All things dark and beautiful: How Euclid may upend astrophysics

6 Jul 2023

Image: © Aaron Alien/

Peter Coles, the only academic in Ireland working on Euclid, tells Vish Gain how the ESA mission will put decades-old predictions on dark matter and dark energy to the test.

Over the past weekend, thousands watched as Europe’s latest Euclid telescope was blasted into space by the powerful SpaceX Falcon 9 rocket to probe some of the most mysterious aspects of our universe – dark matter and dark energy.

And while a lot of people may know that Euclid hopes to test whether our current theories on dark matter and energy are accurate, what may elude the understanding of most is what the two ominous sounding thigs are in the first place. For starters, they’re not aptly named.

“If you say dark matter, that tends to suggest that it’s absorbing light, or that it’s putting a blanket over something that makes it dark,” explained Peter Coles, a professor at Maynooth University and the only Irish academic involved in the Euclid mission.

“In fact, dark matter is actually transparent. It just doesn’t interact with light or electromagnetic radiation at all. But we’re pretty convinced it must be there because of the way we see things. We can detect things in astronomy all the time through the way they affect the way other things move.

“So, I don’t think ‘dark’ is the right word. But we’re stuck with it now, it’s not going to go away.”

What is dark matter and why does it matter?

Scientists only know about dark matter because even though it doesn’t interact with most forces of nature, it exerts gravity. And that “feeble” force it exerts is enough to bend light ever so slightly over long distances.

“It’s nothing like as dramatic as a black hole, but it’s the same physics,” Coles explained. “When we look at really distant galaxies, the light from those galaxies has travelled through the whole universe – which is pervaded with this dark matter – and that dark matter is clumped. So the light is slightly distorted by the effect on the photons of light traveling through this dark matter.”

It is this distortion in light from distant galaxies that Euclid will capitalise on to test our current theories on dark matter, which is believed to account for most of the matter present in the universe.

“We are there, after 12 years of technical development and scientific preparation, and we now are moving to the second phase of the mission that will tell us what is the very nature of dark energy,” said Yannick Mellier, Euclid’s consortium lead, ahead of the launch last week.

In Coles’ words, dark energy is a different kettle of fish. While it is also believed to be transparent like dark matter, that’s where the similarity ends.

“Dark energy is what we think is a vacuum energy. It’s an energy associated with empty space, which changes the expansion rate of the universe,” Coles explained.

“The point about the Euclid experiment is that we’re looking back to distant galaxies. And although the dark energy doesn’t produce the lensing effect of dark matter, it does change the expansion rate, and therefore, the redshift [the wavelength of the light is stretched so the light is seen as ‘shifted’ towards the red part of the spectrum].

“So, by looking again at galaxies at different distances and how they cluster, we can actually measure whether the amount of dark energy there is what is predicted.”

The most exciting outcome for Coles from the European Space Agency’s Euclid mission is rather surprising. “If dark matter and dark energy are not detected as predicted with this experiment, then our theory is wrong. And that’s probably the most exciting thing that could happen is if it’s wrong, because then we have to think of something else.

“But we know what we’re shooting at, you know, because there’s a definite prediction.”

On the bright side

Coles, who is originally from the UK, moved to Maynooth in 2017 in the aftermath of the Brexit vote. He was already involved in the Euclid mission – which Ireland was not a part of – before he moved here.

Today, Coles is part of a consortium of more than 2,500 people working on the Euclid mission across 300 laboratories worldwide. He is working with the galaxy clustering working group that is studying how clustering evolves with redshift through measurements and statistical analyses.

Now on its way to the Earth-Sun Lagrange Point 2, where the James Webb telescope is also located, Euclid will study one third of the sky over the next six years at a cost of around €1.4bn. The whole mission, however, is spread over 10 years and involves more than 15 countries.

And while dark matter and energy will be Euclid’s primary focus over this period, Coles is optimistic the telescope will discover more than just those two mysteries. Unlike the James Webb and other telescopes, Euclid will not give us fresh and beautiful images of distant celestial worlds every week.

“It’s not going to be data every day in the press and pictures and so on. But it is a big map of the sky, and I would say that the first thing that Euclid will discover is likely to be some serendipitous thing that just happens to be in the data when it’s been calibrated,” he said.

“So, maybe a supernova goes off in one of the distant galaxies or something like that. The core science mission is a very long, difficult and painstaking operation, but there will be other stuff in the data that will probably come out.

“You’re looking at such a huge part of the universe, something is bound to happen.”

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Vish Gain is a journalist with Silicon Republic