It eventually parked at the Sun-Earth Lagrange Point L2, 1.5 million kilometers from Earth. This position, "locked" by the combined gravitational forces of the Sun, Earth, and Moon, not only provides it with an unobstructed view free from atmospheric interference but also allows its solar panels to continuously capture stellar energy. Here, Euclid will spend at least 6 years scanning one-third of the sky to create an unprecedented 3D map of the universe for humanity. The ultimate goal of this map is to unravel the mysteries of dark matter and dark energy, which make up 95% of the universe's mass. ### 01. The Silent Observer: A Cosmic Vision in Its Technical Framework Every inch of the Euclid telescope's structure embodies the ultimate pursuit of "wide field" and "high precision." Its 4.7-meter-tall frame is cast from silicon carbide ceramics, a material that can maintain micron-level stability even with temperature differences of ±200 degrees Celsius. This ensures that the 1.2-meter-diameter primary mirror remains aligned with its target — for observation tasks requiring the capture of faint starlight from billions of light-years away, such stability means images won't blur due to material deformation. The telescope's "eyes" consist of two parts: the Visible Light Camera (VIS) and the Near-Infrared Spectrometer and Photometer (NISP). The former is extremely sensitive to light in the 550-900 nanometer wavelength range. The focal plane array, composed of 36 4000×4000-pixel CCD chips, can capture a sky area equivalent to 2.5 full moons in one shot, with a resolution of 0.23 arcseconds — sufficient to distinguish individual stars in a globular cluster 8,000 light-years away. The latter penetrates dust and gas obscuration to measure galaxy redshifts in the 900-2000 nanometer range,推算ing their distance and recession velocity through the degree of light wavelength stretching. These technical details are not isolated parameters but collectively form a "tool" for observing dark matter and dark energy. For example, the stability of the silicon carbide material ensures accurate measurement of weak gravitational lensing — when light from distant galaxies passes through dark matter, gravity bends it like a lens. Only sufficiently clear images can capture such subtle distortions, allowing the reconstruction of dark matter distribution. Meanwhile, NISP's redshift measurement capability enables scientists to label each observed galaxy with a "cosmic timestamp." Combined with positional information, this helps piece together the universe's expansion history from 10 billion years ago to the present — the key to unraveling how dark energy drives the accelerating expansion of the universe. Even the choice of the L2 point holds深意: it avoids atmospheric absorption of infrared light and allows the telescope to maintain its orbit with minimal fuel consumption, providing a stable observation platform for the six-year sky survey. ### 01. The Silent Observer: Cosmic Vision in Technical Skeleton Every part of the Euclid telescope's structure hides the ultimate pursuit of "wide field" and "high precision." Its 4.7-meter-tall body is cast from silicon carbide ceramics, a material that can maintain micron-level stability even with temperature differences of ±200 degrees Celsius, ensuring that the 1.2-meter-diameter primary mirror remains aligned with the target. For observation tasks that need to capture faint starlight from billions of light-years away, this stability means that images will not be blurred due to material deformation. The "eyes" of the telescope consist of two parts: the Visible Light Camera (VIS) and the Near-Infrared Spectrometer and Photometer (NISP). The former is extremely sensitive to light in the 550-900 nanometer wavelength range. The focal plane array composed of 36 4000×4000-pixel CCD chips can capture a sky area equivalent to 2.5 full moons at one time, with a resolution of 0.23 arcseconds, which is sufficient to distinguish individual stars in a globular cluster 8,000 light-years away. The latter can penetrate the obstruction of dust and gas and measure the redshift of galaxies in the 900-2000 nanometer wavelength range, inferring their distance and recession velocity through the stretching degree of light wavelength. These technical details are not isolated parameters but together form a "sharp tool" for observing dark matter and dark energy. For example, the stability of silicon carbide materials ensures the accurate measurement of weak gravitational lensing effect. When the light from distant galaxies passes through dark matter, gravity will twist its shape like a lens. Only sufficiently clear images can capture such subtle deformation, and then infer the distribution of dark matter. The redshift measurement capability of NISP enables scientists to mark each observed galaxy with a "cosmic timestamp". Combined with its position information, it can gradually piece together the expansion track of the universe from 10 billion years ago to now, which is the key to solving the mystery of dark energy driving the accelerated expansion of the universe. Even the choice of the Sun-Earth Lagrange L2 point has profound implications: it not only provides a broad view free from atmospheric interference but also allows the solar panels to continuously capture the energy of stars, providing a stable observation platform for the six-year sky survey. ### 02. Decoding the Dark Universe: From Theoretical Puzzles to Observation Windows Dark matter and dark energy, these two "invisible players" that dominate the universe, have always been the biggest puzzles in modern cosmology. In 1933, when astronomer Fritz Zwicky was measuring the Coma Cluster, he found that the movement speed of galaxies was far faster than that supported by the gravity of visible matter. He speculated that there was a kind of "missing mass" — later known as dark matter. This invisible matter does not interact with light, but shapes the structure of the universe through gravity: it is like an invisible skeleton, connecting galaxies into a filamentary cosmic web, and forming galaxy clusters at the intersections of the filaments. The proposal of dark energy is related to the expansion of the universe: Einstein once introduced the "cosmological constant" to maintain a static universe, but later regarded it as his "biggest mistake". However, observations in 1998 confirmed that the universe is expanding at an accelerated rate. Scientists had to pick up this concept again and call the unknown force driving the expansion dark energy, which accounts for 68% of the total mass-energy of the universe, but its form remains unknown. The mission of the Euclid telescope is to find observational evidence for these theories. For dark matter, it "tracks" in two ways: first, observing the formation of galaxy clusters. A giant structure like the Perseus Cluster, which contains 1000 galaxies, can only be bound together by the gravity of dark matter. The distortion of the shapes of 100,000 background galaxies captured by the telescope can outline the distribution of dark matter around the galaxy cluster. Second, studying the tidal tails of globular clusters. These "tails" composed of stars are the products of the interaction between the cluster and the Milky Way. If there is a dark matter halo, stars will be pulled by gravity and difficult to escape; otherwise, obvious tidal tails will form. Euclid's observation of the globular cluster NGC 6397 is trying to infer the distribution of dark matter in the Milky Way through this structure. The exploration of dark energy focuses on the history of cosmic expansion. By measuring the redshifts of billions of galaxies with NISP, scientists can map the cosmic scales in different periods — the smaller the redshift (the closer the distance), the more the change in the cosmic expansion speed can reflect the role of dark energy. For example, if dark energy is a constant "cosmological constant", the expansion acceleration should remain stable; if its intensity changes with time, the acceleration will also change accordingly. Euclid's observation data will test these models and may even reveal whether dark energy belongs to a new fundamental force. ### 03. Revelations in Light and Shadow: Cosmic Clues in the First Images The first set of scientific images released in November 2023 vividly demonstrated Euclid's capabilities. In the photo of the Perseus Cluster, 1000 foreground galaxies and 100,000 background galaxies weave together the "local network" of the universe. The distortion of the light from those distant galaxies when passing through dark matter filaments, like fingerprints, records the distribution of dark matter — scientists confirmed from it that it is these invisible "filaments" that pull galaxies into clusters, confirming the theory that the "cosmic web" is maintained by dark matter. The spiral galaxy IC 342, obscured by dust, revealed clear spiral arms under the near-infrared lens. Those previously undiscovered globular clusters, like scattered pearls embedded in the galaxy, their ages and distributions provide a key to studying the star formation history of galaxies similar to the Milky Way — after all, the birth and evolution of stars are always regulated by the gravitational field of dark matter. The second batch of images in May 2024 further expanded the depth of observation. In the Abell 2390 cluster, 50,000 galaxies revolve around the dark matter halo, and the light from background galaxies is distorted into arcs by gravitational lensing. The curvature of these arcs accurately reflects the mass distribution of dark matter. The infrared image of the Messier 78 nebula penetrates the dense molecular cloud, capturing newly born stars and substellar objects. These Jupiter-mass objects are potential samples of "brown dwarfs", candidates for dark matter. In the Abell 2764 cluster, the morphology and distribution of those early galaxies formed when the universe was only 700 million years old suggest that dark matter began to shape structures in the early days of the universe's birth. These images are no longer isolated landscapes but a chain of mutually confirming evidence — from nearby globular clusters to early galaxies billions of light-years away, Euclid is using observations at different scales to piece together a panoramic view of the role of dark matter and dark energy. Most notably, the "first page of the 3D map of the universe" released in October 2024 includes 100 million star and galaxy sources, among which 14 million can be used to study the influence of dark matter and dark energy, accounting for only 1% of the total task. But even this 1% of data has allowed scientists to see new possibilities: by analyzing the shape distortion of 14 million galaxies, the local dark matter distribution map they drew is highly consistent with the prediction of the ΛCDM model, but there are also subtle deviations at the edges of some galaxy clusters — these deviations may be the breakthrough point of the existing theory. ### 04. The Unfinished Map: Six-Year Sky Survey and the Future of Cosmology Euclid's journey has just begun. According to the plan, it will scan 15,000 square degrees of the sky in the next 6 years, equivalent to 1/3 of the night sky, observe billions of galaxies, and finally draw a 3D cosmic map spanning 10 billion light-years. The significance of this map lies not only in its huge scale but also in that it will realize the "collaborative research" of dark matter and dark energy for the first time. Previous observations mostly focused on a single phenomenon, while Euclid's data can simultaneously analyze weak gravitational lensing (reflecting the distribution of dark matter) and baryon acoustic oscillations (reflecting the history of cosmic expansion, related to dark energy). This collaboration will greatly improve the reliability of the conclusions.