It Is Too Early for Mars, Let’s Start оn the Moon*
* Edited 29.09: Four months later, we speak to Henk again following the NASA announcement that water has been found on Mars. Read his take on this discovery below.
Henk Hoekstra is an observational cosmologist and associate professor at Leiden Observatory, the Netherlands.
Why did you choose astronomy as your profession?
I have always been curious and wanted to learn how things work, and learning more about the Universe was, therefore, an easy choice. Although I was good in mathematics and physics, I decided to study astronomy, partly because it was less specialized than some of the larger disciplines, but also because you study some really interesting questions. But I was never an amateur astronomer, in case you were wondering, and can only recognize two constellations.
What is cosmology about?
Cosmology is the study of the global questions about the Universe: how did it begin, what is it made of, how will it change in the future. These are very fundamental questions about why there is a Universe, but we try to answer them using scientific methods and our understanding of physics. Amazingly, the Universe turns out to be a strange place, and by trying to answer these basic questions we have learned that our current understanding of physics needs to be revised.
You work in the observatory where Willem de Sitter, Ejnar Hertzsprung, and Jan Oort used to work. Your name is already a part of the observatory’s history. Is that something you were aiming for аnd does the career of a scientist follow such targets?
Astronomers at Leiden Observatory have indeed made important contributions to the field, and every scientist wants to make his or her mark. Therefore, it is an honor to work at such a place. But at the same time, research is always forward looking, and the current work environment is far more important. So the fact that Leiden Observatory is still a world-class institute was far more important for my decision to move to Leiden.
Which are the most important issues in contemporary cosmology?
We’ve known for nearly a century that the Universe is expanding, but everybody expected that the expansion would slow down. In 1998, two teams discovered independently that the expansion of the Universe was actually accelerating. We do not understand this at all, and to reflect our ignorance, we say that this is caused by dark energy. But this is not the only problem: most of the matter in the Universe is invisible, and we still do not understand the very early Universe. So there are many open questions that remain to be answered.
You say that linking the accelerating expansion of the Universe to dark energy is probably the most significant finding in cosmology for the last three decades. Why is that so and why is it important to understand it?
It is so important because this should not happen according to our current understanding of physics. There is simply no good explanation for this observational result. The various solutions that people have put forward all challenge some part of contemporary physics, such as Einstein’s theory of General Relativity. These results tell us that we are doing something wrong, but we do not know what. That is exciting because it can spur new developments, perhaps similar to what happened 100 years ago.
At end of the 18th century, people thought that physics was finished. There were only a few strange things that still needed to be explained. This led to the theory of General Relativity (without which our GPS would not work) and the development of quantum mechanics (which is nеeded to make computers). The physics of the 20th century has completely changed our lives and maybe this will happen again through new discoveries.
Аs for the importance of understanding the expansion:
By trying to answer these questions, we show our curiosity, which is a typical human trait. Most civilizations have developed creation mythologies to explain this, and we are now using modern science to see whether we can understand it.
Because the Universe is so enormous, seemingly nothing changes and therefore people were convinced that the Universe was static. Even Albert Einstein was convinced of this until Edwin Hubble showed that the Universe was expanding. Thanks to astronomy, we learned something we did not expect. By studying the expansion a bit better, we now know that the Universe is 13,6 billion years old and that it will expand forever at a faster and faster rate because of the dark energy. Unless somebody makes a new discovery that shows we got it all wrong. That makes it so exciting. By trying to answer such questions, we discovered that there are problems with our understanding of physics.
If you have to describe dark energy and matter to a non-scientist, as well as your research on the subject, how would you do it?
That is not easy because these are topics that we do not encounter in our everyday life. The dark matter is the easiest: it is like ordinary matter, but it is invisible. What we mean by matter is that it is affected by gravitational pull, and it is dark because it cannot emit or absorb light. What is interesting about this is that we do not have a good theory that explains the existence of dark matter.
Dark energy is more difficult to explain; partly because we do not know much about it other than that it causes the Universe to expand faster and faster. But we believe it may have to do with the properties of empty space (the vacuum). According to quantum mechanics, empty space is a rather strange place and perhaps not quite empty. If the empty space would have some energy, then this would indeed lead to accelerated expansion of the Universe. What we do not understand is why the observed amount of energy is so tiny.
My research tries to measure in detail the effects that dark matter and dark energy have on the Universe. I do that by measuring the deflection of light by massive objects. This effect is called “gravitational lensing” and was predicted by Einstein. If you look through a wine glass, then everything looks distorted. The same is true when we look at the distant Universe, but the effects are more subtle. In this case, the matter in the Universe causes this effect and by measuring this we can figure out how much dark matter there is and how it is distributed. And with very precise measurements we can even learn more about dark energy.
Do you share the opinion that we live in a golden age for Astronomy because of the technological advances? What do you expect to happen in the field in the next 10 years?
Technological advances are definitely important for progress in astronomical research because it is an empirical science: we cannot test our ideas in a laboratory setting, but have to infer new insights from better observations. Therefore, computers allow us to search ever more data more efficiently and create realistic simulations of the Universe. Progress in engineering allows us to build larger telescopes, with construction just starting in Chile on the Extremely Large Telescope. Right now there is also a lot of excitement about the Atacama Large Millimeter Array (ALMA) in Chile, which was just completed and allows us take sharp pictures of the places where new stars and planets are forming.
Looking at the progress over the past century, things are definitely better than ever before, but the next 25 years may be even more of a “golden age”, because several large projects are being developed and should start collecting data within 10 years from now. There will still be lots of progress, especially in the search for other planets, something that has only just started.
For me, personally, the Euclid satellite will be the most important. This is a space mission that will tell us more about dark matter and dark energy. I have a leading role in this project, which requires a lot of work before the launch, planned for 2020. After the launch, we will take images of a large part of the sky. The collection of the data will take about six years, and the final analysis will also take a few. All this should keep me busy for the next 15 years or so.
A robotic probe has become the first craft to land on a comet, what do you expect to find out?
The aim of this mission is to find out where water on Earth came from. There are indications that most of the water was brought to Earth by comets during the formation of our Solar system. Because we cannot live without water, we are therefore learning something about the conditions for life to develop. Perhaps we can use this to find other habitable planets or even life. Another useful aspect is that projects like this one may one day help us deflect an asteroid that would otherwise collide with the Earth.
The Mars One project, aiming to permanently settle people on Mars, attracts more and more participants. What are your thoughts on it?
Space is a very hostile environment. The atmosphere and the Earth’s magnetic field protect us from harmful radiation, but in space you are exposed to it. So a trip to Mars can lead to serious health issues.
On the other hand, space exploration is important for our very long-term future because the Sun will run out of fuel eventually. Hence, we will need to find ways to travel through the Universe, like in the movie Interstellar.
but it is nice to see that people are coming up with ambitious projects. Posing such challenges is the best way to make progress.
How does a scientist like you conceive contemporary “space” movies such as Interstellar, Gravity, etc.? You must have a lot of fun with them.
The fact that such movies are made shows the interest people have in space, and this is very nice. They are, of course, more fiction than science, but that does not make them less enjoyable. What I do find annoying is the stereotypical way scientists are often portrayed in movies. For instance,
Excuse us, but we just have to ask: what is your opinion on the scandal with Matt Taylor’s (Rosetta scientist) controversial t-shirt?
It was very unfortunate that this detracted so much attention away from the amazing scientific results. The whole mission and particularly the landing were a demonstration of what we can accomplish, which is pretty amazing.
Walk us through a typical day of yours.
The nice thing in my job is that you don’t have a typical day. Although certain tasks need to be done regularly, the work is rarely repetitive. As I am typing this, I am on the train from Bonn to Leiden, where I had a two-day document-planning meeting for the space mission I am working on. Tomorrow I have a series of meetings, starting with a discussion of new papers, followed by a meeting to discuss how we can attract more MSc students to our program. Between the meetings I talk with the members of my research group, finish preparing the exam for the course I taught and will work on a proposal for observing time on one of the telescopes on La Palma. And the next day is again completely different.
Most of the work is in an office setting, but also involves teaching and travelling around the world for meetings and conferences. Occasionally I get to observe at one of the telescopes. For instance, I have used them in La Palma (Canary Islands), Chile and Hawaii, but these days most of the data are collected by telescope operators and transmitted via the internet.
Apart from space, what else matters?
These days I am quite busy with the preparations for Euclid and the work on other projects that I have little time for hobbies. But I do enjoy going to concerts and modern dance performances, which are of a very high quality in the Netherlands. On the weekends I like to cook for friends: preparing a nice meal is a great way to focus on something else.
If you could go back 10 years, what would you tell yourself?
I don’t know what I would tell myself. The last really big discovery in cosmology was the discovery of dark energy in 1998, which is more than 10 years ago. I am lucky to work in such an exciting research direction and would not change what I am doing now. But being able to make such a discovery would have been nice. Maybe that will come in the future.
What may the future of the Universe be? What comes next?
We can never be sure, but based on our current understanding, things will get pretty boring eventually. But before that a few interesting things will happen.
Around the same time, our Galaxy will start to merge with our nearest neighbor, the Andromeda galaxy. But the Universe will continue to expand and eventually we’ll only be able to see our own Galaxy. Everything else will disappear from our view due to dark energy. All the stars will run out of fuel, and the Universe will become a very cold and dark place. So let’s hope we make a discovery that changes this rather bleak future.
Four months later, we speak to Henk again following the NASA announcement that water has been found on Mars. Read his take on this discovery below:
NASA scientists have found evidence that water does exist on Mars. How important is this discovery?
The existence of water on Mars was already a known fact, but only in the form of ice, which cannot support the development of life. The new discovery is that there is liquid water as well. Although these last results do not imply there is life on Mars, it does allow for the possibility, and therefore warrants further research. If we can find evidence for (past) life there, it would suggest that extra-terrestrial life may be common, and provide clues about how life on Earth began.
As far as we know, the NASA team are still not certain where the water is coming from. What do you think?
What I understand from the NASA results is that the water is extracted from the Martian atmosphere by chemicals in the soil. But I am not really an expert on this.
As water is necessary for supporting human life, some rumors about “colonizing” Mars already exist — we would like to hear your opinion on that concept.
Mars is not a pleasant planet to live on, as it is a cold place with no atmosphere. Although a colony would need water, it is not the main problem for establishing one there. For now, the best way to explore the Solar system is by more sophisticated robots, such as the rovers that have been driving around Mars already. The next step is to explore larger areas in more detail, in particular drilling below the surface to see what is there. Some projects are already planned, but with this discovery, I am sure more ideas will be proposed.