To the dark side

How would you feel if I told you that we can only identify 5% of the energy and matter in the universe? The atoms in your body are made up of the same substance as the ground beneath your feet and the stars in the galaxies, and all of this can be fully, beautifully and precisely described by the Standard Model of particle physics. The celestial bodies spread across the night sky are visible because they interact electromagnetically as light bounces off them and into our eyes or telescopes, through one of the three forces encoded in the Standard Model. But for all the success of that theory, we are still pretty much stumped as to the nature of the other 95% of the stuff out there.

The remaining mass (and don’t forget that mass = energy thanks to Einstein’s famous and very important formula) around us is made up of dark matter (about 27%) and dark energy (roughly 68%). These names sound mysterious and conjure up images of super villains with nefarious plots, but ‘dark’ just means ‘not interacting with light’. Dark matter was first hypothesised by Fritz Zwicky in the 1930s, but the concrete evidence was given by Vera Rubin in the 1970s using galaxy rotation curves. Spiral galaxies have most of their visible mass concentrated at the centre, and as they spin we would expect the stars and planets closest to the dense centre to spin faster than those on the outermost layers. Vera Rubin showed that this isn’t the case - the rotation of the visible mass within the spiral galaxy is roughly constant with distance from the centre. This can be seen in the plot below (credit Queen’s University).

The behaviour of the rotation curves can be explained with dark matter halos; invisible matter which interacts gravitationally, evenly distributed to hold the galaxy together and providing a large dollop of extra mass to pull the outermost stars and planets around. Here’s an animation. Helpfully, light also interacts with gravity; it bends as it travels past massive objects. Using this, astronomers have been able to map out regions of dark matter in the universe and measure exactly how much is out there. As for dark energy, this name is given to the phenomenon by which the universe is expanding, and fast. This can be accounted for in the cosmological constant, denoted by the Greek letter lambda, which inserts itself in describing how the fabric of space is pulling apart - but let’s not venture down that road, and return instead to dark matter.

Space may seem empty, but it’s really chock-full of activity. Take, for example, neutrinos (or little neutral ones), extremely light particles featuring in the Standard Model; each second, billions of neutrinos originating from the sun pass through every square centimetre of your body. They interact weakly (and here I actually mean via the weak force, the second of the three forces, which happens to be not very weak at all in some cases) and via gravity, but your body does not detect them. There were suggestions that neutrinos could account for dark matter, but that has been ruled out. Just like the neutrinos, dark matter is probably passing through your body right now. If only it weren’t so antisocial.

The evidence for the existence of dark matter is based on gravitational interactions; what if gravity is just wrong, you ask? Some people are working on modified theories of gravity, but since the Ligo experiment first detected gravitational waves (actual ripples in space time due to colliding black holes billions of light years from Earth - I know) which perfectly fit Einstein’s theory of relativity, it’s becoming less likely that our description of gravity is to blame. The suggestion that dark matter is made of black holes has also been largely ruled out; at least, it cannot be the only explanation. This leads to the point that dark matter may not be one single type of particle - maybe it’s a bunch of different things, all of which are indifferent to the photons carrying the electromagnetic force.

There are three ways we can detect dark matter; indirect detection measuring the byproducts of that dark matter colliding in space, direct detection measuring tiny momentum transfers due to collisions with nuclei on earth, and through experiments at CERN, where we smash protons together in the Large Hadron Collider with (hopefully) enough energy to produce new particles (more on this in a separate post), and look for missing energy in our collisions. We haven’t yet spotted anything concrete through any of these avenues. I’m currently working on a theory of heavy dark matter which interacts only with the top quark. I’ll update this post once the results are public, but the way we check those theories is as follows. First, we write down the interactions our candidate (let’s call it S) has with ordinary matter. It needs to be stable (it cannot decay to anything), so we need to include a mechanism that makes that true. Then we run our theory through a simulation that predicts how much dark matter (the relic density) there would currently be in the universe should the theory be true. This is done by solving the Boltzmann equation. If our theory predicts too much dark matter, it’s back to the drawing board, but if the relic density for S is equal to or below the measured quantity in the universe, we are in business. Then we check our theory against current bounds from each of the three experimental avenues, to see whether it has already been excluded. Every time a theory produces a result that has already been ruled out at experiment, we can cross it off the (very long and ever increasing) list. (Edit 28/04/2021: see our recent paper, in which we studied a heavy dark matter candidate coupling only to the top sector, here.)

The hunt for dark matter is one of the foremost quests in particle physics at the moment, and it’s one of the main reasons we know we are not yet ‘done’. So far, we are doing well in figuring out all the things it isn’t. Hopefully, this will help us in soon discovering what it is.