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.
This animation starts with the sky as viewed by ESA’s Gaia satellite, which is mapping more than one billion stars in our own galaxy, the Milky Way. The sky is shown in the galactic coordinate system, with the centre of the Milky Way at the centre of the map, and the plane of the galaxy – where most of its stars reside – oriented horizontally across the map.
Much farther away, the animation shows hundreds of galaxy clusters, which are the largest structures in the Universe bound together by gravity, comprising hundreds of galaxies each and even larger amounts of hot gas and invisible dark matter. The galaxy cluster sample used in this study has been selected to avoid the area close the galactic plane, where the large density of foreground stars and gas blocks the view of the clusters in the background.
The astronomers measured the X-ray temperatures of hundreds of galaxy clusters across the sky and compared it to the clusters’ brightnesses. Whereas they expected clusters of the same temperature and located at a similar distance to appear similarly bright, they noticed that clusters tended to be less bright than expected in one direction of the sky than in all others.
Finally, the animation shows a map of the estimated rate of the Universe’s expansion in different directions across the sky, based on the galaxy cluster data. The rate of the Universe’s expansion, indicated in terms of the so-called Hubble constant, is shown in different colours, with purple hues indicating a slower rate and orange/yellow hues indicating a faster rate.
The direction in the sky where galaxy clusters appeared less bright is represented by the region shown in purple in this map. If confirmed, the result might challenge the isotropy hypothesis, which assumes that the Universe has the same properties in each direction on large scales. This possibly uneven effect on cosmic expansion might be caused by the mysterious dark energy.