They say a good map can change how you travel. What if a new map could change how we imagine the entire universe?

Imagine standing on a dark beach and throwing a handful of pebbles into the water. Each pebble makes ripples. Now imagine the ripples are the pattern of galaxies across the sky. The Euclid space telescope is building a map of those ripples on a scale we have never seen before. That map could tell us what fills most of the universe and how the cosmos has been stretching and bending over billions of years.

This is not science fiction. The Euclid mission is a real space telescope with one clear goal: to help us understand two giant mysteries called dark matter and dark energy. The mission overview and plan are spelled out in a major paper from the Euclid Consortium.

The problem in plain words

From our everyday life we learn about visible things: stars, planets, light. But when scientists add up all visible matter, it is only a tiny fraction of what the universe needs to behave the way we see it. Most of the universe seems to be invisible. We call some of that invisible stuff dark matter. It acts like extra gravity pulling things together. Another part of the invisible content acts like a push that makes the universe expand faster and faster. We call that dark energy.

We do not know what dark matter or dark energy are. That is the mystery. Euclid is designed to gather data that will let scientists test the leading ideas and perhaps rule some out.

A bold plan: map more sky, see more clearly

Euclid is not one of those telescopes that looks at a small patch of sky in great detail. It is a mapper. It will observe about 14,000 square degrees of the extragalactic sky. To give that meaning, that is a huge piece of the sky outside our own galaxy. Over that area Euclid will take sharp optical pictures and measure light at near infrared wavelengths. From this survey the mission expects to measure the shapes of about 1.5 billion galaxies and gather accurate distance information for tens of millions more.

Why so many galaxies? Because tiny patterns in how galaxies cluster and how their light is bent as it travels to us carry clues about the invisible stuff. With many more galaxies and cleaner images from space, the signal gets clearer. The mission aims to reach a precision that would let scientists test dark energy like never before.

How Euclid reads the universe

Euclid uses two main methods that work together.

First, it measures how the images of far away galaxies are subtly stretched by gravity. This effect is called weak gravitational lensing. Think of the universe like a thin glass with small bumps. Light passing through will bend slightly. By measuring the way billions of galaxy images are distorted, scientists can make a map of where the mass is, even when that mass is dark and invisible.

Second, Euclid measures how galaxies are spread out in three dimensional space. It does this by measuring redshift, which is a way to estimate distance from small shifts in the color of light. By looking at the clustering pattern of galaxies across space and time, the mission will trace how structure grew under gravity and how fast the universe expanded.

Together, these two approaches give a powerful combination. One tells where mass sits, the other tells how matter moves and clumps. Combining them reduces mistakes and improves confidence in the results.

Big numbers that matter

Euclid plans to observe 14,000 square degrees of sky and measure shapes for roughly 1.5 billion galaxies. It will collect emission line distances for more than 25 million galaxies in a key range of cosmic history. Those distances come from a type of measurement called slitless spectroscopy at near infrared wavelengths. With that much data, Euclid aims to push the precision on dark energy parameters to a new level. In more technical terms, the mission targets a dark energy figure of merit that is far beyond current surveys and aims to measure the growth of cosmic structure with fine accuracy. These are the numbers that let the science move from rough ideas to strong tests.

Why this matters for everyone

You might ask why billions of tiny points of light matter for life on Earth. The short answer is this: understanding the basic rules that govern how the universe expands and how gravity works shapes our deepest picture of reality. It can affect how we think about the origin of the universe, the ultimate fate of all galaxies, and the laws of physics that also govern particles and energy in our labs.

Policy makers, science funders, and educators benefit too. Big surveys like Euclid drive advances in detectors, in data handling, and in machine learning tools that later find use in medicine, climate science, and industry. The mission is also a model for global scientific cooperation and open data. Euclid data products will be processed and released so researchers everywhere can use them.

A human story in the data

Picture an early career researcher opening the first public data from the mission. They see millions of galaxies with clean, sharp shapes. A pattern stands out that was only a hint in earlier surveys. That one pattern nudges a small theory aside and focuses attention elsewhere. That is how progress often works. One clear map can change the questions we spend the next decade asking.

Limits and caution

No mission is magic. Euclid will face challenges. Measuring tiny image distortions requires exquisite control of the instrument and of the data processing. The mission teams must avoid confusing instrumental quirks with true cosmic signals. Ground based surveys working at the same time will provide supporting data and help check results. The scientists behind Euclid are transparent about uncertainties and make plans to quantify and reduce possible errors before claiming big discoveries.

What could change if Euclid succeeds

If Euclid finds that the simplest idea for dark energy fits the data well, that would tighten the case for a cosmological constant type of energy. If it finds subtle departures, that could point to a new field, a new particle, or a tweak in our picture of gravity. Either outcome is a win for science because we would know more and be able to narrow our theories.

Beyond the big physics questions, the mission will produce a rich catalog of galaxies that will help with many other topics: how galaxies form and evolve, how black holes grow, how matter is distributed at different times, and even the distance to nearby galaxies that set the cosmic distance scale.

A simple way to think about the payoff

If you want to know whether a recipe works, you test it many times with different ingredients. Euclid is the ultimate test kitchen for the recipe that describes the universe. With a billion galaxy images, the mission will either confirm the recipe we think works now, or it will show that a key ingredient is different. Both outcomes push science forward.

Final thought

Big questions need big maps. With Euclid the map will be bigger and clearer than anything before. The mission will not hand us a single answer at once. Instead it will give the field the data it needs to make strong choices about what ideas to keep and what ideas to set aside. For anyone curious about the cosmos, that is an exciting promise.

Publication details
Year of online publication: 2025
Journal: Astronomy and Astrophysics (A&A)
Publisher: EDP Sciences
DOI: https://doi.org/10.1051/0004-6361/202450810

Credit and disclaimer: This article is based on the Euclid mission overview paper in Astronomy and Astrophysics. Readers are encouraged to consult the full research article for complete methods, data, and technical details.