Astronomers are puzzled by the announcement that the masses of the largest objects in the Universe appear to depend on which method is used to weigh them. New work was presented at a specialist discussion meeting on ‘Scaling Relations of Galaxy Clusters’ organised by the Astrophysics Research Institute (ARI) at Liverpool John Moores University, supported by the Royal Astronomical Society.

Clusters of galaxies are the largest gravitationally bound objects in the Universe containing thousands of galaxies like the Milky Way. Their weight is an important probe of their dark matter content and evolution through cosmic time. Measurements used to weigh these systems are  carried out in three different regions of electromagnetic spectrum: X-ray, optical and millimetre wavelengths, giving rise to significantly different results.

Eduardo Rozo, from the University of Chicago, explains that any two of the measurements can be made to fit easily enough, but that always leaves the estimate using the third technique out of line.

Dubbed the ‘Axis of Evil’, it is as if the Universe is being difficult by keeping back one or two pieces of the jigsaw and so deliberately preventing us from calibrating our weighing scales properly.

More than 40 of the leading cluster astronomers from UK, Europe and the US attended the meeting to discuss the early results from the Planck satellite, currently scanning the heavens at millimetre wavelengths, looking for the smallest signals from clusters of galaxies and the cosmic background radiation in order to understand the birth of the Universe.

Planck measurements were compared with optical cluster images from the Sloan Digitised Sky Survey (SDSS) as well as the  new X-ray observations from the XMM-Newton satellite.

ARI astronomers are taking a leading role in this research through participation in the X-ray cluster work and observations of the constituent galaxies using the largest ground-based optical telescopes.

One possible resolution to the ‘Axis of Evil’ problem discussed at the meeting is a new population of clusters which is optically bright but also X-ray faint.

Dr Jim Bartlett (Univ. Paris), one of the astronomers who presented the Planck results, argues that the prospect of a new cluster population which responds differently was a ‘frightening prospect’ overturning age old ideas about the gravitational physics being the same from cluster to cluster.