How the Euclid telescope is mapping dark matter

The truth is out there...
04 June 2024

Interview with 

Carole Mundell, ESA


Messier 78 image taken from Euclid


The Large Hadron Collider is currently unable to shed any further light on the nature of dark matter. So, we must look in a different direction for the moment. Enter the Euclid mission. Euclid is a space telescope developed by the European Space Agency and the Euclid consortium. It was launched in July of last year, with the objective of looking 10 billion years into the past to track how the expansion and formation of galaxies might have been influenced by our two mysterious friends. The European Space Agency’s director of science is Carole Mundell...

Carole - The Euclid mission is an iconic and unique, world leading space mission and combines some unique capabilities. What it's particularly good at is very wide field imaging. We also have spectroscopy in the optical, in the infrared, but combining wide field patches on the sky with very sensitive detectors, so we can look back in cosmic history very far and see very, very faint features in the universe. But also with a very third and very important feature, which is very crystal clear vision, so diffraction limited optics. And the reason that we need to have a wide field of sensitivity and precision is because we are going to map, in the next six years the entire extragalactic sky, back to 10 billions in cosmic time. And what we will deliver to our cosmologists in the Euclid Consortium is this beautiful precise data that they will go in and they will look for tiny little distortions in the shapes of billions of galaxies. And they will use those distortions to look for the distorting signatures of dark matter and how that dark matter couples to dark energy through cosmic history. And so it really is a huge step forward in our technical capability to study the laws of physics beyond where we currently understand them.

Will - The one thing that I've really grasped when talking about dark matter and energy is that we can't see it. Why have you chosen to use a telescope?

Carole - Yeah, so this is a great question and whenever we show the beautiful early release images that we've taken with Euclid to test our engineering, people always say, 'show me where the dark matter is.' And what dark matter does is it changes the way spacetime bends, if you like. And so we can look at big groups of galaxies where we know there's an awful lot of matter there. And when the light from background galaxies comes through that so-called gravitational lens, we actually see slight distortions. When you're very close to the centre of a big concentration of dark matter, you see big distortions. So galaxies might be twisted out into thin arcs. And when you go further away from that concentration, although there is still dark matter there, you will see tiny little distortions in galaxies. And it's being able to do that over big areas of the sky to control and the systematics so that we can then map out that distribution of dark matter through cosmic filaments that we think are where that the galaxy clusters form at the intersections of these filaments and really map that back, that 3D mapping through cosmic time over big areas of the sky.

Will - The fourth dimension of time, is this something new that Euclid is bringing to the research into dark energy and dark matter? By seeing how these things behave over time, we might get more clues as to what they are.

Carole - Yes, absolutely. And that's why we have the spectroscopy on board as well. Just taking photographs isn't enough because obviously that third spatial dimension is also a time dimension. So as we go further out in cosmic distance, we go further back in time. And so by adding on the so-called Redshift, so where the galaxies are in, in space, but also measuring how the universe expands, putting all of that together across these vast areas is what really gives us that three dimensional time machine.

Will - We're very early on in this expedition. Have there been any standouts so far?

Carole - There have actually, I mean we began the survey in earnest in February of this year. We're already 4% of the way. We took one day of data back in the summer once we got through commissioning and we made sure that all of our instruments in our spacecraft were working well. And we've really given those images to scientists to start to mine them. And I think in terms of the dark energy dark matter question, we're really showing that the precise measurements that the cosmologists want to make will be feasible. How you can see in small dwarf galaxies, small star forming nurseries. On the other end of the spectrum, globular clusters, so ancient balls of stars, are probably the oldest structures in the universe. And we've been able to detect the lowest mass stars out to the edge of the globular cluster, but also resolve all the detail in the middle. So you can actually do the physics of dark matter within the globular clusters. And also we've got the exquisite sensitivity to detect what's called the intra cluster light. So these are individual stars that have been torn out of the galaxies in the galaxy cluster by the force of gravity. And they're also a tracer or a proxy for the gravitational potential and the hidden dark matter. So it's a treasure trove. And I think the scientific community is just starting to wake up to how they're going to have to transform the way they mine this data, the way they ask questions of astrophysical and astronomical data because it's really too much with too much detail and too much richness to be able just to gaze at the images. We all gazed at the images to start with. And then you start to think, 'wow, what's the physics you can do here?' So I think it will transform cosmology, but it will also transform modern astrophysics.


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