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Is the expansion of the universe the same everywhere? New research suggests that it is not, which could have huge implications for our understanding of the cosmos.
Scientists led by Konstantinos Migkas at the University of Bonn in Germany analyzed data on X-rays emitted by hundreds of galaxy clusters, revealing that the universe appears to be expanding differently in different parts of the sky. These results seem to verify other past results, which previously looked like abnormalities. And while it is not yet time to topple modern cosmology, scientists are already wondering if there is a paradigm shift on the horizon.
“It is a very exciting result, with great potential for future work,” Migkas told Gizmodo in an email. “It can lead to rethinking cosmology. But before we do that, we have to make sure we eliminate all other possible explanations. ”
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A cornerstone of modern theories for explaining the universe is that it is isotropic, or uniform in all directions, on the largest scales. This means that, barring any more specific interactions between objects, no matter where they are found, innate physical properties, such as the rate of expansion of the universe, are the same everywhere.
Given the importance of this assumption to modern cosmology, scientists have attempted to prove it in several ways. For example, the most distant radiation temperature that astronomers can see is surprisingly uniform, although there appears to be a slight irregularity, called anisotropy. Observations of a type of supernova used as a standard distance gauge have revealed both a uniform and slightly nonuniform universe, which appears to depend on which supernovae are included in the sample. Scientists have made many other observations using various probes, looking at visible light and infrared radiation from distant galaxies, gamma-ray bursts, or distant radio sources, again with varying results.
The team of scientists in this case compared the brightness and the temperature of the X-rays emitted by different galaxy clusters across the sky. These values, as well as the redshift or how much the color of the radiation has changed by the rate of expansion of the universe, are measurable without relying on assumptions about the properties of the universe. They can measure these properties across the sky, then calculate what properties the universe would need to have to produce the results we see with our telescopes. Research is published this month in Astronomy and Astrophysics magazine.
The data for the study comes from various X-ray telescopes, including ROSAT, XMM-Newton and the Chandra X-ray telescope, which observed 313 galaxy clusters scattered across the sky. When they did their analysis, they found that the expansion of the universe differed slightly at different points in the sky. For simplicity, imagine placing galaxies on an expanding watch face, where 12:00 and 6:00 move out faster than expected, while 3:00 and 9:00 move more slowly. than expected. And while the differences from an isotropic universe (aka uniform) weren’t that great if you looked at each individual point, the overall picture was much less likely to be due to chance alone.
Migkas told Gizmodo that the team is not yet ready to declare the universe anisotropic expansion. Perhaps the undetected gas and dust clouds absorbed some of the X-rays, making some of the groups look weaker than they should. Perhaps some of the galaxy clusters are moving together for reasons beyond the expansion of the universe. The team plans to improve its methods, such as taking measurements at other wavelengths of light than those used in this study, Migkas said.
A researcher who was not involved in the study, Professor Subir Sarkar of the University of Oxford, explained to Gizmodo that the results themselves are an interesting departure from expectations, but are more compelling when combined with results analyzing distant galaxies. radio stations, nearby infrared galaxies and distant supernovae. Looking at all those data together, it seems like the universe could really be anisotropic. However, he warned that this type of result could be sensitive to changes in the statistical analysis in the data or in which galaxy clusters are observed.
Meanwhile, the associate deputy director of the High Energy Astrophysics Division of the Harvard-Smithsonian Center for Astrophysics Alexey Vikhlinin tweeted that he was “willing to bet“ that the article is “junk science”. He told Gizmodo in an email to select cosmic objects based on luminosity, as was done in this sample, a well-known type of bias where the sample is brighter than the general population. He said that the document did not adequately correct this bias and that it could also have been the victim of other selection biases.
So is the universe expanding evenly or unevenly? For now, an isotropic universe is an assumption, and astronomers are still working to prove or disprove that assumption. If the universe were truly anisotropic, some of the central theories that govern it might require revision. But, as is often the case, more observations will be needed before we know the answer for sure.
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