Astronomers have assumed for decades that the Universe is expanding at the same rate in all directions. A new study based on data from ESA’s XMM-Newton, NASA’s Chandra and the German-led ROSAT X-ray observatories suggests this key premise of cosmology might be wrong.Konstantinos Migkas, a PhD researcher in astronomy and astrophysics at the University of Bonn, Germany, and his supervisor Thomas Reiprich originally set out to verify a new method that would enable astronomers to test the so-called isotropy hypothesis. According to this assumption, the Universe has, despite some local differences, the same properties in each direction on the large scale.Widely accepted as a consequence of well-established fundamental physics, the hypothesis has been supported by observations of the cosmic microwave background (CMB). A direct remnant of the Big Bang, the CMB reflects the state of the Universe as it was in its infancy, at only 380 000 years of age. The CMB's uniform distribution in the sky suggests that in those early days the Universe must have been expanding rapidly and at the same rate in all directions.In today's Universe, however, this may no longer be true.“Together with colleagues from the University of Bonn and Harvard University, we looked at the behaviour of over 800 galaxy clusters in the present Universe,” says Konstantinos. “If the isotropy hypothesis was correct, the properties of the clusters would be uniform across the sky. But we actually saw significant differences.”The astronomers used X-ray temperature measurements of the extremely hot gas that pervades the clusters and compared the data with how bright the clusters appear in the sky. Clusters of the same temperature and located at a similar distance should appear similarly bright. But that is not what the astronomers observed.“We saw that clusters with the same properties, with similar temperatures, appeared to be less bright than what we would expect in one direction of the sky, and brighter than expected in another direction,” says Thomas. “The difference was quite significant, around 30 per cent. These differences are not random but have a clear pattern depending on the direction in which we observed in the sky.”Before challenging the widely accepted cosmology model, which provides the basis for estimating the cluster distances, Konstantinos and colleagues first looked at other possible explanations. Perhaps, there could be undetected gas or dust clouds obscuring the view and making clusters in a certain area appear dimmer. The data, however, do not support this scenario.In some regions of space the distribution of clusters could be affected by bulk flows, large-scale motions of matter caused by the gravitational pull of extremely massive structures such as large cluster groups. This hypothesis, however, also seems unlikely. Konstantinos adds that the findings took the team by surprise.
