The universe is not frozen in eternal youth. Behind its apparent stability there is a slow transformation, almost imperceptible on a human scale. As the galaxies continue to shine, their light betrays another, more discreet narrative, that of gradual exhaustion. Star formation, the fundamental driver of cosmic evolution, is now showing clear signs of slowing down, revealed by the most recent observations from the Euclid telescope.
What the largest stack of galaxies ever reveals
Until recently, no space mission had made it possible to map so precisely the average temperature of millions of galaxies distributed throughout the universe. This milestone has just been reached thanks to the collaboration between two missions of the European Space Agency: the Euclid telescope, launched in 2023, and its predecessor Herschel, active between 2009 and 2013. By combining the optical and infrared surveys of these two instruments, a team of 175 researchers analyzed more than 2.6 million galaxies to extract thermal data of unprecedented precision.
The study, led by Ryley Hill and published in the journal Astronomy & Astrophysics, relies on a technique called statistical stacking. It consists of superimposing the light signals of all the galaxies observed, even the faintest, in order to identify an overall trend. This approach revealed a slow but measurable drop in the temperature of interstellar dust over nearly ten billion years. These grains of dust, invisible to the naked eye, are nevertheless essential for understanding how galaxies make their stars.
How star formation declined over time
The study highlights a temperature drop of around 10 kelvins over the last ten billion years, a weak but significant signal. Unlike the temperature of a star like the Sun, which well exceeds a million kelvins, galaxies, mainly made up of vacuum, display a much lower average temperature. The oldest observed in this study reached around 35 kelvins, or -238 degrees Celsius.
This temperature is not a simple indicator of residual heat. It is directly linked to the rate of star formation. The more stars a galaxy forms, the more its dust is heated by the presence of young massive stars. Conversely, cooling indicates a slowdown in the creation of new stars. Douglas Scott, a cosmologist at the University of British Columbia, points out that galactic dust, in decreasing quantities, now mainly draws its heat from old and cold stellar populations. This observation shows that the universe is well past its peak of star formation.
Why the cosmos is heading towards a gradual extinction of light
In galaxies, stars are born when clouds of gas and dust collapse on themselves under the influence of gravity. This process, which strongly depends on the quantity of available material and its thermal agitation, today seems to be running out of steam. Several mechanisms can dry out a galaxy of its stellar materials. Collisions between galaxies, winds caused by supermassive black holes, or even the simple aging of stellar systems, contribute to this drying up.
Recent results show that this drying up does not only concern isolated or atypical galaxies. It is a universal phenomenon, detectable in all regions of the universe. According to the analyzes relayed in LiveScience, the peak of cosmic activity is already behind us. The universe continues to exist, but it becomes more and more calm, more and more cold, and less and less fertile. This slow shift will not be completed for billions, even trillions of years, but it is now inevitable.
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