The Bullet Cluster

Chris Lintott ... 17 October 2006

One of the fundamental motivations behind the writing of Bang! was that - for the first time in many decades - most of the observational tests astronomers can think of produce results which agree with a single model of how the Universe evolves. In the trade, this has become known as the 'concordance cosmology', and we concentrated on telling that story alone. (We did chicken out when it came to selecting from the many possible ends of our Universe, but that's another story altogether) However, you shouldn't get the idea that the world's cosmologists are now comfortably resting on their laurels, perhaps with a nice cup of tea (or something stronger); the model is deeply unsatisfactory, relying as it does on two as yet unknown components of the Universe which we label dark matter and dark energy. Any theory which can dispose of the need for such mysterious actors on the Universe's stage therefore deserves to be taken seriously.

The most promising theories fall under the general banner of 'MoND', or Modified Newtonian Dynamics, replacing the need for dark matter with tweaks to our theory of gravity. Mond has achieved major success in the past, accounting for the most of the evidence for dark matter on scales around the size of a galaxy. However, the final nail in the coffin seemed to have been dealt early this summer, when observations of the galaxy cluster 1E 0657-56 - the Bullet Cluster - were released.

The pink region in the image is hot gas, glowing in X-rays detected by the NASA satellite, Chandra. The blue region represents our best estimate of where the matter in the cluster is, as traced by its effect on the light from the background galaxies. Essentially, the 'normal' matter, represented by the x-ray radiation, is at the centre of the object, while the total mass is much more spread out. This is exactly what we would expect if the majority of the mass in the cluster was in the form of Cold Dark Matter.

The Bullet cluster is actually two galaxy clusters which have recently collided. (see the animation, here ) As they passed through each other, the gas in each cluster interacted, producing a drag force similar to air resistance, and slowed down. This is how normal matter behaves. However, cold dark matter only interacts with gravity, and so the dark matter in the clusters is able to move ahead of the gas, producing the shape of the Bullet seen today.

So is this the final nail in the coffin for MOND? It is certainly difficult to explain with a purely MONDian theory. However, as Garry Angus from the University of St Andrews explained at a recent seminar, it may only be 'a nail - not yet the final one'. The latest theories seem to be able to account for the observations of the Bullet cluster, but only if another form of dark matter is introduced. We need not only to alter gravity, but also to introduce dark matter in the form of massive neutrinos. Three types of neutrino are known; it is just about possible that further, heavier, types exist. However, the forthcoming KATRIN experiment will test this possibility.

Meanwhile the message of the Bullet Cluster is that although arguments continue as to what form of dark matter is necessary, it is clear that even modifying gravity doesn't eliminate it entirely.

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