Space, a Few Years Later*

* Edited 29.05.2023: Eight years after our first interview, in Space and Henk’s work a lot and a little has changed. Here you can read his current stance on the future of cosmology, the tech giants’ space ambitions and the place of humans in Space (yes, they are still not built for that).

A few years ago, you shared that the accelerating expansion of the Universe to dark energy is probably the most significant finding in cosmology. Still, we need to be on the right track to explain it. What do you think now?

Little has changed since then. We still do not have a good explanation why the expansion of the Universe is accelerating. The only way forward, for now at least, is to collect vast amounts of data about the distribution of galaxies and dark matter in the Universe. The launch of Euclid, which is an ESA satellite to study the nature of the dark Universe, will be big step in this direction. One interesting recent result is that there is some disagreement how fast the Universe is currently expanding – direct measurements in the local Universe differ slightly from what is inferred from measurements from the afterglow of the big bang. This may be a first sign of something new to be discovered, but there may very well be a problem with the data. These are difficult measurements to do, but Euclid can quickly shed light on this. Hence, the coming years are going to be exciting!

Using data from the Hubble Space Telescope and ground-based eyes on the sky, the astronomers finally detected a stellar-mass black hole 5,150 light-years away from Earth. It likely isn’t the only one in the Cosmos?

We already knew for some time that very massive black holes exist in the central regions of galaxies, including on in our own Milky Way. In fact, last year a direct image of this black hole was published. It is supermassive – four million times more massive than the Sun. That is a lot, but we also know that black holes exist that are a thousand times more massive. Other evidence for the existence of black holes comes from the ripples in the fabric of space, so-called gravitational waves, that are created when a pair of black holes comes together. This was first detected in 2015, but since then many more events have been seen.

The discovery you are referring to is a single stellar mass black hole, which is much more difficult to spot, because it is invisible. Fortunately, Einstein’s theory of gravity shows how mass distorts the space around it. One consequence is that light from a more distant star gets enhanced as the light is bent around the black hole. This is how this black hole was discovered. People have been searching for this for a long time as a possible source of dark matter. If that were the case, we would see many more black holes. So dark matter is something different, but this discovery is still important. This is just the beginning and more black holes will be discovered. These will provide a census of these objects and help us to understand how they formed.

Dark energy, dark matter, black holes – Space seems pretty dark. How can you explain the three discoveries to a broader audience?

The study of the Universe is humbling and the fascination for the Cosmos and its origins goes back a long time. For most of history, however, people projected their experiences to create their ideal models of the Universe. Advances in science, in particular in the 20th century, have resulted in a very different Universe. Some might even say the discoveries have been surprising, but there is no reason why the Universe as a whole should be anything like what we experience in our daily lives.

Black holes are perhaps the easiest to explain, because they are a consequence of Einstein’s theory of gravity: if enough mass is compressed into a small volume, space is distorted so much that even light can no longer escape its gravitational pull. We do not fully understand what happens inside or at the boundary of the black hole, but these remarkable objects were once predicted and we now see that they actually do exist.

Dark matter is already more difficult to explain, because the evidence is purely empirical. When we look at the motion of stars in the outskirts of galaxies, we see that they move too fast. We can predict how fast they should move based on the amount of starlight we observe, because their motion should be such that it balances gravity. The fact that they move much faster means something must be causing extra gravity. Similarly, we can see how the mass of galaxies distorts the space around them, and again we see that the amount of gravity is too large. We think that this extra gravity is caused by a massive halo of dark matter that surrounds each galaxy, but we do not know what the dark matter itself is. The leading idea is that is a new fundamental form of matter, that is literally invisible. Experiments are trying to detect the dark matter particles, but so far without success.Dark energy is the most difficult of all. Since 1998 we know that the expansion of the Universe is accelerating. This discovery came as a real surprise, and we have no good idea what is causing this. Each explanation creates new problems. A possibility is that it is related to fundamental properties of empty space, the vacuum: it may contain some form of “energy”, which is where the term “dark energy” came from. It is by no means a good description of the physics, but is catchy and the name stuck. But we are really in the dark about its nature. It is one of the biggest problems in contemporary physics together with the nature of dark matter.

You have been working on the Euclid satellite project for years. Tell us more about your role in the ESA mission, and are you ready to start in 2023?

Euclid is a large, mostly European project, involving about 2000 people, from engineers to scientists. My role is to coordinate the research on the nature of dark matter and dark energy using weak gravitational lensing. As mentioned earlier, a massive object distorts the space around it. The dark matter around galaxies does the same, resulting in a small change in the shapes of galaxies. Euclid is designed to measure this extremely well, and my role has been to specify the scientific needs, to check if the designed mission can achieve these, and to help solve problems when they occur. Another part of the work is people management. For instance, we define policies that hopefully will make sure that everybody can work on the data in harmony.

The project postponed its launch, which was for 2020? What was the main reason – the pandemic, the war? How do these events change your plans and way of work?

By the time the pandemic hit, the schedule had already slipped for various reasons. Luckily the pandemic itself had only a minor impact: the main instruments were delivered right before the lockdowns started. However, as the elements of the telescope were integrated and tested, some problems were discovered. One problem was that some of the thrusters that are used to orient the telescope were prone to leakage. These had to be replaced, which caused a delay, which then affected the subsequent testing programme. About a year ago, we were on track to launch in March 2023, but then the war in Ukraine started. That had a major impact, because Euclid was supposed to be launched with a Soyuz rocket from French Guyana. This was no longer possible, and we had to look for alternatives. Fortunately, thanks to hard work by people at ESA and SpaceX it became clear that it is possible to launch with a Falcon 9. Remarkably, this work was completed in about 6 months, something that normally takes several years! So now we are planning to launch in July this year. In the end, this still leaves us with little time to prepare, as so many small issues, in particular with the analysis software, remain to be solved.

European Space Agency will partner with Elon Musk’s SpaceX Falcon 9 to launch the Euclid space telescope mission and the Hera probe. This partnership is replacing Russia’s Soyuz on the Euclid and Hera missions. What do you think about SpaceX? Why did ESA choose it for the assignments?

For other reasons ESA had already started exploring the possibility of using the new Ariane 6 as a backup option for the launch of Euclid. However, the first launch of Ariane 6 has been significantly delayed, and in April Amazon signed a large contract for a series of launches, which would move us further back. The characteristics of the Falcon 9 are similar to Soyuz, in particular both use liquid fuel, and both are very reliable. This is why this option was explored. Within a remarkably short time, SpaceX showed that things should work just fine and ESA decided to move forward. This is really an exceptional situation, but postponing the Euclid launch would have a big impact on the project.

First of all, the telescope is ready and storing it is very expensive. Moreover, we would lose our competitive advantage (we are still ahead of the competition) and team members would move onto other projects. I am very happy that things worked out fine after all. It was quite the rollercoaster ride last year.

There is a race among a few private Space Tech Companies worldwide. What is your opinion on that? Should there be some kind of space regulation?

It is clear that the private companies, in particular SpaceX, have brought new ideas into the industry, but it is important to realise they did not start from scratch: they benefited tremendously from existing knowledge gained over the years by the “old” space industry. Clearly competition helps move things forward, but it can also lead to a reluctance in sharing of knowledge, which is the case in pharmaceutical research. Moreover, the commercial exploitation of space can have adverse effects. For instance, Starlink satellites contaminate astronomical observations, but the community cannot do much to stop this. Another concern is that with more satellites in space, the chance of accidents increases. So, regulation is definitely needed before it is too late.

Which are the best countries for science Space work? We know that India also invests a lot in space science.

Nonetheless, it is nice to see other countries getting involved and collaborating with established partners. Countries, such as India have a long way to go, and the best places for space science are Europe and the USA. NASA recently launched the James Webb Space Telescope, which is a major step forward, while ESA has a very nice line-up of space telescopes.

You are a scientist, and you are dedicated to looking beyond the stars. However, obviously, nowadays, humans are still violent and primitive as the humans from hundred years ago. What do you think about humanity and its progress in terms of emotions?

This is a genuine concern, and also depressing.

But perhaps closer to home, social media excesses show that we are not always aware of the impact that technology can have, especially in the wrong hands. People abuse these tools to advance their own agenda’s, spreading disinformation and plain lies. We saw this perhaps most clearly during the pandemic: all the scientists took the vaccine, whereas large groups “did not believe” in the pandemic or the benefits of the vaccines.

A similar problem is how we are (not) dealing with global warming. To a large extent it is caused by technological advances that have improved our daily life. Unfortunately, we cannot continue as we did. Scientists have been warning about this for a very long time, yet the majority of people are not able to change their behaviour. Many decisions we make in our daily life are driven by emotions. I am aware it is difficult to do otherwise, but we collectively need to act more rationally if we are to solve the problems we face. If not for ourselves, then for future generations.

What is the role of creativity in your work? In our previous interview, you mentioned that you love contemporary dance performances. Where do science and art meet?

In the end science is reductionist – we start with many possible ideas and home in on the single best idea that can explain the data most efficiently. Ultimately, the outcome is independent of the scientist. In practice, however, many problems have to be solved and new ideas explored. This is where creativity does play a role. Importantly, science can also inspire. In the past years I participated in a number of collaborations with artists. For instance, in 2020 I consulted Thijs Biersteker on a project on the discovery of dark matter that was commissioned by the Dublin Science Gallery. It is called “Dark Distortions”. Unfortunately, the public was not able to experience this amazing art work because of the pandemic. Another highlight was the collaboration Open Space with the Nederlands Dans Theater, a world-leading modern dance company. During two weeks two colleagues and myself inspired dancers, choreographers and composers to explore the Cosmos through dance. I was able to see the creative process developing, which was an amazing experience. I hope to continue the collaboration in the coming years and see where creativity leads us.

Do you still believe “Humans are not built for space travel”?

TNothing has changed there – biological systems like our bodies evolve slowly and are adapted to living on Earth, where the atmosphere protects us from radiation. Space is an unforgiving environment, where distances are vast. Interstellar travel takes much longer than a human lifetime, and who would want to spend their entire life in a cramped spacecraft with no hope of experiencing all the things that we take for granted every day. For most people, the lockdowns during the pandemic were tough enough, imagine your whole life like that…