Dark Matter Due for Discovery?

The Large Hadron Collider (LHC) at CERN will be up and running again soon following its winter shutdown. As well as seeking the Higgs Boson, scientists are also hoping that the accelerator will provide insights into the nature of dark matter.

In my last post, Torsion and the Matter-Antimatter Asymmetry, I discussed a recent article which concluded that dark matter might consist of antiparticles left over from the Big Bang.  But this is only one of many theories.  What follows is an introduction to dark matter and a brief summary of the theories that have been proposed to answer to the question: what is dark matter?

Introduction to Dark Matter

The concept of “dark matter” was introduced in 1934 by Fritz Zwicky. By making measurements of the motions of galaxies on the edge of a cluster, he calculated that the total mass of the cluster was 400 times greater than the amount that he observed.

The problem was taken up by astronomers Vera Rubin and Kent Ford at the Carnegie Institute during the 60s and 70s. They measured the velocity curve of spiral galaxies and produced results that also seemed to imply the presence of a large amount of non-visible mass.

The Evidence for Dark Matter

Astronomical observations clearly show that most of the stars in a spiral galaxy lie in a rotating circular plane, with the density of stars being much greater towards the centre of the plane than further out. The stars in the galaxy are kept together by the gravitational attraction between them.

F is the gravitational force,

m1 and m2 are the masses

of the two bodies, r is the

distance between them,

and G is a constant

As stated in Newton's famous gravitational law (shown to the right), the gravitational attraction between two bodies decreases as the distance between them increases. This means that, if most of the mass of the galaxy is located in its centre, then a star orbiting at the edge of the galaxy should experience a weaker gravitational pull than a star that is close to the dense galactic core.

The speed at which a star orbits depends on how much gravitational force is exerted on it by the rest of the galaxy: a star orbiting too slowly will gradually spiral in towards the centre, and a star that orbits too fast will spiral outwards as the gravitational force is not strong enough to keep it reined in. Therefore, we would expect stars that are further out to have lower velocities than those close to the centre. This expectation is plotted on the graph below as curve A. However, Rubin and Kent's results (curve B) show that the velocity of the stars is about the same for all distances from the centre.

There are two possible explanations for this. The first is that the theory of Newtonian gravity, despite its success in predicting the orbits of planets in our own solar system, is flawed. It has been suggested that Newton's law of gravity should be modified to better fit the galaxy rotation curves. This hypothesis is known as MOND – Modified Newtonian Dynamics. Although MOND is very successful in predicting galaxy rotation curves (as one would expect, since this is the situation it was designed to model), it is less good at predicting effects at the larger scale of galactic clusters.

The more popular explanation is dark matter theory: the idea that there is more mass in galaxies than what we can see through our telescopes. The problem is that no-one knows what this dark matter actually is. It was first thought that it might consist of non-luminescent objects such as black holes and dead stars such as brown dwarfs or neutron stars, known collectively as MACHOs – Massive Compact Halo Objects. However, it is now thought that the majority of dark matter is in the form of WIMPs – Weakly Interacting Massive Particles.

But what are WIMPs?

Particle physics has suggested many candidates for WIMPs. Any particle that makes up dark matter would have to be stable – i.e. it must not decay into other particles. Neutrinos, which rarely decay or interact with other particles, have been suggested as a possible candidate.

Another possibility is the theoretical Lightest Supersymmetric Particle (LSP). Supersymmetry is a theory of physics which predicts the existence of a much heavier partner to each known particle. The lightest of these super-particles (nicknamed “sparticles”) is expected to be stable so would be a good candidate for dark matter.

Experiments are currently underway at the LHC to try to produce sparticles, and we could see results as soon as this year. Will dark matter finally reveal its identity?

The discovery of supersymmetry would have many exciting consequences for theoretical physics beyond the identification of dark matter... but that's the subject of a future post.