Probing Dark Matter with Gravitational Lensing and Citizen Science


What is dark matter? We assume that there is more in the universe than visible matter like stars, gas and dust we can directly observe. Various indications on very different scales point to the existence of an invisible i.e. dark matter. In the year 1933 one of the first clues came from Swiss physicist Fritz Zwicky. According to his calculations the individual components of the Coma Cluster – a collection of galaxies – were moving way too fast. Adding up all the visible matter in the Coma Cluster, the resulting gravitational force would be 400 times too weak account for the paths of the galaxies on their orbits given their velocities. There are two possible explanations for this: either the gravitational laws are incorrect or there is more mass than we can see, i.e. dark matter.

If we assume that our gravitational laws are true (Einstein's General Relativity) and there is a special kind of matter that we cannot see because it neither radiates nor blocks light, how can we observe dark matter in order to try to understand it? We follow Zwicky’s example and observe the gravitational effect of this invisible matter. Einstein explained the effect of a mass not by means of a gravitational force but with a geometric argument: Let us imagine space itself being distorted by the presence of a mass, like a trampoline is distorted when a person stands in the middle. In school we explained motion as the effect of a gravitational force – the moon revolves around the earth because it is attracted by it. Using general relativity we can explain i.e. the orbit of the moon differently: it moves in a straight line, but the space in which it moves is distorted, turning the straight line into a circle.

If this is indeed the case, then mass also influences massless particle that fly by close enough, like a light beam. It would have to change its direction when flying through the edge of such a distortion. We call this effect a gravitational lens, because a mass can disperse light almost like an optical lens, a phenomenon that was first observed in 1919.

With the help of volunteers, we are trying to use this effect for our research – albeit on a somewhat larger scale. We are looking at the bending of light from a source around a galaxy. This effect is significant – the images it provides are spectacular. Unfortunately, these constellations are quite rare. We have to trawl through thousands of photos until we find something exciting. We are happy to have several volunteers helping us in this search in the SpaceWarps project. However, a few Citizen Scientists want to go further and want to know more: They assist in post processing the lenses they found in the SpaghettiLens project – where they create models of these lenses. These models represent new fascinating scientific data that can be published directly with only very little post processing. With their work, our fellow Citizen Scientists not only provide basic data for our dark matter research, but have become experts themselves. We consider our fellow Citizen Scientists more as co-workers and thus we publish scientific articles together.


Project members:


Prof. Dr. Prasenjit Saha (UZH), Rafael Küng (UZH), Philipp Denzel (UZH)

SpaceWarps / Zooniverse:

Dr. Phil Marshall (Stanford University), Dr. Aprajita Verma (University of Oxford), Dr. Anupreeta More (University of Tokyo) and others

Citizen Scientists:

Elisabeth Baeten, Claude Cornen, Christine Macmillan, Julianne K. Wilcox and others


Graphic artist:

Lazar Jeremic