The Euclid telescope: On the trail of dark energy and matter

You've probably heard of dark matter and dark energy, right? They sound like something straight out of a Star Wars movie. Terms like these can sound so foreign to us that we just don't bother thinking about them. That's what often happens in science. Getting our heads around some of its concepts can be very challenging — yes, I'm looking at you, quantum mechanics.

Unraveling these mysteries is one of the main goals of the new Euclid space telescope. Euclid successfully launched on July 1 on a SpaceX Falcon 9 rocket from Cape Canaveral, Florida, USA. Its mission is to deepen our understanding of dark matter and dark energy. Euclid won't answer all the questions that remain about the dark universe, but it's a big step on the path to do so. 

Ok, cool, but why should we care? Well, because ultimately Euclid's task is to explore some of the most profound and fundamental questions in cosmic history. Like, how did the universe originate and what is it made of? Or, what are the fundamental physical laws of the universe? Take a second to pause, look up into the stars, and ask yourself these questions. Let the mind wander.

The building blocks of the universe

"Both dark matter in particular, but also dark energy, ultimately our existence traces back to those," Hans-Walter Rix, astronomer and director of the Max Planck Institute for Astronomy, in Heidelberg, Germany, told DW.

Dark matter and dark energy provided the right conditions for enough material to come together and form stars, planets, galaxies, life, you and me. So understanding them is a step closer to understanding where we come from.

This is what makes the mission such an historic one. The Euclid telescope is led by the European Space Agency (ESA) with contributions from NASA. It took more than ten years of development, €1.4 billion and more than 3,500 scientists from 21 countries.

Euclid was originally planned to launch from a Russian Soyuz rocket in 2022, but Russia's invasion of Ukraine called for a change of plans.

The distant universe in unprecedented detail

Good news for the James Webb Space Telescope — it's getting company. Euclid, like the Webb, will also orbit the second Lagrange point (L2), 1.5 million kilometers (about 930,000 miles) from Earth.

Imagine there's a straight line coming from the Sun and passing through Earth and beyond. L2 would be in this line, behind the Earth. Always aligned as both orbit the sun.

The questions Euclid is tasked with addressing are pretty ambitious ones. The primary goal is to create a map. "It is the biggest and most accurate map of the universe, what we are basically aiming to do, that has never been done before," Guadalupe Cañas Herrera, a cosmologist working on the Euclid mission at ESA, told DW.

It takes its name from the Greek mathematician Euclid of Alexandria, who lived in the 3rd century BC and is known as the father of geometry — and the terror of high school students. The mission is named in his honor because better understanding of how everything is distributed in the universe tells us a lot about its geometry.

To make the map, Euclid is going to look at and measure billions of galaxies — yes, that's a thousand million — with extreme precision over a span of at least six years. Euclid is equipped with a visible light sensor and a near-infrared instrument capable of accurately measuring the distance of these galaxies.

"We are aiming to do something really challenging or something that is really ambitious, which is mapping a third of the full sky," said Cañas.

Euclid vs. the James Webb

One third of the sky is huge. Just take a look at the deep field image from the James Webb, down here. 

That was just a portion of the sky the size of a grain of sand on your fingertip on a stretched arm, so imagine how many stars and galaxies there are in a third of the whole sky, — and how many planets.

They are both space telescopes, but they have different purposes. Euclid's mirror is 1.2 m in diameter, compared to the Webb's huge 6.5 m. But the James Webb is like a precise pencil that sketches fine details, it looks at a very small portion of the sky with amazing detail. Euclid is more like a brush that can cover big patches of sky quickly. 

But don't be fooled by Euclid's size, "Euclid will actually take a very high-resolution picture of the sky that will deliver gorgeous, gorgeous pictures," added Rix.

How the shape of the universe has changed over time

Euclid will be glimpsing into the past. That's because light takes a while to travel through the immensity of space. For example, it takes light eight minutes to travel from the sun to us. So, at larger distances, the farther we look, the earlier we see into our universe.

And these galaxies can be very far away, with their light taking up to 10 billion years to reach us. For reference, the age of the universe is around 13.8 billion years.

The goal here is to have the best understanding to date of where galaxies are in the distant universe up to 10 billion light-years away, but also how the shape of the distant universe has evolved over time. 

But to better understand how this can help answer the previously mentioned questions and how all of these is related to dark matter and dark energy, we first need to talk about the dark universe.

The dark universe

Everything we know and see — bacteria, plants, animals, stars, planets, galaxies — is made of atoms that you can pinpoint in the periodic table. Astronomers refer to this as baryonic matter, in case you want to dazzle your cosmologist cousin.

But this visible matter is just a fraction of what the universe is made of — just 5%. The rest is dark matter and dark energy.

We might not know what they are, but there are many different lines of evidence that tell us that for sure they are there.

Dark matter seems to be keeping galaxies together, making the stars inside orbit faster than we would expect them to, and preventing galaxies from falling apart. It doesn't emit or absorb light, but we can also tell it's there by how seemingly empty regions of space bend the light of objects behind it — what's called gravitational lensing. And there's a lot of dark matter, around five times more than ordinary matter.

The idea of dark energy was first mooted in the 1990s, when scientists discovered that, contrary to what they thought, everything in the universe, on a larger scale, is moving apart from everything else at an accelerated rate. Just as your speed increases every second when you press down on the gas pedal, a very distant galaxy is moving away from us faster every second. Dark energy is what we call the unknown driving agent of this accelerating expansion.

A good analogy, and something you can try at home, is to grab a balloon that's just slightly inflated, draw a few dots with a marker on it and then start blowing it up. You'll see every dot getting further apart from all the other dots. That's exactly like what happens in our universe.

We don't experience this expansion in our daily life because our galaxy, for example, is tightly bound by gravity — mostly from dark matter.

Euclid, dark matter and dark energy

"You are not just mapping where all the stuff you can see is, but we will also map where all the stuff we can't see is," Becky Smethurst, a Royal Astronomical Society Research Fellow at the University of Oxford, told DW.

Euclid will not only map the visible, "ordinary" matter, but also dark matter in the universe. A more detailed map of where dark matter is and how it is distributed can be a huge help in trying to better understand what dark matter is.

About that gravitational lensing mentioned earlier. This occurs when a gravitational field distorts light just like a lens, or a glass of water. The light of very far away galaxies gets distorted — often in a sausage shape. And depending on the level of distortion, gravitational lensing can indirectly tell us how much dark matter there is. 

Well, Euclid is going to exploit the phenomenon to look into billions of distorted galaxies and infer the amount and distribution of dark matter there is, effectively creating a gigantic 3D map.

Mapping all these millions of galaxies requires very precise measurements of how distant they are. This is measured by the red shift.

The expanding universe stretches light waves. In visible light, such a stretch would shift light towards the red side of the spectrum, hence red shift. But this can happen with all light waves, like infrared or radio, and it can be measured. This way, Euclid is going to provide the most precise measurements ever of how the larger cosmic structures have changed over time, effectively tracing the effects of dark energy, which ultimately will tell us much more about what dark energy is and what it is not.

This article was originally published on June 29, 2023. It has been updated on July 3, 2023, to reflect the launch of the telescope.

Edited by: Jane Paulick