the universe may not expand evenly in all directions

The study thereby brushes aside a long accepted theory, which may require us to revise our current cosmological model.

Astronomers largely agree that the cosmos continued to expand after the Big Bang. It means that the distance between galaxies in our universe is getting a little bit bigger every second. In addition, scientists have for decades assumed that the universe is expanding in all directions at the same speed. But a new study published in Astronomy & Astrophysics is now sweeping that long-accepted theory on the table.

Raisin cake
The expansion of the universe is best compared to a raisin cake. As the cake – representing the universe – bakes and expands in the oven, the raisins – representing cosmic objects such as galaxies – move away from each other. With an even mixture, the cake should expand in the same way in all directions, just like an isotropic universe. And astronomers have assumed this for decades.

Researchers decided to challenge the so-called “isotropic hypothesis” about the universe. Because is it actually true that the universe is expanding evenly in all directions? “We looked at more than 800 clusters in the current universe,” says researcher Konstantinos Migkas. “If the isotropic hypothesis is correct, the properties of the clusters would be the same everywhere. But we soon saw significant differences. ”

Four out of hundreds of clusters analyzed in the study. Image: NASA / CXC / Univ. or Bonn / K. Migkas et al.

The astronomers used X-ray temperature measurements of the extremely hot gas that penetrates the clusters and compared the data with how bright the clusters appear. Clusters with the same temperature and at a similar distance should – according to the theory – look equally clear. But that’s not what the researchers noted. “We saw that clusters with the same properties and with similar temperatures seemed much less bright in one direction than expected, while in the other they were much brighter,” said Thomas Reiprich. “The difference was quite big, about thirty percent.”

The researchers decided to look for possible explanations. Because perhaps unnoticed gas or dust clouds obstructed the view, making the clusters in a certain area appear darker. However, that turned out not to be the case. Some other causes that could invalidate the new study results were also investigated, but were dismissed. It means that the universe may be expanding unevenly. But what underlies this? The scientists are now speculating that possibly dark energy has a finger in the pie. Most of the universe may consist of this mysterious form of energy, although we don’t know much about it at the moment. All scientists assume is that dark matter is somehow responsible for accelerating the expansion of the universe. In this case, dark energy may be stronger in some parts of the universe than in others, which may cause differences in how fast the universe expands in certain directions.

A map of the speed of the expansion of the universe in different directions. Image: K. Migkas et al. 2020, CC BY-SA 3.0 IGO

The findings have major consequences. Because until now, researchers actually assumed by default that the universe is isotropic. “If the universe is truly anisotropic – if only in the past few billion years – it would have enormous consequences,” says Migkas. “When we analyze the properties of objects, the direction of each object must be taken into account. For example, today we estimate the distance to very distant objects in the universe by applying a series of cosmological parameters and equations. We believe that these parameters are the same everywhere. But if our findings are correct, that would no longer be the case, so we have to revise all our previous conclusions. ”

Future research will have to show whether this is really necessary. “The findings are really interesting, but the sample in the study is still relatively small to draw such far-reaching conclusions,” said René Laureijs of ESA. “If we really need to rethink the generally accepted cosmological model, we need more data.” And they may also be there. ESA’s upcoming telescope Euclid is designed to image billions of galaxies. This telescope will be able to more accurately investigate the expansion of the universe, the acceleration and the nature of dark matter. The telescope is scheduled to be launched in 2022. We are anxiously awaiting whether this telescope could help solve the great mystery in the future.

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