In the publishing world, a common promotional practice when a book is released is to unveil extracts from it. At the 75th International Astronautical Congress, held in Milan, Italy, from October 14 to 18, the European Space Agency (ESA) played the same game, unveiling the first pages of the sky atlas currently being compiled by its Euclid probe.
Launched in July 2023 in a bid to unravel the two great cosmological riddles of dark matter and dark energy, the spacecraft photographed a small patch of the southern sky in detail this spring. The definition of Euclid's images is such that you can zoom right in. The photograph accompanying this article, showing two interacting galaxies in the center, is the result of 150x magnification. On the right is a cluster of galaxies separated from us by 678 million light-years.
This first "panel" revealed by ESA covers 132 square degrees of sky, more than 500 times the size of the Moon as we see it from Earth. No fewer than 260 observations were required to create this mosaic. However, it represents only a fragment – around 1% – of what Euclid will map throughout its mission. The probe will focus on the entire section of the firmament that is not too "polluted" by our own galaxy, the Milky Way – that is, a little more than a third of the sky. If all goes to plan, the entire survey will be published in several stages between now and 2030.
For the time being, this is "just 1% of the [final] map, and yet it is full of a variety of sources that will help scientists discover new ways to describe the universe," said Valeria Pettorino, an Italian astrophysicist working on Euclid at ESA.
It may seem paradoxical, but the main aim of the mission is to understand two features... that we can't see in the images! The first is dark matter, which holds galaxies together: Without it, the stars gathered in these spinning groupings would be ejected into the cosmos like children not holding tightly to a merry-go-round. Dark matter has an effect on mass, but it does not interact at all with electromagnetic waves, making it invisible.
The second target of the cosmologists' investigation is dark energy. According to theory, this is responsible for the accelerated expansion of the universe and is equally invisible. Between them, dark matter and dark energy account for 95% of the cosmos' content (compared with just 5% for the "classical" matter of which we are made). In other words, there's a real challenge in understanding their properties.
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