Why are galaxies growing colder, and what does that mean for the cosmos? The latest results from the European Space Agency’s Euclid mission, combined with archival data from the Herschel Space Observatory, provide the clearest answer yet: the era of maximum star formation is behind us, and the universe’s thermal engine is winding down.
The team of 175 scientists on the research team examined the heat emanating from stardust in 2.6 million galaxies across a time frame of 10 billion years. Euclid’s visible and near-infrared instruments charted the optical structure of these galaxies, while Herschel’s far-infrared detectors captured the faint thermal glow of dust grains. By combining these datasets, astronomers achieved the most comprehensive galactic temperature measurements ever taken, spanning wavelengths that reveal both the hot, massive stars driving star formation and the cooler interstellar medium.
These results reveal a subtle yet telling decline: the average galactic dust temperatures have fallen from about 35 kelvins in the early universe to about 25 kelvins today-a 10-kelvin drop over 10 billion years. Though modest in absolute terms, this cooling is strongly coupled with the observed slowdown of star formation rates. Hotter dust indicates copious, short-lived massive stars; cooler dust signals fewer such stars and a reduced capacity for fresh stellar births.
Dust plays a very fundamental role in this process. As described in studies related to infrared astronomy, dust grains serve as the catalyst for molecular hydrogen formation, allowing the collapse of cold clouds under their self-gravity to ignite nuclear fusion. This decrease in dust mass-owing to both reduced stellar production and less efficient replenishment in the interstellar medium-means fewer star-forming regions are able to achieve the densities required for collapse. While supernovae and asymptotic giant branch stars enriched galaxies with metals and dust early in the universe, over time the balance flipped. Once a galaxy’s metallicity surpassed the critical threshold, dust growth in the ISM dominated production. With star formation now slowing, even that growth is tapering off.
The cooling trend is also a reflection of the rising predominance of “quenched” galaxies-those that have lost or ejected their star-forming material. Galaxy quenching may be caused by a variety of astrophysical mechanisms. Mergers can disrupt gas inflows, while AGN feedback from supermassive black holes can heat or expel interstellar gas and thus suppress star formation. Observations and simulations, such as models of cosmic reionisation produced for Thesan, demonstrate that AGN-driven outflows can quickly remove cold gas and leave behind massive quiescent galaxies that continue to grow in mass but are inert in terms of star formation.
Euclid’s wide-field capabilities are central to detecting these trends, with its field of view 100 times larger than that of the James Webb Space Telescope, capturing the large-scale structure of the universe while resolving fine details in individual galaxies. The ultimate aim of this mission is a 3D map of 1.5 billion galaxies, covering one-third of the sky, to probe how matter and energy-particularly dark matter and dark energy-drive cosmic evolution.
This first dataset already spans 10.5 billion light-years, providing a statistically robust foundation for tracking how dust temperatures and star formation rates change with time. The implications extend beyond astrophysics: understanding the timeline of star formation informs models of galaxy evolution, the distribution of heavy elements, and the future habitability of planetary systems. As co-author Douglas Scott summarized, “The Universe will just get colder and deader from now on.” While that fate lies tens of billions of years ahead, the mechanisms driving it are already in motion, etched into the cooling dust and fading light of galaxies across the sky.
