The "Cosmological Principle" states that the universe is homogeneous and isotropic, which means everything looks the same in all directions on a sufficiently large scale, including the expansion rate of the universe which should be the same in any direction. This assumption is a fundamental concept in modern cosmology. However, a latest study reveals that this assumption may not be true.

The research team estimated the X-ray luminosity of distant galaxy clusters from the temperature of their gas. On the other hand, they calculated the expected X-ray luminosity of the same galaxy clusters with the effect of the expansion of the universe. The rate of expansion of the universe can hence be found after comparison. By collecting data form galaxy clusters in multiple directions, the expansion rate of universe in different directions can be deduced.

The Big Bang theory, as the mainstream cosmological model widely supported by astronomers, indicates that the universe is expanding in all directions evenly, but the result of this study reveals that the expansion rate might be higher in some directions. However, more evidence is needed before calming further conclusion. If the expansion of the universe is indeed not the same in all directions, it will be a big challenge for cosmology, and we may even need to build a new cosmological model.

More information: https://www.nasa.gov/mission_pages/chandra/news/universe-s-expansion-may-not-be-the-same-in-all-directions.html