Ian Sample speaks to the cosmologist Dr Andrew Pontzen about the European Space Agency’s Euclid mission, which hopes to uncover more about two of the universe’s most baffling components: dark energy and dark matter. Pontzen explains what the probe will be looking for and how its findings will contribute to our understanding of the structure and evolution of the cosmos. Help support our independent journalism at theguardian.com/sciencepod

The trillions of stars and billions of galaxies that light up our telescopes – that's just

Scientists have some pretty good theories though.

Our cosmic home appears to be composed of 3 things normal matter, all the stuff we can see, dark matter and dark energy.

Dark matter it's the most mysterious substance in the universe little is known about it scientists are pretty sure it exists but still don't know what it is.

Astrophysicists don't know what dark matter is made of, but they do know there's a lot of it in the universe.

No 1 has observed dark matter or dark energy directly, or knows what they are.

But we have seen the forces they exert on the Universe, shaping its evolution, creating vast structures and bending light.

So, At 4.11pm British summer time this Saturday in French Guiana, European Space Agency scientists are scheduled to launch a new billion-dollar spacecraft, Euclid, which will map the dark universe and probe its expansion over the last 10 billion years, hopefully providing data that can begin to unravel the mystery and answer 1 of science's most fundamental questions, what is our universe really made from?

I'm the Guardian's Science Editor, Ian Sample, And this is Science Weekly.

Andrew Ponson, you're a professor of cosmology at University College London and you've recently written a book, The Universe in a Box, A New Cosmic History, which dives into how scientists use computer simulations to explore vast cosmic questions.

But of course, the test theories and simulations, you have to compare them to real world data.

And that's where this new spacecraft, Euclid, comes in.

Where's it going and what's it going to be looking at?

Richard Williams The idea of the Euclid spacecraft is that it's going to be looking at the whole sky, trying to map it out, looking to find quite distant galaxies in our universe to kind of use them as tracers of what else is out there.

The central goal for Euclid is to figure out more about dark energy, which is this kind of mysterious force that seems to be pushing apart our universe, about which we know very little other than the fact that it's there.

It's also going to be looking at dark matter.

I mean, just give me a little bit more on these 2 things, dark energy and dark matter.

We'll dive into them in detail later, but what are they in essence?

They're not dark in the sense that you might imagine they're creating shadows or sort of blocking out our view of the cosmos or anything like that.

They are invisible or transparent and they seem to be sculpting the universe in different ways.

So dark matter is the name we give to something which seems to be pulling stuff together.

It's extra stuff, it's got mass and therefore it's creating actually a large amount of gravity that seems to be creating galaxies and sculpting those galaxies and pulling them into what we call the cosmic web, which is sort of an overarching pattern for the galaxies in our universe.

Whereas dark energy appears to be then pushing that apart.

So we've got these sort of almost competing forces.

So let's dig into how this mission is going to explore these ideas.

It's being sent 1.5 million kilometres from Earth in the opposite direction to the Sun and on board it will have a 1.2 metre telescope collecting light which will be split into visible and infrared and channeled to 2 instruments a visible light camera and a near-infrared spectrometer and using these it will study the evolution of the universe on this vast scale.

Now you've already mentioned the cosmic web, but say a bit more about that.

What is it and what can it tell us about dark matter and dark energy?

Hitchcock The cosmic web is the name that we give to the largest structures that are in the universe.

And I mean, these really are so big that even for a cosmologist like me, it can be hard to get your head around what we're talking about.

We are talking about things on the scale of hundreds of millions of light years, far, far, far bigger than any individual galaxy.

And look, galaxies are already vast structures.

So, the cosmic web is that sort of organisation of the galaxies in the universe.

And the name is given to it because it looks almost like a cobweb.

If you sort of zoom out, then you find that the galaxies are arranged in this way that is very reminiscent of a sort of tangly web with sheets, filaments, and there are also large voids.

They're just sort of empty regions of space where there aren't very many galaxies at all.

So if you can map this out in a lot of detail, and especially if you can map it out as Euclid will, over time, by peering a large distance into our cosmos because the light's just taken so long to reach you, once you start to do that you really get a handle on how has our universe been sculpted, how has it changed over time.

That in turn starts revealing the physics at the heart of the universe's creation and its evolution.

That physics is very intimately tied up with dark matter and dark energy.

1 of the other main goals of Euclid is to accurately map galaxy redshift.

Tell us what that is and what we might learn from studying that.

The redshift is 1 of the primary ways we have to turn a picture of the night sky, which is a sort of 2D thing, into something more like a 3D map that has depth.

That's absolutely crucial if you want to do the kind of work that we're talking about.

You don't really understand the structure that you're looking at.

And redshift is the primary way that we do that.

It's based around something very much like the Doppler shift.

So if you stand and an ambulance passes you, then famously the pitch of the siren is going to change as it goes off into the distance.

You're hearing that siren lower than its true pitch.

And in the universe, something very similar happens, but with light.

So if you are looking at something that's receding away from you quite quickly, then its light waves will have their frequency shifted.

And the way that we perceive that is a shift in their color.

That's where the idea of redshift comes from.

And the sort of final part of this jigsaw I suppose is that the universe is expanding.

What that means is that as you look at things that are further and further away they are receding from us faster and faster.

And therefore, the extent of the reddening of a galaxy is very closely tied to how far away it is.

Now you have all of the pieces to create a 3D map rather than just a 2D image.

So as well as getting some sense of the distance from that redshift, those measurements will also give us some idea of how the universe, the galaxy has evolved over time.

I mean, the further away you look, the greater the redshift, but also the further you're looking back in time, just because the light has taken that long to reach you.

Dark energy in particular changes over time.

Actually in the early universe, its effect was very modest, mainly because the earlier universe was a lot denser and so dark energy's effect wasn't felt so keenly.

So when you look at how things have changed over time, you're quite directly getting to see what is dark energy doing because it's over time that its effect starts to kick in.

And so reconstructing that history definitely tells you a lot more about dark energy.

MA2 Other spacecraft and surveys have previously looked at similar things.

I'm wondering what Euclid is doing that sort of hasn't been done before?

I mean, how could it add to the information that we already have about dark energy, dark matter?

1 of the primary targets of Euclid is an effect called gravitational lensing.

So, gravitational lensing is just a statement of the fact that light, as it makes its way through the cosmos, is tugged around slightly by gravity in just the same way that, you know, if I drop something here on Earth, its path is determined by the gravitational force of the Earth.

Actually light is also subject to those gravitational forces.

But light is zipping so fast that the effect of gravity is very, very subtle.

And if you can measure these slight deflections of the light, then you get a sense of what is gravity doing in the universe.

Now because dark matter has such a sort of overwhelming amount of mass associated with it, it therefore has a huge effect on gravity in the universe and that means that by measuring this gravitational lensing effect you are able to reconstruct really what the dark matter is doing.

So Euclid is specifically designed to go after that effect.

It has absolutely beautiful optics and it's also sort of tuned to pick up infrared light which is particularly good for getting light from the distant universe, which has therefore passed through a lot of the universe and picked up a lot of these distortions along the way.

Let's talk a bit about dark matter and dark energy.

I mean, first of all, with dark matter, I don't know if you can give me a sort of a potted history of how dark matter came about within the sort of scientific astronomical community?

I mean, when did scientists really first realise that something was off with their observations if the universe was just made out of normal matter?

The first sort of really quite convincing evidence started to appear in the 1970s and it came from what we call galaxy rotation curves.

Now what they are is a measure of how fast galaxies are spinning.

Hundreds of kilometres per second is the kind of movements that we're talking about.

It doesn't appear to be moving very much because the distances involved are also huge.

And the thing that got people really thinking was the fact that those movements appeared to be quite fast, even far into the outer reaches of galaxies.

And that seemed quite surprising because gravity is the force that is giving rise to that spin.

And if you go far into the outskirts of a galaxy, gravity should be becoming very weak and that should mean that things would be moving quite slowly out there in the outskirts of the galaxy.

So Vera Rubin was an astronomer who became famous for making this point and taking really exquisite observations that demonstrated that this is a real issue.

In the 1980s, people kind of took the idea and ran with it and they said, well, if it's true that there's extra material out there, then it should have other consequences as well.

And this is where computer simulations come to the fore, because it's very hard to figure that out just with pencil and paper.

But if you instruct a computer to start simulating a universe, trying to calculate what would a universe be like if it had all of this dark matter, this extra stuff, it's very massive, got a big tug of gravity in it, then the computer can follow that through.

And that's where predictions started to come out.

ROGER You're saying that we don't know what dark matter is, but are there sort of leading theories as to what it might be?

We're at a very interesting stage of this search.

I think, you know, if you'd asked this 5 years ago, we were fairly confident it's probably something known as a WIMP, a weakly interacting massive particle.

Things have changed a bit in recent years.

It seems like searches for these particles, they're not turning anything up.

And so now we're at this interesting stage where it seems like it's wide open again.

Without getting into all the details of all these possible ideas, they're just different particles that would have subtly different effects on the universe.

So it's actually quite an exciting time for this whole field.

In some sense, counterpoint to dark matter, this repulsive energy pushing the universe apart.

SIMON The idea that there would be something pushing as well as gravity pulling, you can trace that all the way back to Newton, in fact, who, as well as writing down his sort of famous theory about the way gravitation works, he also said it's equally possible that out in space there may be a sort of subtle push as well as an overall pull.

And then you can trace that idea through Einstein, who also had a sort of similar set of ideas.

The evidence that such a push actually happens, first of all, came actually from the cosmic web.

Looking at the way that galaxies are strung through the universe and comparing that to simulations of how they ought to be spread through the universe, it became clear as people made better and better maps that it wasn't quite right.

In some sense, the structures that they were seeing were a bit too big, a bit too strung out.

So then in the early 90s, people started speculating, well, you know, what about if there's this push?

This would start pushing things apart in the universe and kind of make any structures that you have blow up to larger scales.

And then the deal was really sealed with the Hubble Space Telescope, which in 1998, 2 teams using the Hubble Space Telescope used supernovae to quite directly map out the expansion of the universe and quite directly show our universe is not just expanding, but that expansion is speeding up.

Yeah, I mean, at that point, you really have to believe there is something pushing our universe apart.

What's quite incredible in a sense about these 2 things is that they're essentially invented by scientists to explain these observations.

Yet now the evidence is building for those things being true, for being out there and active.

But I mean, how convinced are researchers that both of these things are as we imagine them to be?

Yeah, there are plenty of scientists out there who are trying to think beyond dark matter and dark energy and think about can we just imagine this from sort of broader perspective and say well we made up these these materials but maybe they're not real, maybe that maybe they're standing in for something else, some other law of nature that we as yet just don't understand.

And that's definitely a possibility and I think that's important work that's going on.

But those ideas, despite, as you say, being made up, they've proved to be extremely predictive.

So they've been very good guides to what we should discover in the universe.

That doesn't literally make them true, but it does mean that even if they're not literally true, even if something comes along and ultimately supplants them and we go, oh yeah, you know, that just wasn't quite the right idea.

It's not going to be so different from the dark matter, dark energy picture that it was a waste of time.

With so many of these mysteries to solve and questions to answer, we've no idea what most of the universe is made from.

Could there be some kind of revolution coming along here in our understanding of the universe?

I mean, I think revolutions in science are notoriously very hard to predict.

I think what we can say for sure is that Euclid is adding to a very rich landscape.

In the coming years, we are going to be inundated with data and Euclid will be part of that.

There are other things like the Vera Rubin Observatory that are going to be playing a large part in that and that will be coming online I believe next year.

You know where you can be certain that revolutions are coming is in our understanding of things like black holes, which we haven't spoken about much, but they also play a very large part and a sort of growing part in our understanding of the universe.

We know very little about where black holes came from, especially the big black holes that are at the centers of galaxies.

That I think we will definitely learn a lot new in the coming 10 or 15 years.

Thanks to Andrew Ponson and what a prediction from him.

Today you'll be able to read my colleague Hannah Devlin's article on the first evidence of ultra-low frequency gravitational waves from supermassive black holes, which have never been seen before and may hold answers about some of the questions Andrew and I were discussing.

You can find that and all our coverage of the Euclid mission at theguardian.com.

The producer was Madeline Finlay, the sound design was by Tony Onochukwu and the executive producer was Ellie Burey.


Speaker 0: This is The Guardian.

Speaker 1: The universe isn't what it seems. The trillions of stars and billions of galaxies that light up our telescopes – that's just

Speaker 0: 5%

Speaker 1: of what's really out there. The rest? We don't know. Scientists have some pretty good theories though. Our cosmic home appears to be composed of 3 things normal matter, all the stuff we can see, dark matter and dark energy.

Speaker 2: Dark matter it's the most mysterious substance in the universe little is known about it scientists are pretty sure it exists but still don't know what it is.

Speaker 3: Astrophysicists don't know what dark matter is made of, but they do know there's a lot of it in the universe.

Speaker 1: No 1 has observed dark matter or dark energy directly, or knows what they are. But we have seen the forces they exert on the Universe, shaping its evolution, creating vast structures and bending light. So, At 4.11pm British summer time this Saturday in French Guiana, European Space Agency scientists are scheduled to launch a new billion-dollar spacecraft, Euclid, which will map the dark universe and probe its expansion over the last 10 billion years, hopefully providing data that can begin to unravel the mystery and answer 1 of science's most fundamental questions, what is our universe really made from? I'm the Guardian's Science Editor, Ian Sample, And this is Science Weekly. Andrew Ponson, you're a professor of cosmology at University College London and you've recently written a book, The Universe in a Box, A New Cosmic History, which dives into how scientists use computer simulations to explore vast cosmic questions.

But of course, the test theories and simulations, you have to compare them to real world data. And that's where this new spacecraft, Euclid, comes in. Give me the basics of this mission. Where's it going and what's it going to be looking at?

Speaker 0: Dr. Richard Williams The idea of the Euclid spacecraft is that it's going to be looking at the whole sky, trying to map it out, looking to find quite distant galaxies in our universe to kind of use them as tracers of what else is out there. The central goal for Euclid is to figure out more about dark energy, which is this kind of mysterious force that seems to be pushing apart our universe, about which we know very little other than the fact that it's there.

Speaker 1: It's also going to be looking at dark matter. I mean, just give me a little bit more on these 2 things, dark energy and dark matter. We'll dive into them in detail later, but what are they in essence?

Speaker 0: They are invisible substances. So dark is a bit of a misnomer. They're not dark in the sense that you might imagine they're creating shadows or sort of blocking out our view of the cosmos or anything like that. They are invisible or transparent and they seem to be sculpting the universe in different ways. So dark matter is the name we give to something which seems to be pulling stuff together.

It's extra stuff, it's got mass and therefore it's creating actually a large amount of gravity that seems to be creating galaxies and sculpting those galaxies and pulling them into what we call the cosmic web, which is sort of an overarching pattern for the galaxies in our universe. Whereas dark energy appears to be then pushing that apart. So we've got these sort of almost competing forces.

Speaker 1: So let's dig into how this mission is going to explore these ideas. It's being sent 1.5 million kilometres from Earth in the opposite direction to the Sun and on board it will have a 1.2 metre telescope collecting light which will be split into visible and infrared and channeled to 2 instruments a visible light camera and a near-infrared spectrometer and using these it will study the evolution of the universe on this vast scale. Now you've already mentioned the cosmic web, but say a bit more about that. What is it and what can it tell us about dark matter and dark energy?

Speaker 0: Dr. Richard D. Hitchcock The cosmic web is the name that we give to the largest structures that are in the universe. And I mean, these really are so big that even for a cosmologist like me, it can be hard to get your head around what we're talking about. We are talking about things on the scale of hundreds of millions of light years, far, far, far bigger than any individual galaxy.

And look, galaxies are already vast structures. So, the cosmic web is that sort of organisation of the galaxies in the universe. And the name is given to it because it looks almost like a cobweb. If you sort of zoom out, then you find that the galaxies are arranged in this way that is very reminiscent of a sort of tangly web with sheets, filaments, and there are also large voids. They're just sort of empty regions of space where there aren't very many galaxies at all.

So if you can map this out in a lot of detail, and especially if you can map it out as Euclid will, over time, by peering a large distance into our cosmos because the light's just taken so long to reach you, once you start to do that you really get a handle on how has our universe been sculpted, how has it changed over time. That in turn starts revealing the physics at the heart of the universe's creation and its evolution. That physics is very intimately tied up with dark matter and dark energy.

Speaker 1: 1 of the other main goals of Euclid is to accurately map galaxy redshift. Tell us what that is and what we might learn from studying that.

Speaker 0: The redshift is 1 of the primary ways we have to turn a picture of the night sky, which is a sort of 2D thing, into something more like a 3D map that has depth. That's absolutely crucial if you want to do the kind of work that we're talking about. A 2D picture is almost useless to you. You don't really understand the structure that you're looking at. So you need to get that third dimension.

And redshift is the primary way that we do that. It's based around something very much like the Doppler shift. So if you stand and an ambulance passes you, then famously the pitch of the siren is going to change as it goes off into the distance. You're hearing that siren lower than its true pitch. And in the universe, something very similar happens, but with light.

So if you are looking at something that's receding away from you quite quickly, then its light waves will have their frequency shifted. And the way that we perceive that is a shift in their color. That's where the idea of redshift comes from. And the sort of final part of this jigsaw I suppose is that the universe is expanding. What that means is that as you look at things that are further and further away they are receding from us faster and faster.

And therefore, the extent of the reddening of a galaxy is very closely tied to how far away it is. And so there you are. Now you have all of the pieces to create a 3D map rather than just a 2D image.

Speaker 1: So as well as getting some sense of the distance from that redshift, those measurements will also give us some idea of how the universe, the galaxy has evolved over time.

Speaker 0: Yeah, that's absolutely right. I mean, the further away you look, the greater the redshift, but also the further you're looking back in time, just because the light has taken that long to reach you. Dark energy in particular changes over time. Actually in the early universe, its effect was very modest, mainly because the earlier universe was a lot denser and so dark energy's effect wasn't felt so keenly. So when you look at how things have changed over time, you're quite directly getting to see what is dark energy doing because it's over time that its effect starts to kick in.

And so reconstructing that history definitely tells you a lot more about dark energy.

Speaker 1: MA2 Other spacecraft and surveys have previously looked at similar things. I'm wondering what Euclid is doing that sort of hasn't been done before? I mean, how could it add to the information that we already have about dark energy, dark matter?

Speaker 0: 1 of the primary targets of Euclid is an effect called gravitational lensing. So, gravitational lensing is just a statement of the fact that light, as it makes its way through the cosmos, is tugged around slightly by gravity in just the same way that, you know, if I drop something here on Earth, its path is determined by the gravitational force of the Earth. Actually light is also subject to those gravitational forces. But light is zipping so fast that the effect of gravity is very, very subtle. And if you can measure these slight deflections of the light, then you get a sense of what is gravity doing in the universe.

Now because dark matter has such a sort of overwhelming amount of mass associated with it, it therefore has a huge effect on gravity in the universe and that means that by measuring this gravitational lensing effect you are able to reconstruct really what the dark matter is doing. So Euclid is specifically designed to go after that effect. It has absolutely beautiful optics and it's also sort of tuned to pick up infrared light which is particularly good for getting light from the distant universe, which has therefore passed through a lot of the universe and picked up a lot of these distortions along the way.

Speaker 1: Let's talk a bit about dark matter and dark energy. I mean, first of all, with dark matter, I don't know if you can give me a sort of a potted history of how dark matter came about within the sort of scientific astronomical community? I mean, when did scientists really first realise that something was off with their observations if the universe was just made out of normal matter?

Speaker 0: The first sort of really quite convincing evidence started to appear in the 1970s and it came from what we call galaxy rotation curves. Now what they are is a measure of how fast galaxies are spinning. Hundreds of kilometres per second is the kind of movements that we're talking about. It doesn't appear to be moving very much because the distances involved are also huge. And the thing that got people really thinking was the fact that those movements appeared to be quite fast, even far into the outer reaches of galaxies.

And that seemed quite surprising because gravity is the force that is giving rise to that spin. And if you go far into the outskirts of a galaxy, gravity should be becoming very weak and that should mean that things would be moving quite slowly out there in the outskirts of the galaxy. So Vera Rubin was an astronomer who became famous for making this point and taking really exquisite observations that demonstrated that this is a real issue. In the 1980s, people kind of took the idea and ran with it and they said, well, if it's true that there's extra material out there, then it should have other consequences as well. And this is where computer simulations come to the fore, because it's very hard to figure that out just with pencil and paper.

But if you instruct a computer to start simulating a universe, trying to calculate what would a universe be like if it had all of this dark matter, this extra stuff, it's very massive, got a big tug of gravity in it, then the computer can follow that through. And that's where predictions started to come out.

Speaker 1: ROGER You're saying that we don't know what dark matter is, but are there sort of leading theories as to what it might be?

Speaker 0: Dr Matthew Fletcher I mean, there were. We're at a very interesting stage of this search. I think, you know, if you'd asked this 5 years ago, we were fairly confident it's probably something known as a WIMP, a weakly interacting massive particle. Things have changed a bit in recent years. It seems like searches for these particles, they're not turning anything up.

And so now we're at this interesting stage where it seems like it's wide open again. Without getting into all the details of all these possible ideas, they're just different particles that would have subtly different effects on the universe. So it's actually quite an exciting time for this whole field.

Speaker 1: Let's get on to dark energy. In some sense, counterpoint to dark matter, this repulsive energy pushing the universe apart. Where did the idea for that come from?

Speaker 0: SIMON The idea that there would be something pushing as well as gravity pulling, you can trace that all the way back to Newton, in fact, who, as well as writing down his sort of famous theory about the way gravitation works, he also said it's equally possible that out in space there may be a sort of subtle push as well as an overall pull. And then you can trace that idea through Einstein, who also had a sort of similar set of ideas. The evidence that such a push actually happens, first of all, came actually from the cosmic web. Looking at the way that galaxies are strung through the universe and comparing that to simulations of how they ought to be spread through the universe, it became clear as people made better and better maps that it wasn't quite right. In some sense, the structures that they were seeing were a bit too big, a bit too strung out.

So then in the early 90s, people started speculating, well, you know, what about if there's this push? This would start pushing things apart in the universe and kind of make any structures that you have blow up to larger scales. And then the deal was really sealed with the Hubble Space Telescope, which in 1998, 2 teams using the Hubble Space Telescope used supernovae to quite directly map out the expansion of the universe and quite directly show our universe is not just expanding, but that expansion is speeding up. Yeah, I mean, at that point, you really have to believe there is something pushing our universe apart. That's what we call dark energy.

Speaker 1: What's quite incredible in a sense about these 2 things is that they're essentially invented by scientists to explain these observations. Yet now the evidence is building for those things being true, for being out there and active. But I mean, how convinced are researchers that both of these things are as we imagine them to be?

Speaker 0: Yeah, there are plenty of scientists out there who are trying to think beyond dark matter and dark energy and think about can we just imagine this from sort of broader perspective and say well we made up these these materials but maybe they're not real, maybe that maybe they're standing in for something else, some other law of nature that we as yet just don't understand. And that's definitely a possibility and I think that's important work that's going on. But those ideas, despite, as you say, being made up, they've proved to be extremely predictive. So they've been very good guides to what we should discover in the universe. That doesn't literally make them true, but it does mean that even if they're not literally true, even if something comes along and ultimately supplants them and we go, oh yeah, you know, that just wasn't quite the right idea.

It's not going to be so different from the dark matter, dark energy picture that it was a waste of time. We're definitely on the right lines.

Speaker 1: With so many of these mysteries to solve and questions to answer, we've no idea what most of the universe is made from. Could there be some kind of revolution coming along here in our understanding of the universe?

Speaker 0: I hope so. I mean, I think revolutions in science are notoriously very hard to predict. I think what we can say for sure is that Euclid is adding to a very rich landscape. In the coming years, we are going to be inundated with data and Euclid will be part of that. There are other things like the Vera Rubin Observatory that are going to be playing a large part in that and that will be coming online I believe next year.

You know where you can be certain that revolutions are coming is in our understanding of things like black holes, which we haven't spoken about much, but they also play a very large part and a sort of growing part in our understanding of the universe. And that's wide open at the moment. We know very little about where black holes came from, especially the big black holes that are at the centers of galaxies. That I think we will definitely learn a lot new in the coming 10 or 15 years.

Speaker 1: Thanks to Andrew Ponson and what a prediction from him. Today you'll be able to read my colleague Hannah Devlin's article on the first evidence of ultra-low frequency gravitational waves from supermassive black holes, which have never been seen before and may hold answers about some of the questions Andrew and I were discussing. You can find that and all our coverage of the Euclid mission at theguardian.com. And that's it for today. The producer was Madeline Finlay, the sound design was by Tony Onochukwu and the executive producer was Ellie Burey.

We'll be back on Tuesday. See you then.

Speaker 0: This is The Guardian.

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Euclid: will the mission uncover the secrets of dark matter and dark energy?