The European Space Agency’s Euclid space telescope has provided new evidence that the Universe is gradually cooling and that the era of its most intense star formation lies in the distant past. By combining fresh observations from Euclid with archival data from the Herschel Space Observatory, an international team of 175 scientists has traced how galaxies have changed over the last 10 billion years. Their findings, currently available as a preprint and submitted to Astronomy & Astrophysics, suggest that the cosmos has moved beyond its most active phase.
The researchers analyzed thermal radiation emitted by cosmic dust in more than 2.6 million galaxies. Interstellar dust absorbs ultraviolet light from young, massive stars and re-emits it in the infrared range. By measuring this infrared glow, astronomers can estimate how actively galaxies are forming new stars. The larger and hotter the dust emission, the more vigorous the star-forming activity.
The results reveal that the average temperature of galaxies has dropped by about 10 Kelvin over cosmic time. The oldest galaxies in the sample exhibited average temperatures of roughly 35 Kelvin (about –238°C), while more recent galaxies show lower values. Although a 10-Kelvin decrease may appear small, on cosmological scales it signals a substantial decline in star formation activity across billions of years.
By combining visible, near-infrared, and far-infrared observations, the team achieved some of the most statistically robust measurements of galactic temperatures to date. The vast sample size strengthened the reliability of their conclusions, allowing scientists to track long-term trends rather than isolated cases. The study confirms a clear relationship between dust temperature and star formation rate: galaxies with warmer dust tend to produce stars more rapidly, while cooler galaxies are gradually becoming quiescent.
Several processes likely contribute to this slowdown. As galaxies evolve, they consume or lose the cold gas required to form new stars. Galactic mergers can disrupt the steady inflow of gas, while powerful outflows driven by supermassive black holes may expel star-forming material into intergalactic space. Over time, as available gas reserves diminish, galaxies transition from vibrant star factories to quieter systems dominated by aging stellar populations.
Despite the dramatic phrasing sometimes used to describe these findings, scientists emphasize that the “cooling” of the Universe does not imply an imminent cosmic end. The decline in star formation has been unfolding gradually over billions of years and will likely continue for billions more. Our Sun, for example, will exhaust its nuclear fuel and evolve into a red giant long before the Universe reaches any ultimate fate. Black holes may persist for unimaginable stretches of time, outliving stars and galaxies alike.
Euclid’s observations represent one of the mission’s first major scientific achievements. Launched to map the large-scale structure of the cosmos, Euclid aims to create the most detailed three-dimensional map of the Universe ever assembled, covering roughly one-third of the sky and cataloging around 1.5 billion galaxies. Beyond studying galaxy evolution, the mission also seeks to shed light on dark matter and dark energy, the mysterious components that dominate the Universe’s mass-energy content.
Some theoretical models propose that the Universe’s total lifespan could be finite—on the order of 33 billion years—suggesting that cosmic expansion might eventually slow and reverse. According to such scenarios, expansion could continue for another 11 billion years before gravitational effects overcome the influence of dark energy-like fields. However, these models remain speculative and are the subject of ongoing debate within the scientific community.
What Euclid’s data clearly show is that the Universe is no longer in its youthful, star-forming prime. The peak of cosmic activity occurred billions of years ago, and since then, the cosmos has been entering a quieter, cooler phase of evolution. Rather than signaling catastrophe, this discovery deepens our understanding of how the Universe changes over time—slowly, majestically, and on scales far beyond human experience.
