Laura Baudis – TRANSCRIPT
How often do you find time to gaze into the stars?
Last February, I spent a few cold days high up on a mountain, here, in Switzerland, and when I looked out at the sky in a dark, clear night, I saw thousands of glimmering light sources and even the extended nebulous band of the Milky Way. This nebula is made of 100 billion stars and forms a spiral system – our home galaxy.
I am sure that everyone of you had at least once such an awe-inspiring experience. We cannot easily see the entire structure of the Milky Way for we are part of it. We, together with our Solar System, are located in the stellar disc, and we move around the Galactic Center with 220 kilometers per second. But to give you an idea, it would look very much like our close neighbor, the Andromeda Galaxy.
You can see here its beautiful, spiral arm structure, but you cannot see such an image with the naked eye. With today’s advanced telescopes, we can map not only Andromeda in the Milky Way but our entire observable universe in many different wavelengths from radio waves to the very high-energy gamma rays gathering information through the observation of electromagnetic radiation. We can produce countless, beautiful, and useful pictures of the cosmos. Today, we also know that what we see is not the whole picture.
In fact, why would we even assume that what we can directly observe is all that there is? History has overthrown our most basic assumptions many times, it has told us that the Earth is not at the center of the Solar System, that our Sun is at the outskirts of the Milky Way, and that our own galaxy is but one in two trillions of galaxies in the observable universe. That’s why would matter that radiates or that absorbs electromagnetic radiation be the dominant form of matter in the cosmos.
It was back in 1933, when a Swiss-American astronomer, Fritz Zwicky, discovered that something strange was going on in a rich cluster of galaxies, the Coma Cluster. He studied the motion of individual galaxies in this cluster and found out that their speeds were far too high that galaxies should simply fly apart. The speeds were much higher than predicted, based on the luminous matter alone. He concluded that there must be some new form of matter that he called “dark matter,” and which, like a glue, keeps all these galaxies in the cluster together. Zwicky was brilliant and very much ahead of his time, but he was not an easy person to get along with.
He had remained true to his nature, the mountaineering, and was often inclined to provocation. For instance, he called his colleagues, “spherical bastards,” and perhaps, these are some of the reasons why his results went largely unnoticed until the 70s when Vera Rubin and her team measured the rotation and velocities of stars and gas in spiral galaxies. She came to the same astonishing conclusion: galaxies were filled with this new substance, with dark matter. The stars and the gas at the outer edges, including our own Sun, would simply fly apart if galaxies were dominated by visible matter alone.
Today, many decades later, we have an overwhelming number of observations on all astronomical scales and we know that most of the matter in our universe, namely, 85% is dark or invisible. We know that dark matter is out there because of its gravitational pull on visible matter, such as stars, and galaxies, and interstellar gas. Dark matter influences and distorts how visible matter moves and clusters. These are effects that we can directly observe, but quite incredibly, more than 80 years after Fritz Zwicky’s initial discovery, we are yet to answer the most fundamental question of all: what is dark matter? What is it made of? What is its true nature?
Before we continue, let’s pause for a moment and remind ourselves what normal matter is made of: stars, gas, planets, and people. Everything is made of atoms, and atoms are made of protons, neutrons, and electrons; protons and neutrons are made of quarks. In this beautiful, colorful chart, you can see all the known elementary particles: the quarks in the top half, the leptons in the bottom half, the force carriers and the Higgs boson discovered here at CERN, in 2012. Can any of these particles be the dark matter? No.
Today, we know that none of the familiar particles is a good candidate. However, physicists have a vivid imagination, and many other possibilities were proposed. The most popular candidates are weakly interacting massive particles also known as WIMPs. As their name suggests, they scatter only rarely with normal matter, and they are heavier than a proton; perhaps 100 or 1,000 times heavier. WIMPs are an example for the so-called cold dark matter, which means, that under the influence of gravity, they produce exactly the structures that we observe today. All these elementary particles were produced in our young and hot universe about one billionth of a second after the Big Bang.
Our young cosmos was very energetic, and other fundamental particles, heavy particles such as WIMPs, were likely produced at the same time. If these particles are stable, they might still be around today and form the halos of galaxies, including the halo of our own Milky Way, We thus have very good reasons to believe that a new, yet undiscovered particle might be a prime candidate for this mysterious dark matter. This is really the simplest solution of all: the dark matter made of a new particle which does not emit nor absorb any light.
Then, how can we possibly make it visible? Here at CERN, my colleagues are trying to produce such new particles by creating conditions as they existed shortly after the Big Bang. They look for WIMPs emerging in collisions of very-high-energy protons. We, on the other hand, look for collisions of dark matter particles – those that are here and out there forming the halo of our Milky Way – with atoms in a terrestrial detector.
In this rare process, which you can picture like an elastic collision between billiard balls, a tiny amount of energy is transferred onto the atom, which then gets excited and emits a faint flash of light. We have calculated about ten million dark matter particles pass through your hand every second. So, hold out your hand and just imagine this; can you feel anything? No. You don’t feel the billions of neutrinos either and all the other cosmic ray particles that pass through you and your body in very high numbers. These cosmic rays would create a huge noise in our detectors preventing us from listening to the rare and faint dark matter interactions.
So, where is the best place to look for something that’s invisible? At the bottom of a dark, deep mine. To have even the slightest chance of detecting WIMPs, we have to go deep underground. We use mountains as shields against this noise caused by cosmic radiation. I work in one of the largest underground laboratories in the world, the Gran Sasso Laboratory in Italy. It is located beneath the Cardinal Grand Massif of the Abruzzi mountains and offers about 1, 400 meters of shield against noise; both real and cosmic.
In this quiet location, we recently placed a large detector filled with three and a half tons of liquid xenon kept cold at 100 degrees Celsius below zero. The liquid xenon emits a tiny flash of light when a particle collides with a xenon atom. We can detect this light, and we can also measure the few electrons produced in such a collision. The trick is then to make sure that no other particle ever makes it to the center of your detector, and that only dark matter particles can leave the faint trace we are looking for.
So we have to surround the xenon by various shields. It is contained in an inner detector itself, in a large stainless steel cryostat which then, is surrounded by ten meters of water. The water protects it against those cosmic rays that can still penetrate deep underground. The core of this detector, its inner part, is called time projection chamber. It delivers a precise, three-dimensional image of any interaction that happens in the liquid by means of 248 photo-sensors, or eyes, that are constantly monitoring the cold liquid.
So, considering now the flux of the dark matter particles, the ten millions per second through your hand, and the fact that we haven’t seen any yet with the previous generation of detectors, we can estimate that only a handful will scatter in our new detector in one full year of data; maybe, even just one. We are looking for very rare events indeed, but there was another challenge that we had to overcome; it is not sufficient to bring your detector underground and to surround it by water, we also had to ensure that every material we employed in its construction had incredible low levels of natural radioactivity.