This astronomer from the University of Edinburgh was the first to create a map of dark matter, that elusive and invisible material that makes up most of the universe.

Astronomer Catherine Heymans, on part of her dark matter map.

The things we can observe, like stars or planets, only make up 4.9% of the entire universe. The rest is divided between dark matter and dark energy. We cannot see or measure it, but physicists are convinced that it plays a role in the formation and location of galaxies.

One of the main recent contributions to understanding the dark universe was the map that two young astronomers, Meghan Gray and Catherine Heymans, began to make almost a decade ago. What they did was observe images of more than 60,000 galaxies located thousands of light years from Earth. The light emitted by these galaxies was captured by the Hubble Space Telescope, and it arrived curved due to the effect of gravity caused by the dark matter that surrounded them.

Illustration of a dark matter map.

Illustration of a dark matter map.
NASA

The result of that map of the dark universe, presented in 2012, was something like a set of neurons, dendrites and synapses, and revealed something surprising: dark matter. formed a kind of scaffolding that supported the visible universe. Since then, Heymans and his team have focused on using new telescopes, new detectors and, above all, new cosmological theories to further advance our understanding of the most mysterious part of the universe, which is its vast majority.

Astrophysics has passed through Madrid this week to delve from the BBVA Foundation on the need for a “new physics” in order to continue offering answers to these questions.

What questions were you trying to answer when you created that first map of dark matter in the universe?

Five years ago we did this project and we were the first to produce a large-scale map of dark matter for the first time. What we found met our expectations, we saw that dark matter commands when and where galaxies form in our universe. Dark matter is completely invisible, you cannot see it and the only way to know if it is there is by the effect it has on the things you can see. We spent five years making deep observations of four pieces of the sky and focusing on ten million galaxies we were able to create this map of dark matter, thanks to the effect it had on the light emitted by these very distant galaxies. It met our expectations but was actually just a very small piece of heaven, so we set out to make it bigger and better.

I find it very interesting to find that dark matter has a kind of… I don’t know whether to call it “structure”, but that it is not randomly distributed.

Yes, it is like a giant cosmic network, it has dense nodes which is where you find thousands of galaxies and then a kind of sparse filaments that connect some nodes with others. You can almost imagine it as a road map, where you find cities, which would be these nodes, and the filaments as roads that connect some cities with others. We were the first to see that dark matter formed such shapes and structures.

It is a very good metaphor. And from that point to now, how has that map improved?

In science there are always two paths you can take: you can use better instruments – telescopes or bigger detectors – or you can advance theories, theoretical knowledge, to explain what you are observing. We, our groups, have gone both ways. We have a wonderful new telescope on Cerro Paranal in Chile, the VST, belonging to the European Southern Observatory. Whenever the weather is perfect, we look at two pieces of the sky, one in the north and one in the south. Each piece has a size like from the elbow to the wrist, if you bend your arm looking at the sky. But when we finish the observations, the map will be ten times bigger than the last one we made. That is giving us wider and deeper maps of dark matter, and it is allowing us to confront many different theories about the dark universe. Why is dark matter there, why does it behave like this? What about that mysterious dark energy that affects how that cosmic web appears every time we look? And then we have different theories about the origin of matter and dark energy. And with this larger set of data we can confirm whether these theories are correct or not.

Do you think that we are reaching a technological limit, that we need new devices to be able to answer certain questions about the cosmos?

Right now, I think all the major advances in science come from an advance in technology, and they really go hand in hand. We have a very important question for science, and that is that we do not know what makes up 95% of our universe. Knowing the answer would be a huge advance for science, so this question is driving technology forward to build the instruments we need to make observations.

In science there are always two paths you can take: you can use better instruments or you can advance theories.

We are currently preparing for two new projects, in one of which Spain will play a fundamental role: the Euclid will be launched in three years and is a telescope that reaches above the atmosphere taking images of the entire night sky. It’s going to be an unprecedented view of the universe. At the same time, on the ground we will have a new telescope called LSST that is going to be mapping the entire night sky for ten years. It will be a set of exquisite observations and, although they are competitors, the sum of these two instruments will allow us to demonstrate theories that go as far as questioning the theory of gravity itself, these new instruments will allow us to take the knowledge much further. In short, I do not believe that technology limits us but that science is taking technology further.

When we think of “dark matter” we think of something homogeneous, as composed all of the same, but this also happened centuries ago with matter and that dark universe may actually be something different. Do you think in the future could we reveal this unknown?

There are many different theories as to what that “dark matter particle” could be and you are correct that it could be a collection of different particles. Currently, the main candidate is the so-called Supersymmetry Theory, which says that every particle that exists according to the Standard Model has a supersymmetric pair, so there would be a lot of dark matter particles. Currently, the problem with this theory is that, in its simplest scenario, we expected to have found some of these particles in the Large Hadron Collider at CERN, and we have not seen them yet. I’m not saying they won’t show up, but people are starting to get nervous. [Risas] They were hoping they had seen them already, so the simplest explanation may not be the correct one after all. It would be something very exciting, on the other hand …

Exciting but troublesome.

When the Higgs boson was found it was something fantastic, an incredible advance and we all thought that next would come the discovery of all these new particles, and that has not happened … for now. There is a throttle upgrade that will take place in the next two years, it will increase the energy and they may appear then.

Can gravitational waves and their study offer answers about dark matter?

Absolutely. The discovery of gravitational waves was an achievement of engineering and technology and also demonstrates a key aspect of Einstein’s theory of general relativity, which precisely said that these waves existed. One of the momentous theories that we physicists are trying to prove right now is that of gravity itself, and gravitational waves are one of the tools we have for this. However, that’s only part of the theory so what we’re doing right now is combining all the different tests out there in the universe, seeing how many different ways we can test gravity, and gravitational waves are one of the parts, like the gravitational lenses that we have built to make these maps.

Heymans, posing in front of a galaxy.

Heymans, posing in front of a galaxy.
FBBVA

In the past, what was the way to study dark matter?

Dark matter has the problem that it does not interact with the electromagnetic spectrum, neither emits light nor absorbs it. So if you want to detect these particles you have to do it with its gravitational pulsar, there is a lot of dark matter that can be seen by the effects they have on gravity, or if you are a particle physicist you can observe the collisions: they have huge tanks under the ground full of chemicals and wait for some dark matter particle to collide with something, something that happens very rarely, and release a little energy that these scientists can detect. In this room right now, there are between one and a billion dark matter particles passing through your big toe every second. But it may take three to four hours for them to collide with one of their atoms, but this is because these particles are so small compared to what we are made of that they only collide with them occasionally.

How did you become interested in starting to study all of this?

I love great questions! I like to know why. I am an eternal three-year-old girl. Why, why, why That’s me. And this is one of the big questions in science today, we only understand 5% of the universe. It’s shocking! Of course, we understand that 5% very well and we can explain things in our daily lives well, but there is so much to understand in the universe. Just calculating how much dark matter there is is already a great achievement, but where did it come from, why is it there … it is disturbing not to know anything, but at the same time it is very exciting, because it means that there are pieces that are missing in our understanding of the universe, and if they are missing … you have to find them!

And has what we know about dark matter changed a lot from your college years to now?

The zoo of different theories and models that try to explain what we have seen has grown a lot. Do we need a new Einstein? Not necessarily, we already have thousands and thousands out there coming up with new theories. What we really need is the data to confront these theories and thus progress.