The cornerstone of our understanding of the cosmos is that the universe is expanding at the same rate in all directions. If so, this would make it "isotropic" as whichever direction we look, the universe is the same. If the universe is instead different in various directions, it would be called "anisotropic" – a finding that would force researchers to rethink a lot of cosmology. This idea is now being put forward in a new hotly debated study published in Astronomy & Astrophysics.
The team investigated the X-ray emissions of extremely hot gas present in 842 clusters of galaxies. The assumption is that clusters with similar temperatures at similar distances should appear equally bright in the sky, but the researchers did not observe this to be the case.
"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," co-author Thomas Reiprich, from the University of Bonn, said in a statement. "The difference was quite significant, around 30 percent. These differences are not random but have a clear pattern depending on the direction in which we observed in the sky."
Assuming that the difference observed is real, there are two possible scenarios for why: either something is wrong with how we have measured the properties of celestial objects using X-rays or the universe doesn't behave like we believe it to.
"If the Universe is truly anisotropic, even if only in the past few billion years, that would mean a huge paradigm shift because the direction of every object would have to be taken into account when we analyse their properties," he said. "For example, today, we estimate the distance of very distant objects in the Universe by applying a set of cosmological parameters and equations. We believe that these parameters are the same everywhere. But if our conclusions are right than that would not be the case and we would have to revisit all our previous conclusions."
While the finding is intriguing, the sample size is small and more observations are needed to explain what’s happening. It is important to note that if the anisotropy is real, it would not just affect X-ray measurements but would be detectable across the board. The isotropy of the universe is one of the crucial results in observations of the cosmic microwave background, the first light that shone in the universe, often referred to as the echo from the Big Bang. This light is an imprint of how the universe started as well as how it has changed over the last 13.8 billion years. It is not the last word on the true nature of the Universe, but researchers believe that if the universe had a strong anisotropy as suggested in this study, it should have been observed in the latest CMB detections.
Researchers hope that upcoming observation campaigns aimed at imaging billions of galaxies to understand the nature of the expansion of the Universe will get to the bottom of it.