Exclusive Interview with Margot Brouwer

With Einstein’s theory of general relativity passing its 100th birthday, the new documentary, Chasing Einstein, challenges his equations as scientists from across the globe question the laws governing our universe. One of these scientists is Margot Brouwer, whose research on dark matter is bound to change the way in which we perceive the world around us. With an interest in cosmology from a young age and after completing her PhD in Leiden, Brouwer has since determinedly investigated the mystery surrounding dark matter and her ambition will be sure to inspire many curious generations to come.

To start off, what sparked your interest in researching cosmology, or specifically dark matter?

Ever since I was a child, I was interested in big philosophical questions from ‘Where do we come from?’ to ‘How did the world start and does it have an end?’. I already knew from when I was eight or nine years old that I wanted to become an astronomer. Then, after I had finished high school and started studying astronomy, I found that the field where these questions are answered is cosmology. While I think all cosmological questions are interesting, from the Big Bang to the question of multiple universes, you have to pick one sub-topic. The reason I chose dark matter is that it actually turns out to be one of the more tangible questions in cosmology, in the sense that you can really observe the effects of dark matter everywhere in the universe, in lots of different places, and that there’s really strong evidence for it. I like that it combines big questions like ‘What is the universe made of?’, which is really theoretical, but then has the observations to keep my feet on the ground and to show me that it’s not just a fantasy and that I’m working on something that is actually there.

As the XENON dark matter research project has been underway since 2002, how and when did you get involved with this project?

I don’t know how to break it, but I’m actually not involved with the XENON experiment, I’m doing something completely different (laughs). My role is doing astronomical observations, so I’m part of a big collaboration that uses the Very Large Survey Telescope in Chile. I use the data from the telescope to study the distribution of dark matter around galaxies in order to find out how dark matter behaves and hopefully this leads us to some answers as to what it is, what it consists of. Or it might be that there is no dark matter at all, rather there is something wrong with the way that we view gravity. That’s the whole enigma in the movie. The question is if there’s some “invisible” stuff out there, whether it’s dark matter that is causing this extra gravity, or if there’s something wrong with the equations of Einstein’s theory of general relativity. We know that Einstein’s equations do work very well on Earth and within our Solar System, yet the theory could be incomplete and on very large scales, like scales of galaxies, where gravity works in a different way. So those are the different theories that are discussed in the documentary.

My history of how I got into this is that I was doing my PhD in Leiden and used telescope observations to study the distribution of dark matter around galaxies. While I was doing my PhD research I heard from some old friends in Amsterdam, where I completed my undergrad, that one of the Professors, Erik Verlinde, who’s also in the movie, had a new theory of gravity. He didn’t believe that dark matter existed and that he could solve the problem with his new theory of gravity. I was, of course, quite skeptical! So I thought, okay, if I’m studying the distribution of dark matter around galaxies, and one of my old Professors says, “Well, dark matter does not actually exist”, then maybe I should go check it out (laughs). And at that moment it struck me that I could just as easily take his equation and test it on the observations that I was doing. So I didn’t really tell anyone about it, it was just like, “Let’s try it out”! When I actually compared my observations to his theory, it was like BAM, spot on. I looked at my computer and just thought, “Okay, maybe this guy has an interesting point-of-view”. And then I had to sort of come out to my colleagues saying “Hey, I tested this guy’s theory and it actually works quite well”. Some of my colleagues were, well, (laughs) not that supportive, although my PhD supervisor was. I came to him and said, “Okay, this is the case, what do I do now?” and he said, “Well, what do you think? Go tell this Erik Verlinde guy!”. I was, of course, super nervous because he was my old Professor and a big shot theoretical physicist and I was just a fresh PhD student. But then I just emailed him and showed him the result. He invited me to come over and yeah, it all sort of started.

As you also enjoy popular science writing, outreach and education, has this been your first participation in a documentary?

Well, this is definitely the biggest movie with me in it. When I was doing the research and my test of Erik Verlinde’s theory came out, there was also quite a lot of media attention, but that was mainly in the Netherlands. There were a few short interviews with me for television, magazines and some newspapers, so I’ve had some experience with that. But this movie is definitely the biggest media experience that I’ve had so far. It was really amazing to go to the premiere in Copenhagen to see myself on the big screen, and people coming up to me saying, “You were so amazing in the movie” or young girls who are doing science studies coming up to me and saying, “It was so inspirational for me to see a young woman in that role”. It was a really amazing experience.

Are you planning on participating in any more projects to further this aspiration?

Absolutely, this experience definitely drove me more in the direction of science communication and I would really like to be involved with that. I don’t have any films lined up, but I’m just doing small things here and there. Actually, for my current job, as I’ve finished my PhD and now work as a post-doc in Groningen, seventy-five percent of my contract is research, while the other twenty-five percent is being a Dutch ambassador and science communication officer for the Euclid telescope, which is the next big thing in terms of cosmology telescopes and will be launched in space end of 2022, or beginning of 2023. Many different European countries are building this telescope and contributing to it, so I’m trying to support the communication within the collaboration and also the communication to the general public. In Groningen, for instance, we have a big planetarium where I sometimes give shows for the public about this telescope and describe how we are going to study dark matter with it, so that’s quite a significant part of my job now which I really enjoy.

How did you react to the release of the first ‘image’ of the black hole, was it something you expected or a happy surprise?

The image of the black hole was definitely a big milestone in the astronomical community and we were all celebrating together on that day, all over the Netherlands, all over Europe and all over the world. What’s sort of funny, or what I find funny, is that even though we didn’t have an image of the black hole before, the existence of black holes was quite generally accepted in the science community.

Is this image clear evidence that the whole universe is filled with millions of black holes?

Well, Albert Einstein’s theory of general relativity was created around 1915, so it’s almost exactly hundred years after he created the concept of black holes, which is why it didn’t come as a surprise for the people in the astrophysics community. Also, over the past decades, we’ve found lots of indirect evidence of black holes by tracing the orbits of stars in the center of the Milky Way. We then do calculations that make it very probable that this object is indeed a black hole. As far as I know, most astrophysicists didn’t really question the existence of black holes, so I find it quite amusing that people who are not in the science community were so excited about this picture of the black hole because we’re so used to having indirect evidence. After all, ‘seeing’ in science can take on many different forms. But still, it’s a milestone that we can now make an image and of course it gives you this extra piece of evidence, making Einstein’s theory even stronger today.

Why are we sure that dark matter has to exist so that our galaxy functions in the way that it does?

Well, one of the first pieces of evidence for dark matter is how galaxies rotate. Actually, a female astronomer, Vera Rubin, who was studying lots and lots of galaxies and how they rotate, was one of the first people who discovered this problem. Basically, the more mass there is in the galaxy, the stronger the gravity that holds the stars in the galaxy together. So the gravity that holds the galaxy together has to balance the outward force, so the centrifugal force resulting from the rotation from the galaxy. If it rotates too fast, it is not strong enough to balance the centrifugal force, making all the stars in the galaxy just fly out in different directions. Like, when you have a kid on a merry-go-round and you spin it too fast, and it just flies out (laughs). We found that we need about five times more mass to balance that centrifugal force of galaxies than we can see with our telescope, so it became one of the first pieces of evidence that there was something going on. A Swiss astronomer, Fritz Zwicky, also did a similar study, but he was looking at groups of different galaxies that were rotating around each other and found the same result, namely that galaxies were moving around each other too fast.

 The Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, USA, in Chasing Einstein.

Assuming that dark matter exists, when do we expect to scientifically prove this? In 2,10, 20 or more years?

Mhm, when I think of in my field, so the astronomical research of dark matter, but also Cern and the XENON experiment, all three experiments are now at their maximum capacity. Every few years they have a shutdown period, where they work on the experiment and try to improve it. As far as I understand it, CERN wants to build quite a big upgrade called the High Luminosity LHC, which they expect to be working around 2026, so it’s still a few years away. There is, of course, the Euclid telescope that we are now building, which will hopefully be launched by the beginning of 2023, although it will then still take a year or so before we have collected a good amount of data. The same goes for the XENON experiment: you see in the movie that they have the last big release from the current run of the experiment and then, this year, it stopped taking data in order to prepare for the next big upgrade. To go to the next level will take a lot of effort and years of building, so I would say that in the next 2, 3, maybe even 5 years it will be quite unlikely that there is a big breakthrough. I mean, of course something could happen at any moment, in a way, but we’re now at a point where we’ve done everything that we can with these experiments. But then, once we’re at the next level, then I think we have quite a good chance of shedding a lot of light on this problem. We will give everything that we have in terms of technology that is available to us. If we don’t find dark matter then, well, we have to seriously reconsider our options in terms of whether there will be a next run. Of course there are different scenarios but in the end there’s only one truth, so either the dark matter particle exists and there is some way we can find it apart from the gravitational effect. Or, the dark matter particle does not exist and there’s some new theory of gravity that we can find or create. The darkest scenario, however, is that there is in fact a dark matter particle but there is just no other way to detect it except through gravity – you can just never find it no matter how hard you try. I think this scenario would be quite sad for the whole community and all the work we spent, but it’s just a reality we have to face and it is a possibility that we will never find the dark matter particle even if its exists. So there’s no way in which I can say, “Oh, we will have found it  10 years from now or 20 years from now, or even within my lifetime”. I would be quite pissed if the dark matter problem is still not solved when I die. I’d be like, “Come on, guys!” (laughs). 

Could dark matter be fragments of a black hole? Is there a clear correlation between dark matter and black holes?

When dark matter was first discovered, or when these problems first came to light, there were lots of different theories about what dark matter could be, and black holes were indeed one of the theories. Black holes are of course very difficult to see, but what you can do is this: if a black hole passes in front of something that is luminous, such as a star, then the gravity of the black hole acts as a lens for the light of the star. So the light of the star gets bent by the gravity of the black hole, which sort of focuses on your telescope. You see the star becoming brighter for a moment before fading again as the black hole passes between the Earth and the star. They did a lot of observations trying to see these brightening stars and over the past decade or so, they’ve made it very improbable through this method that black holes can solve the dark matter problem. Yeah, so the answer is no (laughs). Still, it’s quite funny that, because of popular media, when people come to me and ask, “What do you do?” and I say, “I study dark matter”,  then they say: “Oh, is that the same as black holes?”.  Then I’m like “Oh, not again” (laughs). Usually, I just say no. I guess it’s because of the words dark matter, black holes, dark and black… it’s actually kind of our fault that we don’t really give good names. I mean, it’s not even a good name because the matter is not dark, it’s not that we see something that’s dark – we see absolutely nothing. It’s just transparent matter, or invisible matter. I guess dark matter sounds cooler? We’re just completely in the dark about what it is, so it’s more of an intellectual darkness (laughs).

If dark matter does exist outside our solar system, would it attract or repel different solar systems?

The thing with dark matter is that it’s very diffuse, so looking at our Milky Way or other galaxies, we can observe the extra gravity as it is sort of shaped like a cloud around the galaxy. The cloud is more than ten times bigger than the galaxy itself, yet the distribution of dark matter is equally distributed around the galaxy. Since both solar systems are drifting in a homogenous sea of dark matter, they will not attract each other or repel each other – they’re just both floating around in the same sea of dark matter. On the other hand, when you zoom out and look at the dark matter floating around a galaxy, and then you have another galaxy with a dark matter cloud, that indeed causes these two galaxies to gravitationally attract each other. That’s also what you see when I talked about Fritz Zwicky’s observations of groups of galaxies that are rotating around each other. The attraction between these galaxies is much stronger than what you would expect from just the visible matter, and that’s caused by the dark matter. Also, you have our Milky Way and Andromeda galaxy which are attracting each other and might even collide at some point. The interaction between different galaxies is much stronger because of the dark matter. So the question is a good one, except not for solar systems but for galaxies!

CHASING EINSTEIN celebrated its premiere at CPH:DOX on Saturday March 23 and will have its UK Premiere at the Sci-Fi-London Film Festival on Sunday 19th May, 1pm at Stratford Picturehouse. For more information visit: www.chasingeinsteinfilm.com

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