Focus on research: Building on the shoulders of giants
If UvA’s brand new GRAPPA initiative could be described in one single word, it would be collaboration. Founded in October 2011 as a partnership between UvA’s Institute for Theoretical Physics (ITFA), the Anton Pannekoek Institute and the Institute for High Energy Physics (IHEF), GRAPPA is a center of excellence in which astrophysicists, particle physicists and theorists all come together. With their combined knowledge, its members hope to answer some of the biggest questions of the universe.
Every scientist is looking for the unknown, but that doubly applies to the five newly appointed members of GRAPPA. Patrick Decowski, Shin’ichiro Ando, Gianfranco Bertone, Jacco Vink and Ben Freivogel all came to Amsterdam to shed their light on some of the darkest mysteries of the universe. The problem of dark matter for instance, is one of GRAPPA’s core subjects that each member takes on from their own perspective and expertise, but it certainly isn’t the only one. The quest for dark matter is exemplary for the spirit of cooperation within GRAPPA. As the name suggests, we know very little of dark matter, except that it consists of a new subatomic particle that has not yet been identified, even though it’s supposed to be around in enormous quantities. Theory suggests that of all the mass and energy in the universe – stars, planets, asteroids, etc. – 74 percent is comprised of dark energy, 22 percent of dark matter and only 4 percent of normal matter – the stuff we know from the standard model of particle physics.
The term ‘dark matter’ was coined by the Swiss astronomer Fritz Zwicky in 1933. To explain the high rotational velocity of some galaxies like our Milky Way, the universe should contain much more mass – and therefore much more matter – than was previously assumed. This dark matter particle probably interacts with only two of the four fundamental forces of physics: it reacts only to gravity and the weak nuclear force but it is immune to the strong nuclear force and electromagnetism. Since dark matter neither absorbs nor gives off light, it is invisible to the eye, hence Zwicky’s apt characterization as ‘dunkele Materie’. Although the dark matter problem is one of the hottest issues in contemporary physics, the particle itself has never actually been observed. Yet. But if it’s up to the new GRAPPA members, that is about to change.
Some people look up to find dark matter; others look down. Particle physicist Patrick Decowski goes deep underground to work on the XENON Dark Matter Experiment in the Gran Sasso National Laboratory in Italy: “There are several ideas about what dark matter could be”, says Decowski. “One of those is the WIMP hypothesis. We don’t notice it, but each second, billions of these Weakly Interacting Massive Particles move through our body and the earth. Every once in a while, these particles hit something else, like balls on a pool table. This is very rare, but we hope to raise our chances by using the element xenon. If we can detect the collision of dark matter with xenon, we may be able to find this WIMP particle. To avoid useless noise from other particles, we go underground and use the earth as a natural filter. The XENON Dark Matter Experiment is a large international endeavor. Fifteen universities and institutes are involved, including GRAPPA and the National Institute for Subatomic Physics (Nikhef) in Amsterdam. The actual detector will be a barrel of about one by one meter, filled with liquid xenon. It will be equipped with super sensitive light sensors. Whenever a WIMP particle hits a xenon nucleus, it will produce two tiny flashes of light. By measuring and studying these flashes, we should be able to detect this particle directly. In the quest for dark matter, time is an issue: the detector should be operational in 2014.”
Patrick tells about two other GRAPPA members, Stan Bentvelsen and Paul de Jong, who are also looking for dark matter. They work on the ATLAS Experiment at CERN, the European Center for Nuclear Research in Switzerland, which zooms in on the production of dark matter. Decowski: “The Large Hadron Collider (LHC) at CERN may be able to produce this new, hypothetical particle by smashing two highly energetic protons into each other. Already, the ATLAS experiment has produced some great data that we can combine with those from our xenon experiment. This way, we extend our knowledge even further. We are building on the shoulders of giants. That’s the great thing about GRAPPA: We blog, discuss each other’s projects and meet regularly to talk about the papers we read. All knowledge naturally comes together.”
Instead of searching for dark matter in an underground experiment, the Japanese astrophysicist Shin’ichiro Ando looks up at the skies. He scans the universe for traces that dark matter may leave behind. Using public observation data, he studies areas in the universe with supposedly high concentrations of dark matter, like the center of our Milky Way. There, these particles smash into each other every once in a while because there are so many around. Ando: “When two dark matter particles collide, they destroy each other. The particles themselves disappear, but this annihilation produces new particles that we already know, like high-energetic photons – gamma rays – which we can measure quite accurately. Instead of looking for this unknown dark matter itself, we search with telescopes and satellites for a signature that dark matter annihilation leaves behind. This method is called indirect detection.”
Ando is not only optimistic about his own search for dark matter, but also about those of his colleagues: “This research field has become very dynamic in recent years. Patrick’s experiment, the work at CERN, the observations in space; we can do it all with a solid theoretical base. There are many mysteries in the universe, but for dark matter we at least have some ideas to work with. We are going for it.”
Ando shares his optimism with the Italian astroparticle physicist Gianfranco Bertone: “Shin’ichiro recently told me about an ancient model from India”, says Bertone. “It showed the earth resting on an elephant, the elephant standing on a turtle, and the turtle on a giant snake. Every civilization always had a depiction of how the skies were organized, some kind of explanation. Since we now live in this technological era, we want to base this model on real observations, on things we can measure. In hindsight it is obvious, but in the late seventies and early eighties astrophysicists and particle physicists realized that they were essentially dealing with the same questions. They were looking at the same
things, only from a different perspective. These people established a big and important connection in this field of research. And we will benefit from it. Right now, we mainly know what dark matter cannot be, but in a few years we will get some answers. If we find something, it would mean an enormous breakthrough. It will open a brand new branch of physics that we can study for decades to come. We are on a worldwide treasure hunt.”
Bertone plays a key role within GRAPPA: “In our field, there is a broad distinction between theorists and experimentalists. That gap can be very wide, so you need people who act like a bridge between the two. I’m like a translator. I analyze data from all kinds of experiments, like those at CERN, direct and indirect detection, and see how this connects with the theory. This way I hope to find out how the theory will manifest itself in an experiment. On the one hand, I use theoretical physics to make predictions on experiments, while on the other I use the data of those experiments to further constrain the theory.”
Bertone deals with data from not just one, but from a whole range of experiments and observations: “I feel comfortable with a holistic approach; to see the bigger picture, I like to combine things. And it’s not just limited to dark matter. I also studied general relativity and black holes, for instance. Right now, I’m fascinated by the dark matter problem and this is definitively the right time to work on it, but astroparticle physics and GRAPPA have much more to offer.”
The Dutch astrophysicist Jacco Vink is not directly involved with the dark matter problem. He studies cosmic rays: high-energy particles in space that continuously bombard the earth. “At the moment I’m looking into supernova remnants, the leftovers of exploded stars”, says Vink, as he shows a crystal clear image of a multicolored cloud in space, enclosed by a bright blue ring. It’s the supernova remnant Tycho, shot by the CHANDRA satellite. It owes its name to the Danish astronomer Tycho Brahe, who witnessed the explosion in 1572 and wrote a book about it. Vink: “Whenever such a cosmic bomb goes off, it sets in motion all kinds of processes. One of the reasons why supernova remnants are so interesting is that they accelerate particles. The shock boosts these particles to tremendous energies, even higher than what they can do in CERN. These energetic particles get their energy from crossing the shock multiple times. The bright blue ring consists of energetic electrons that are very close to shock front and are able jump back and forth across the shock. I’m trying to figure out the conditions under which these particles accelerate and how much energy this requires.”
At first glance, cosmic rays have little to do with dark matter, but still, there is a connection: “GRAPPA is all about astroparticle physics”, says Vink. “I study high-energetic phenomena in the universe. Shin’ichiro is doing indirect detection of dark matter. The energy of the gamma rays produced by the annihilation of dark matter particles should correspond with the mass of those particles. But if you look through a telescope, you see lights everywhere. Some of it may come from two dark matter particles colliding, but how do you know which light is which? Which highly energetic phenomena cannot be attributed to cosmic rays and may therefore point to dark matter? GRAPPA has only just started, but I think that’s one problem where we will find an overlap between my research and that of others. Already there is a lot of interaction within the group. Patrick and I teach a course at Amsterdam University College, and everybody is talking to everybody all the time.”
Theory of everything
The American theoretical physicist Ben Freivogel only recently joined GRAPPA but already he follows the work of his colleagues with great interest: “Of course I’m very curious what comes out of these experiments, also at CERN with the LHC. My particular interest is in trying to make a connection between the results of those experiments and the fundamental theory. Right now, our best hope for this fundamental theory is string theory, a theory of everything. It is really beautiful and mathematically self-consistent, but it’s been really hard to get from that fundamental theory to predictions for what will come out of these experiments. The theory produces a landscape of possible solutions that we don’t quite understand yet. It’s a process of trying to figure out more things directly from theory and trying to learn some hints from experiments, to try to build a connection there.”
Although Freivogel has just arrived in Amsterdam, he already has lots of plans: “One of the things I want to think about more is the axion, a particle that may be dark matter. Maybe we can find a way to detect it. But that’s not the only thing. There are some people involved in GRAPPA, like Sera Markoff, who work on black holes. There are some interesting connections with fundamental physics there. My interests have always crossed over from theoretical physics to things more closely connected to observation. But generally, the way things are organized, you know, you’re working in a string theory group or whatever, and that’s whom you have lunch with. So this is really a special thing, to try to build a team across so many areas of physics. It will definitely be a very exciting learning process.”