For 80 years, we have been aware of a substance in the universe known as dark matter. This mysterious stuff explains a lot about the universe, such as the movements of stars and the rotational speed of galaxies. We can also see the affects of dark matter in gravitational lensing, giving us a way of ‘mapping out’ dark matter densities across the universe. What is it? Nobody knows. It’s one of the greatest mysteries in astrophysics. It consists of about 80% of the mass of the universe and seems to behave gravitationally like everything else, but we haven’t a clue what it is.
Theories are constantly being proposed to explain what dark matter is. Personally, my favorite explanation for dark matter (simply because it’s cool) is that dark matter is actually gravity leaking between different dimensions. So, dark matter halos could be said to be galaxy-like objects – or at least very large, gravitationally bound objects – in other dimensions. As far as I’m aware, such a theory is currently untestable, so this new explanation ranks a little higher in the realm of being a good hypothesis (plus, it was too good to pass up).
Physicists have been looking at the standard model of particle physics, specifically supersymmetry, for explanations of dark matter for several years now. Supersymmetry is an idea that elementary particles with a spin of one have a corresponding heavy superpartner whose spin differs by half. Scientists have yet to discover any direct evidence for supersymmetry, but it’s interesting that supersymmetry continuously pops up as a solution to many of our problems.
In this particular case, Are Raklev of the University of Oslo proposes a hypothesis that both explains dark matter and gives us a method to discover particles making up dark matter experimentally. This is where gravitinos come in.
Now, a gravitino is the superpartner of the graviton – the force carrier of gravity. That is a lot of ‘hypothetical’ happing right there, Raklev puts it best when he says, “the gravitino is the hypothetical, supersymmetric partner of the hypothetical particle graviton, so it is also impossible to predict a more hypothetical particle than this.” He goes on to say, “A graviton is the particle we believe mediates gravitational force, just like a photon, the light particle, mediates electromagnetic force. While gravitons do not weigh anything at all, gravitinos may weigh a great deal. If nature is supersymmetric and gravitons exist, then gravitinos also exist. And vice versa. This is pure mathematics.”
So, what’s the catch? Of course, now you’re probably thinking ‘wait, they are talking about using a hypothetical symmetry for the hypothetical superpartner of a hypothetical particle and that isn’t the catch? This must be a trick question.’ Nope, this isn’t a trick question. The catch is physicists are unable to demonstrate the relationship between gravitinos and gravitons (you know, by using math) without unifying the forces of nature. That’s right, now enters grand unification.
Scientists are already three-quarters the way there. In the mid twentieth century, electricity and magnetism were unified to form the electromagnetic force. Likewise, in the 70s, electromagnetism was unified with the strong and weak nuclear forces forming the standard model. Now, we just need to unify the standard model with gravity and we’ll be able to describe every possible (and imaginary) interaction between any possible particle in nature – no pressure. This theory is referred to as Grand Unification Theory or The Theory of Everything (though, usually the latter).
In order to achieve unification, we need to understand how gravity works at the quantum level. This basically means we need to find the graviton a home within the atomic nucleus. Of course, finding the graviton would be a big help.
According to Raklev, “Supersymmetry simplifies everything. If the ToE Theory exists, in other words if it is possible to unify the four forces of nature, gravitinos must exist."
So, when I started, I said that this theory was testable, you’ve either forgotten that by now or you’re wondering how you manage to test the existence of a hypothetical superpartner of a hypothetical particle in a hyperthetical state of nature called supersymmetry when you first need a hypothetical theory to unify the forces of nature. The cool thing about math is we can still test this theory, even with all of that hypotheticalness.
Shortly after the big bang, you basically had a large soup of particles that were colliding with each other. When gluons (the carriers of the strong nuclear force) collide with other gluons, they emit gravitinos. This is when and how most of the gravitinos in the universe were created.
For a long time, physicist predicted there were too many gravitinos – this was a problem. So, they attempted to create models and theories which didn’t require the existence of these troublesome particles. By unifying supersymmetry and dark matter with gravitinos, we create a situation where dark matter is not stable; it simply has a long life span (in contrast to other models which say dark matter has no lifespan). In a universe where dark matter has a lifespan, in means the particles that make up dark matter (in this case, gravitinos) must eventually be converted into something else – either by colliding with each other or by decaying.
Enter the experiment. If gravitinos collide, they should convert to photons or antimatter. Unfortunately, it seems like gravitinos either don’t collide or collide very, very rarely. The second observational part of this experiment is to observe the decay of gravitinos. Using mathematical predictions, when they decay these particles should emit a gamma ray. That has potentially been observed.
The Fermi-LAT space probe has picked up a ‘small, suspicious surplus of gamma rays from the center of our galaxy.’ These initial observations go on to give the model some credence since these numbers both match the hypothesis and are exactly what you would hope to see when gravitinos decay.
So, let’s get this right. We have a hypothetical system of symmetry, a hypothetical supersymmetric partner particle to another hypothetical particle, interacting with each other in a hypothetical unifying theory known as the Theory of Everything – yet we can collect observation evidence and potentially prove this theory, or at least give us some pretty strong evidence to believe it’s valid.
Isn’t science awesome?
Image explanation: The image shows all the gamma rays recorded by the Fermi-LAT space probe as a map of the entire universe. The red band through the middle of the image is radiation from our own galaxy. The centre of the galaxy is almost at the centre of the image. "It is here that a small surplus of gamma rays has been seen that one cannot immediately explain by the radiation one expects from ordinary matter. The observations may fit our dark matter models. This surplus of gamma rays is not visible to the eye, but can be found by a time consuming analysis of the data," says Are Raklev, who reminds us that the analysis is still a little uncertain.