Dark Matter in a Nutshell

In the early thirties of the 20th century, Fritz Zwicky inferred the existence of a mysterious form of matter not emitting light: the dark matter [1]. In the sixties and seventies, more and more robust evidence for the existence of an unseen form of matter accumulated thanks, in particular, to the work of Vera Rubin [2]. In more recent years, detailed astronomical observations confirmed this picture with a very high degree of precision. However, while more than eighty years passed since the early work by Zwicky, we still do not know much about this mysterious form of matter. So, what do we mean when we talk about dark matter [3]?

What we do know

In the Universe, we observe the effects of gravity due to something that we do not see. We cannot observe this directly with our eyes or a telescope because it does not emit light, i.e., it does not interact electromagnetically. This sounds very mysterious. However, this is similar to how astronomers in the past centuries used to infer the presence of planets beyond the reach of their “primitive” telescopes. To give an example, imagine that the planet Jupiter did not emit any kind of light, or, more correctly, did not reflect the light from our Sun. We would not be able to see it, but we would conclude that it is there just by studying the motion of the other planets of the Solar system. This is exactly how we investigate dark matter. We cannot see it, but we do observe a number of gravitational effects in the Universe that suggest its presence.

The first evidence came from the study of the motion of stars in galaxies, and the motion of galaxies in clusters of galaxies. Astrophysicists noted that these motions could not be easily explained by simple gravitational arguments. For example, we would expect that the velocity of stars decreases as the square root of radius moving away from the centre of a galaxy. However, this does not happen, instead it remains constant. This is explained by the presence of some sort of invisible matter permeating the galaxy up to farther out from its centre than the visible matter, therefore allowing the velocity of stars to remain constant.

There is strong proof for the existence of dark matter, based on different lines of evidence. I will further explain two of my all-time favourites: colliding clusters of galaxies, and the cosmic microwave background.

Clusters of galaxies are a bunch of galaxies held together by gravity, and the space between them is filled with a very hot gas. A famous cluster of galaxies is the so-called “Bullet” cluster, a system constituted by a main cluster being hit by a smaller one: the bullet (see Figure 1).


 Figure 1: the “Bullet” cluster (credits: NASA)

Astrophysicists measured the total matter distribution of the “Bullet” cluster, using two different methods. The first method employs the X-ray light emitted by the cluster’s gas, which tells us how the gas itself is distributed. The second method is based on a phenomenon described by Einstein, the so-called gravitational lensing: a light ray from a given source is deflected from its path toward us by the matter present along the way. The gas was thought to represent most of the mass of such a system; however, the two methods gave very different results. While the visible matter traced by the X-ray emitting gas shows a certain distribution, the total (galaxies, gas and dark) matter of the system measured via gravitational lensing shows a very different one, implying the existence of a dark matter component [4].

Another piece of very important evidence for the existence of dark matter comes from the observation of the so-called Cosmic Microwave Background: a radiation coming from the entire Universe that is remarkably similar in all directions [5]. The very small differences in the temperature of this radiation – a factor of 1 over 100.000 – trace the differences in the primordial matter distribution (see Figure 2). In other words, the Cosmic Microwave Background tells us how the matter was distributed before any structure – stars, planets, galaxies, and so on – actually formed.


 Figure 2: Cosmic Microwave Background measured by Planck (credits: ESA)

At some point, this nearly uniform matter distribution started collapsing around the locations with the highest density, triggering the formation of structures. If all matter in the Universe were merely light-emitting matter, this formation process would have been too slow to develop all the structures that we observe nowadays. A “catalyst” for the gravitational attraction during structure formation is needed to make the whole process faster, and the dark matter is that catalyst.

These are some of the reasons why we are so confident that dark matter is out there. However, it remains a mystery on other levels.

What we do not know

To be fair, we do not know much else about dark matter itself. We know that it is out there, and we can measure its distribution. We know that it does not emit light, and how much there should be with respect to normal matter, about 80%. Consider this; we do not know what 80% of the matter in the Universe is made of!

The commonly accepted scenario is that dark matter is made of some sort of – yet unknown – particle that does not interact electromagnetically and therefore does not emit light. We do not know if dark matter is made of only one or several of such particles. Following Occam’s razor, which tells us that when confronted with different hypothesis we should select the one with the fewest assumptions, our working hypothesis is that dark matter is made of one kind of particle. Under such assumption, we can infer some of the characteristics that this particle should have. For example, we can estimate its mass and its velocity to allow for a “correct” structure formation process.

In this context, the most studied particles are the so-called weakly interacting massive particles, a hypothetical group of particles not described in the Standard Model of Particles. We search for these particles with several techniques both on Earth and in Space. For example, we hope to gain some indications from the Large Hadron Collider in Geneva. We search for them directly, looking at possible weak interactions with normal matter in underground experiments. Finally, we also try to detect them indirectly, looking for possible products of its annihilation or decay, from places in Space where we know that dark matter is abundant. For this, we use both satellites and ground-based instruments. This is a very lively and exciting field of research, but it is very hard too. It is difficult to search for something, when you do not really know what you are searching for.

Alternative theories invoke a modification of the laws of gravity, rather than yet unknown particles. So-called modified gravity theories try to explain the dark-matter-related effects that we observe mainly as result of modifications of gravity, that is gravity should be different in different places in the Universe. While the hypothesis that dark matter is a particle of some sort is the most commonly accepted hypothesis by the scientific community, there are growing attempts to understand whether modifications of gravity exist and how we can detect them.

What we do

As mentioned before, we search for dark matter in different ways, either by looking for possible weak interactions of the dark matter particle with normal matter or for possible products of dark matter annihilation or decay.

As dark matter might interact weakly with normal matter, if it interacts at all, our detectors need to use a large “piece” of material if we want to see the possible interactions with dark matter. Additionally, all kind of particles can interact with this material, and therefore we have to screen our experiment from this “noise”. An example is the XENON1T experiment that has been recently inaugurated in the underground laboratory at Gran Sasso in Italy. The Gran Sasso laboratory is located under a mountain that is indeed used as a natural screener for the “noise”. This exciting research is executed in collaboration with the National Institute for Subatomic Physics (Nikhef) in Amsterdam.

Another example is the planned Cherenkov Telescope Array (CTA), a large international collaboration – with Dutch participation led by the University of Amsterdam – which is planning to build about a hundred telescopes in two different sites in the southern and northern hemisphere. This array of telescopes will be looking at photons of the highest energies, so-called gamma rays. In several models, the products of the particle dark matter annihilation or decay can generate very specific gamma-ray signals. CTA will be looking for such signals, for example from the centre of our own Galaxy where we know a large concentration of dark matter is present.

In the last couple of decades, there have been numerous experimental efforts aimed at detecting the dark matter particle and to understand its basic nature. However, so far, we have been able only to put constraints on several models. Therefore, we need to go a step further with the next generation of instruments in the hope to shed some light on the mystery of dark matter. Hopefully, the future will be bright for the dark side of the Universe.

Fabio Zandanel (article originally prepared for the 41st number of online journal Blind)

[1] Zwicky, F., “Die Rotverschiebung von extragalaktischen Nebeln”, Helvetica Physica Acta, 6, 110, 1933; Zwicky, F., “On the Masses of Nebulae and of Clusters of Nebulae”, Astrophysical Journal, 86, 217, 1937.
[2] For example: Rubin, V.C. & W.K. Ford “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions”, Astrophysical Journal, 159, 379, 1970.
[3] See the book aimed at the general public by Gianfranco Bertone (Institute of Physics, University of Amsterdam) Behind the Scenes of the Universe – From the Higgs to Dark Matter, Oxford University Press, 2013.
[4] Clowe, D., A. Gonzalez & M. Markevitch, “Weak-Lensing Mass Reconstruction of the Interacting Cluster 1E 0657-558: Direct Evidence for the Existence of Dark Matter”, Astrophysical Journal, 604, 596, 2004.
[5] For example: Planck Collaboration, “Planck 2013 results. XVI. Cosmological parameters”, Astronomy & Astrophysics, 571, A16, 2014.

Dark Matter Experiments Around the World

Most of the matter in the Universe is dark. In fact, there is about five times more dark matter than ordinary matter. However, we do not know much about the nature of dark matter apart from the fact that it does not interact electromagnetically, and for its gravitational effects on astrophysical objects that we can observe with our telescopes.

If dark matter is a new type of particle, or more than one, we might be able to detect it. There are several experiments carried out all over the world that try to detect dark matter particles with different techniques. Some experiments are placed deep underground, for instance those at the Gran Sasso Laboratory in Italy, and some on-board satellites orbiting around the Earth. This represents an international effort of the scientific community to eventually unravel the properties of this mysterious form of matter.

To visualize all the dark matter experiments in the world, GRAPPA master student Veerle Tammer developed an interactive webpage as part of her master’s project. Click on this link, or the image below, and discover more about the various dark matter experiments carried out around the world.


GRAPPA Display @ Science Park 904

We have recently installed a small exhibition describing the activities of the GRAPPA institute on the first floor of the UvA Science building at Science Park 904.


The top shelf contains an overall description of GRAPPA and some public science book by our members.

The third shelf from the top currently has a LEGO model for the ATLAS detector at LHC, while the bottom shelf contains some theses from GRAPPA students.

On the second shelf from the top, we have a digital display showing several images/animations: the Bullet cluster; a dark matter ring around a galaxy cluster; a movie from the Millennium dark matter simulation; a picture of the recently discovered Fermi bubbles; an artistic impression of the planned Cherenkov Telescope Array (CTA); a movie of the real-time operation of the High Energy Stereoscopic System (HESS) provided by our member Arnim Balzer; an illustration of the Cosmological Inflation Theory; a picture of the Planck satellite and its recently published sky map; a simulation of the decay and merging of a double black hole binary system; a picture of the Cassiopeia A Nebula, the gaseous remnant of a supernova; a combined X-ray/optical image of a Pulsar Wind Nebula; an image of Sagittarius A* as obtained by the NASA Chandra satellite; and, finally, a simulation of cooling losses during accretion into the Super Massive black hole Sagittarius A* is provided by our member Solame Dibi.

Enjoy our display!

CERN: glorious past, exciting future

Today, 60 years ago, the visionary convention establishing the European Organization for Nuclear Research – better known with its French acronym, CERN – entered into force, marking the beginning of an extraordinary scientific adventure that has profoundly changed science, technology, and society, and that is still far from over.

With other pan-European institutions established in the late 1940s and early 1950s — like the Council of Europe and the European Coal and Steel Community — CERN shared the same founding goal: to coordinate the efforts of European countries after the devastating losses and large-scale destruction of World War II. Europe had in particular lost its scientific and intellectual leadership, and many scientists had fled to other countries. Time had come for European researchers to join forces towards creating of a world-leading laboratory for fundamental science.

Sixty years after its foundation, CERN is today the largest scientific laboratory in the world, with more than 2000 staff members and many more temporary visitors and fellows. It hosts the most powerful particle accelerator ever built. It also hosts exhibitions, lectures, shows, meetings, and debates, providing a forum of discussion where science meets industry and society.

What has happened in these six decades of scientific research? As a physicist, I should probably first mention the many ground-breaking discoveries in Particle Physics, such as the discovery of some of the most fundamental building block of matter, like the W and Z bosons in 1983; the measurement of the number of neutrino families at LEP in 1989; and of course the recent discovery of the Higgs boson in 2012, which prompted the Nobel Prize in Physics to Peter Higgs and Francois Englert in 2013.

But looking back at the glorious history of this laboratory, much more comes to mind: the development of technologies that found medical applications such as PET scans; computer science applications such as globally distributed computing, that finds application in many fields ranging from genetic mapping to economic modeling; and the World Wide Web, that was developed at CERN as a network to connect universities and research laboratories.

CERN Control Center (2).jpg
“CERN Control Center (2)” by Martin Dougiamas – Flickr: CERN control center. Licensed under CC BY 2.0 via Wikimedia Commons.

If you’ve ever asked yourself what such a laboratory may look like, especially if you plan to visit it in the future and expect to see building with a distinctive sleek, high-tech look, let me warn you that the first impression may be slightly disappointing. When I first visited CERN, I couldn’t help noticing the old buildings, dusty corridors, and the overall rather grimy look of the section hosting the theory institute. But it was when an elevator brought me down to visit the accelerator that I realized what was actually happening there, as I witnessed the colossal size of the detectors, and the incredible degree of sophistication of the technology used. ATLAS, for instance, is a 25 meters high, 25 meters wide and 45 meters long detector, and it weighs about 7,000 tons!

The 27-km long Large Hadron Collider is currently shut down for planned upgrades. When new beams of protons will be circulated in it at the end of 2014, it will be at almost twice the energy reached in the previous run. There will be about 2800 bunches of protons in its orbit, each containing several hundred billion protons, separated by – as in a car race, the distance between bunches can be expressed in units of time – 250 billionths of a second. The energy of each proton will be compared to that of a flying mosquito, but concentrated in a single elementary particle. And the energy of an entire bunch of protons will be comparable to that of a medium-sized car launched at highway speed.

Why these high energies? Einstein’s E=mc2 tells us that energy can be converted to mass, so by colliding two protons with very high energy, we can in principle produce very heavy particles, possibly new particles that we have never before observed. You may wonder why we would expect that such new particles exist. After all we have already successfully created Higgs bosons through very high-energy collisions, what can we expect to find beyond that? Well, that’s where the story becomes exciting.

Some of the best motivated theories currently under scrutiny in the scientific community – such as Supersymmetry – predict that not only should new particles exist, but they could explain one of the greatest mysteries in Cosmology: the presence of large amounts of unseen matter in the Universe, which seem to dominate the dynamics of all structures in the Universe, including our own Milky Way galaxy — Dark Matter.

Identifying in our accelerators the substance that permeates the Universe and shapes its structure would represent an important step forward in our quest to understand the Cosmos, and our place in it. CERN, 60 years and still going strong, is rising up to challenge.

Gianfranco Bertone

Gianfranco Bertone is Associate Professor at the University of Amsterdam and spokesperson of the GRAPPA Institute, where he investigates topics at the interface between Particle Physics and Cosmology. After a PhD at the University of Oxford and the Institute of Astrophysics in Paris, he has held teaching and research positions at the Fermi National Accelerator Laboratory, the University of Padova and the University of Zurich, before going back to Paris as a permanent CNRS researcher. He joined in 2011 the new center of excellence in Gravitation and Astroparticle Physics of the University of Amsterdam. He is the editor of the book Particle Dark Matter: Observations, Models and Searches, the editor-in-chief of the journal Physics of the Dark Universe and the author of Behind the Scenes of the Universe: From Higgs to Dark Matter.

See more at: http://blog.oup.com/2014/09/cern-glorious-past-exciting-future/#sthash.eRclU02e.dpuf

History & Future of Dark Matter

We are pleased to announce the “History & Future of Dark Matter” symposium which will take place in the 17th century Koepelkerk in downtown Amsterdam on 22 June 2014. The symposium is open to the scientific community and the public alike, so everybody is welcome to attend! However, attendance is by registration only (tickets cost €15,00).

An extraordinary discovered has recently revolutionized Particle Physics and Cosmology. The understanding of the Universe had proceeded rather linearly from the beginning of the 20th century, when Hubble had discovered the expansion of the Universe. But when, in the 1970s, scientists tried to put together the many pieces of the cosmic puzzle to come up with a consistent cosmological model, these pieces just didn’t seem to fit. To complete the puzzle, the existence of a new form of matter, dark matter, had to be postulated. [Text adapted from the book “Behind the Scenes of the Universe”, by Gianfranco Bertone]

Eight world-leading scientists who pioneered the discovery of dark matter and its detection strategies will discuss the history and future of dark matter studies. The event will consist of two series of 3 presentations, each followed by a round table discussion with experts in Cosmology and in the History of Science.

We hope top see you at Koepelkerk on 22 June 2014!

GRAPPA Outreach webpage goes live!

Welcome! This is a newly created space where GRAPPA scientists will share their research for the general public and prospect students. We plan to regular post articles and multimedia material to explain the research carried out here at GRAPPA.

To inaugurate this webpage we would like to share with you a public lecture recently given by Gianfranco Bertone at the Perimeter Institute:

Enjoy and stay tuned!