Euclid Telescope: Mapping the Dark Universe

The European Space Agency (ESA) has officially opened a new window into the cosmos. With the successful release of its first full-color images, the Euclid space telescope has begun its primary mission to create the most accurate 3D map of the universe ever produced. By observing billions of galaxies out to a distance of 10 billion light-years, scientists aim to solve one of the biggest mysteries in astrophysics: the true nature of dark energy and dark matter.

The Mission to Illuminate the Invisible

Launched on July 1, 2023, aboard a SpaceX Falcon 9 rocket from Cape Canaveral, Euclid traveled to the second Lagrange point (L2). This is a stable gravitational point located 1.5 million kilometers (about 1 million miles) from Earth. From this vantage point, Euclid has a clear, blocked view of the cosmos, free from the light pollution and atmospheric distortion that plague ground-based telescopes.

While telescopes like the James Webb Space Telescope (JWST) focus on peering deeply into very small sections of the sky to see the earliest galaxies, Euclid has a different job. It is a survey telescope. Its goal is to scan more than one-third of the entire sky over the next six years.

The data gathered will help scientists understand the “dark universe.” Ordinary matter—stars, planets, gas, and us—makes up only about 5% of the universe. The rest is comprised of two mysterious components:

• Dark Matter (approx. 25%): An invisible substance that exerts gravity and holds galaxies together.
• Dark Energy (approx. 70%): A mysterious force that is accelerating the expansion of the universe.

Unprecedented Images: What Euclid Has Seen

The first images released by the Euclid Consortium were not just engineering tests; they were scientifically significant observations that proved the telescope exceeds its design expectations. These images showcase Euclid’s unique ability to capture sharp, wide-angle views in both visible and near-infrared light.

The Perseus Cluster

One of the headline images captures the Perseus Cluster of galaxies. In a single shot, Euclid captured 1,000 galaxies belonging to the Perseus Cluster and more than 100,000 additional galaxies in the background. Some of these faint background galaxies are so distant that their light has traveled for 10 billion years to reach us. This image is vital because galaxy clusters like Perseus formed within massive halos of dark matter. By mapping the visible galaxies, astronomers can map the invisible dark matter distribution.

The “Hidden Galaxy” (IC 342)

Euclid also imaged IC 342, often called the “Hidden Galaxy” because it sits behind the dusty disc of our own Milky Way. While dust usually obscures visible light, Euclid’s Near-Infrared Spectrometer and Photometer (NISP) can peer through this dust to reveal the cool, low-mass stars dominating the galaxy’s mass. This proves the telescope’s capability to study galaxy structure even in difficult regions of the sky.

Messier 78 and Abell 2390

In a subsequent data release in May 2024, ESA revealed more stunning visuals, including the star-forming region Messier 78 and the galaxy cluster Abell 2390. The image of Abell 2390 is particularly useful for studying “intracluster light,” which is faint light from stars that have been ripped away from their parent galaxies. This helps trace the history of violent interactions within the cluster.

How Euclid Measures Dark Energy

Euclid does not detect dark energy or dark matter directly. Instead, it measures their influence on visible objects. The telescope uses two primary techniques to do this.

Weak Gravitational Lensing

Mass bends light. This is a prediction of Einstein’s theory of general relativity. When light from a distant galaxy passes massive clumps of dark matter, the light path bends, slightly distorting the shape of the galaxy as seen from Earth. This effect is known as gravitational lensing.

Euclid measures these tiny distortions in the shapes of billions of background galaxies. By analyzing how distorted these galaxies appear, scientists can reverse-engineer the location and amount of dark matter that the light passed through. This creates a 3D map of the dark matter density across the universe.

Baryonic Acoustic Oscillations (BAO)

Euclid also measures the precise distance to galaxies using redshift data. By mapping the 3D distribution of galaxies, researchers look for specific patterns left over from the early universe, known as Baryonic Acoustic Oscillations. These patterns act as a “standard ruler” to measure how the universe has expanded over time. By tracking the expansion history, scientists can calculate how the influence of dark energy has changed over the last 10 billion years.

The Technology Behind the Survey

Euclid is equipped with a 1.2-meter diameter mirror and two cutting-edge scientific instruments that work simultaneously.

1. VIS (Visible Instrument): This acts as a massive 600-megapixel digital camera. It operates in the visible spectrum and provides the sharp shape measurements required for weak lensing studies.
2. NISP (Near-Infrared Spectrometer and Photometer): This instrument takes images in near-infrared light and measures the redshift of galaxies. NASA provided the critical detectors for this instrument.

The telescope generates a massive volume of data. Over its six-year mission, Euclid will send approximately 100GB of compressed data to Earth every day. This data is processed by the Euclid Consortium, a group of more than 2,000 scientists from 300 institutes across 13 European countries, the US, Canada, and Japan.

Why This Matters Now

The current model of cosmology works well, but it has cracks. For example, measurements of the expansion rate of the universe (the Hubble Constant) often disagree depending on the method used to measure them. This “Hubble Tension” suggests we might be missing something fundamental about physics.

Euclid provides the scale needed to resolve these tensions. By surveying a third of the sky rather than small pockets, it reduces statistical errors. If dark energy is a constant force (the Cosmological Constant), Euclid will confirm it with higher precision than ever before. If dark energy changes over time, Euclid is the instrument most likely to detect that shift, which would require us to rewrite the laws of physics.

Frequently Asked Questions

How is Euclid different from the James Webb Space Telescope (JWST)? JWST is designed to look very deep into small patches of the sky to see the first stars and analyze exoplanet atmospheres. Euclid is a survey telescope designed to look wide. It covers a much larger area of the sky to map the large-scale structure of the universe.

How long will the Euclid mission last? The nominal mission is scheduled to last six years. However, if the hardware remains healthy and fuel reserves allow, the mission could be extended to gather even more precise data.

Does Euclid take color photos? Euclid captures data in specific filters (visible and infrared). Scientists combine these filters to create the full-color images we see. While they are “false color” in a technical sense, they accurately represent the physical properties of the light being emitted by the celestial objects.

Where can I see the images taken by Euclid? All images and scientific data are released by the European Space Agency (ESA). The public can view high-resolution versions of the Perseus Cluster, the Horsehead Nebula, and other targets on the official ESA website.