According to a NASA press release in August, the AMS-02 will “use the unique environment of space to advance knowledge of the Universe, leading to a better understanding of the universe’s origin by searching for antimatter, dark matter, strange matter and measuring cosmic rays.”
So. Everybody clear on that then?
For those without a strong background in particles physics or cosmology, the jargon filled press releases and summaries of this important experiment are not easy to decipher. Not surprising when you consider that this project has involved over 500 physicists from 56 institutes representing 16 countries. That is an extremely large collaboration and this article aims to make the goals and science behind the AMS-02 project easier to understand. Perhaps then the excitement being felt in the physics world about the potential results from the AMS-02 can be shared by all.
Orbiting the Earth on the ISS at an altitude of about 300 km, AMS-02 will study with an unprecedented accuracy of one part in 10 billion the composition of primary cosmic rays, exploring a new frontier in the field of particle physics, searching for primordial antimatter and studying the nature of dark matter.
We can break down three main scientific goals of the AMS-02 experiment as follows:
- Cosmic rays: Understanding the cosmic ray environment in space and making precise measurements of the composition and flux (essentially the rate at which a surface is bombarded) of these particles.
- Searching for Dark Matter or “missing matter”
- Searching for Antimatter
- Searching for the “strangelet”
I will outline the basic science behind these goals in the sections below.
To understand the first goal, we need to understand what a “cosmic ray” really is.
Cosmic rays are very energetic particles which come from anywhere in space (i.e. anywhere outside our Earth’s atmosphere) They are particles which travel individually so the term “ray” which suggests a beam of particles together is misleading. Cosmic rays are constantly bombarding the Earth’s atmosphere. The majority of cosmic rays (around 89 percent) are simple protons that travel incredibly fast – near to the speed of light. Another 10% of these particles are so called “alpha particles” which are actually the nuclei of Helium atoms. Less that 1% of the particles are electrons (which we term “beta particles”). The remaining fraction is made up of the other heavier nuclei which are abundant end products of stars’ nuclear synthesis. All of the natural elements in the periodic table are present in cosmic rays, in roughly the same proportion as they occur in the solar system.
The term “cosmic rays” usually refers to Galactic Cosmic Rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy. However, these particles can also originate from as nearby as our Sun.
The sheer speed of these particles means that they can easily penetrate matter. When cosmic rays hit the Earth’s atmosphere they collide and react with the atmospheric gases and produce a “shower” of particles with far less energy. These particles are called “secondary cosmic rays” and every piece of matter on Earth is constantly being hit with these incoming particles- including humans. Luckily for us, the secondary cosmic rays have so much less energy than the original primary cosmic rays that impinge on our atmosphere , that we suffer few ill effects. Astronauts in space however do not have the benefit of this protection and so methods to shield humans from cosmic rays during long term space exploration missions require an excellent understanding of these particles.
The main focus of cosmic ray research has been directed towards investigating where cosmic rays originate, how they get accelerated to such high velocities, what role they play in the dynamics of the Galaxy, and what their composition tells us about matter from outside the solar system. To measure cosmic rays directly, (i.e before they have been slowed down and broken up by the atmosphere), research is carried out by instruments carried on spacecraft and high altitude balloons, using particle detectors similar to those used in nuclear and high energy physics experiments.
The AMS-02 is one such detector. Without the atmosphere in the way, primary cosmic rays can be studied like never before. Also up until now most cosmic ray studies are done by balloon-borne satellites with flight times of days. Even on such a brief time scale we have seen great variation. With a nominal mission time of 3 years, the AMS-02 can gather an enormous amount of accurate data and allow measurements of the long term variation of the cosmic ray flux over a wide energy range.
The Mystery of Dark Matter
If you’ve read popular science books or tuned into the latest documentaries about our universe, chances are you’re heard the terms “dark matter” and “dark energy” at some point. But what is all this dark stuff we keep hearing of and why is it so important?
We use the term “dark” for anything we can’t detect by the radiation it emits or reflects. When we talk of the “matter” in the universe we usually think of what we can see: stars and galaxies. These luminous objects have usually been considered as the most significant collections of matter in our universe.
Amazingly this “ordinary” matter can only account for about 5% of the total matter in the universe. That means that there’s another 95% of something out there that we just can’t see. This 95 percent is dark, either dark matter, which is estimated at 23 percent of the mass of the universe, or dark energy, which makes up the balance. We still do not know the exact nature of both these dark components .
In the words of AMS’ Robert Battitson “Never in the history of science were we so aware of our ignorance: we know that we do not know anything about what makes up 95% of our universe”
So how do we know it’s there if we can’t see it?
By its gravitational force. Anything that has mass also exerts a gravitational force.
Dark matter and Dark Energy are not the same thing. In fact, they can be regarded as opposing forces – each necessary to explain the effects we see in the Universe. Dark matter is thought to be the “stuff” that binds stars and galaxies together, while dark energy exerts a repulsive force driving galaxies further and further apart from each other. Understanding this universal “tug of war” could reveal the ultimate fate of our Universe.
The AMS-02 will be helping in the search for Dark Matter from the unique environment of space.
Dark Matter is unlike anything we have encountered on Earth. It doesn’t emit or reflect light and yet its presence seems to be evident everywhere we look in space.
When we observe certain objects in our universe they behave as though there must be a lot more mass there than we can detect. For example, spiral galaxies spin faster than they should, and clusters of galaxies stick together even though the velocities of their constituent galaxies suggest they should be flying apart. Dark matter also reveals itself when its gravitational mass causes the trajectories of light from stars to bend making the positions of the stars seem to differ from their “true” positions.
Scientists have answered this problem by proposing the existence of some hidden mass – “dark matter”. This solution is widely accepted despite the fact that nobody has ever seen it and nobody is quite sure what it might be. You could say dark matter has been indirectly observed but never captured. The hunt for a dark matter particle continues to accelerate and has become a Holy Grail of particle physics research.
Physicists do have some suggestions as to what form this “missing matter” might take. One candidate you may have heard of is the WIMP or “Weakly Interacting Massive Particle.” These are a class of particles that, as their name suggests, interact via the weak nuclear force and have mass. A popular WIMP is the neutralino. More on that later.
To add to the WIMPS, we also have the possibility of MACHOS (Massive Compact Halo Objects). These objects are less confusing because they are made up of protons and neutrons just like you and I. The term “halo” just refers to the region of a galaxy in which we tend to find these dim/dark objects. MACHOs can be any non-luminous stellar object, including black holes, neutron stars and brown dwarfs.
Because they have mass, MACHOs can be detected using gravitational lensing – a method that looks at how light coming from a distant star has been bent and distorted by an object. Using this technique, limits have been set on the mass of individual MACHOs in our own galaxy, the Milky Way and it has been found that these objects make up no more than a few percent of the mass in our galactic halo. This is not nearly enough to make up the amount of dark matter we know must be present.
So it seems that dark matter must be mostly made up of matter that is very different from that which we are made.
The most popular hypothesis is that dark matter is made up of particles that were created way back in the very early universe and that are stable enough to have made it to this day. The best candidate particle are the neutrino and it’s superpartner, the “neutralino” mentioned earlier.
Neutrinos are abundant and were created in the early universe in numbers similar to electrons and positrons. For a long time we thought that neutrino did not have any mass but it turns out that they do have a very small mass. Although they are difficult to detect because they travel at the speed of light and do not have an electric charge, we know that they really exist . You can imagine that is a relief amongst all this unknown. Billions of neutrinos are currently bombarding your body but don’t be concerned – you won’t feel a thing!
But there’s a problem. Neutrinos are known as “hot” dark matter because they travel so fast. WIMPS on the other hand did not travel at relativistic (fast) speeds in the early universe and we say they are an example of “cold” dark matter
Why does this matter? Early in the universe there must have been fluctuations in the matter density (the way matter was distributed). Otherwise we would not be able to explain the formation of galaxies (clusters of matter). This isn’t a problem if we consider cold dark matter but hot dark matter would have acted similarly to an iron – smoothing out any fluctuations. This would make it impossible to explain the formations of the structure and galaxies we see today. Again we are stuck with a limit on how much dark matter could be made up of neutrinos.
What next? We have one more candidate so far and that is the axion. But these again are “hypothetical” particles and opinion is highly divided on whether they exist at all.
Indeed, we are left with the WIMP being the favoured constituent of dark matter – and the favourite WIMP is the neutralino. Now, we just need to detect it – and this is where the AMS-02 comes in. If neutralinosexist, they could collide with each other producing excesses of charged or neutral particles which can be detected by AMS-02. Any peaks in the background positron, anti-proton, or gamma ray flux could signal the presence of neutralinos or other dark matter candidates. Two previous cosmic ray experiments: PAMELA (an orbiting satellite experiment launched in 2006) and ATIC (a balloon-borne experiment) have given some evidence that these peaks will be found at energies that the AMS-02 instrument is able to detect.
The Search for Antimatter
According to today’s best theories, when the Big Bang occurred about 15 billion years ago, matter and antimatter were created in equal amounts.
What do we mean by antimatter? Antimatter, as the name implies, can be described as the opposite of ordinary matter. Every particle in the universe has characteristics such as mass and charge. With antimatter, the mass remains constant, but the sign of the charge is reversed. An example is the positron – or positive electron. The positron has the same mass as an electron but a positive charge.
It is therefore surprising that our Earth, the solar system, and our galaxy (the Milky Way) do not contain any antimatter. Where did all the antimatter go?
Theories which attempt to explain the present matter-antimatter “asymmetry” (meaning we see enormously more matter than antimatter) are not compatible with many other measurements. Whether or not antimatter still exists in the Universe is a fundamental question in modern physics and cosmology.
Two possibilities exist: either antimatter completely disappeared during the history of universe, or matter and antimatter have been separated from each other to form different regions of the universe. In the second scenario, we would be located in a region where only matter exists but some antimatter coming from an ‘anti’ region outside our galaxy could still have a chance to reach us. This antimatter would be in the form of anti-nuclei (like anti-Helium, anti-Carbon, etc.). The best way to search for this extragalactic antimatter is to use a particle detector in space – the AMS-02.
The AMS-02 will try to detect and prove the existence of this antimatter. Observing just one antihelium nucleus would provide evidence for the existence of large amounts of antimatter somewhere in the Universe. The sensitivity of the instruments aboard the AMS will allow a larger volume of the Universe than ever before to be tested for the existence of antimatter.
Don’t forget about strangelets!
As if all this Dark matter and antimatter wasn’t strange enough already, the AMS-02 will also be searching for something physicist call a “strangelet”. To understand this we need to know a little about particle physics and quarks.
In the 1960’s Murray Gell-Mann hypothesized the existence of “quarks” as one of the fundamental building blocks of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.
There are six types of quarks: up, down, charm, strange, top, and bottom. Up and down quarks have the lowest masses of all quarks. Stable matter is made up of two types of quarks: the “up” quark and the “down” quark.
Heavier quarks rapidly change into up and down quarks through a process of particle decay. Because of this, up and down quarks are generally stable and the most common in the universe, whereas charm, strange, top, and bottom quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators).
It is however possible that there may be stable matter made up of “up”, “down” and the “strange” quark. Physicists have called such particles “strangelets” and they are yet to be detected. Strangelets can have extremely high mass and a small charge to mass ratio. The AMS-02 could give us a definitive answer as to whether this new and extraordinary type of matter exists!