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Issue

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.

 

Background

Researchers 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.

 

Details

• 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.

• Researchers say that if the isotropy hypothesis was correct, the properties of the clusters would be uniform across the sky. But they 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.

• Before challenging the widely accepted cosmology model, which provides the basis for estimating the cluster distances, researchers first looked at other possible explanations such as 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.

• The scientists speculate this possibly uneven effect on cosmic expansion might be caused by dark energy, the mysterious component of the cosmos which accounts for the majority of its overall energy.

• ESA's upcoming telescope Euclid, designed to image billions of galaxies and scrutinise the expansion of the cosmos, its acceleration and the nature of dark energy, might help solve this mystery in the future.