Columbia. The authors report that both the amount of dust in galaxies and their characteristic dust temperatures have been falling for billions of years.
The study has been submitted as a preprint to Astronomy & Astrophysics and is available prior to formal peer review. Lead author Ryley Hill (postdoctoral fellow, UBC) emphasized that combining Euclid’s enormous sample with Herschel’s far-infrared sensitivity enabled “the most statistically robust calculations to date,” letting the team detect subtle but consistent trends across millions of systems.
Why dust temperature matters
Dust plays a central role in star formation. Stars form when clouds of gas and dust collapse under gravity and heat up until nuclear fusion ignites. Massive, short-lived stars heat surrounding dust, producing far-infrared emission that telescopes like Herschel can detect. Conversely, galaxies that have lost or exhausted their cold gas and dust form stars more slowly and appear cooler on average. The measured ~10 K decline (earliest galaxies in the sample had average dust temperatures near ~35 K, about −238 °C or −396 °F) therefore maps onto a long-term decline in star-formation activity.
How galaxies become "quenched"
Galaxies can be starved of the cold gas and dust needed to form new stars in several ways: mergers can disrupt gas inflows, powerful outflows driven by supermassive black holes can expel star-forming material, and heating processes can prevent gas from cooling and collapsing. Over cosmic time, such processes transform star-forming galaxies into "quenched" systems with little or no ongoing star formation.
The new results indicate that, on a global scale, the Universe is on a long-term trajectory toward such quenching. That said, these changes unfold over staggeringly long timescales: estimates for a final cosmic “shutdown” vary widely (tens of billions of years to unimaginably larger numbers depending on definitions and processes considered). Local astrophysical events — for example, the Sun’s evolution or the long lifetimes of compact objects like black holes — will persist long after global star formation fades.
Beyond the headline, this analysis provides one of the most precise probes yet of galactic conditions across cosmic history and will serve as a valuable input to Euclid’s ongoing mission to assemble the largest-ever 3D map of the cosmos.
Data & scope: ~2.6 million galaxies analyzed for dust temperatures, drawn from Euclid’s first public release of ~26 million galaxies; Euclid aims ultimately to map roughly 1.5 billion galaxies across ~1/3 of the sky.
