Surprise: the expansion of the Universe may not be uniform

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Wherever we look, exactly the same rules are followed throughout the Universe. Countless calculations of astrophysics are supported by this basic principle. Now, however, a team of researchers from Bonn and Harvard Universities have questioned this “truth.” If their results, recently published in Astronomy & Astrophysics, are confirmed, many assumptions about the properties of the Universe in which we live would be literally shattered.

From the moment the Big Bang, the Universe is expanding, as a cake full of raisins would do inside a oven. And cosmologists believe that this constant increase in size occurs uniformly in all directions, as a good pastry would. Scientists call this “isotropy,” a hypothesis that has been supported by observations of cosmic background radiation (CMB), which is the direct remnant of the Big Bang and reflects the state of the Universe in its earliest days, when it was hardly 380,000 years old. The uniform distribution of the CMB in the sky suggests that, in those early days, the Universe was expanding rapidly and at the same speed in all directions. Although, according to the calculations carried out by Konstantinos Mikgas and Thomas Reiprich, from the University of Bonn, in the current Universe this may no longer be true.

For the first time, the team led by these researchers has tested the isotropy of the Universe with a new method that allows obtaining more reliable data than previous ones. And the results were most unexpected. Some areas of space, in effect, would be expanding faster than they should, while others, on the contrary, would be expanding more slowly than expected. “Our measurements,” explains Mikgas, “lead us to that conclusion.”

Mikgas and his colleagues have developed a new way to test isotropy. It is based on the observation of the galaxy clusters, which in a way would be the “raisins” of the sponge cake in the previous example. Those galactic groups emit X-ray radiation that can be captured by instruments from Earth, in this case by the Chandra and XMM-Newton orbital telescopes. The temperature of these clusters can be calculated based on certain characteristics of these cosmic X-rays and that, in turn, allows their brightness to be measured. The hotter the clusters, the brighter they will be as well.

In an isotropic Universe a very simple rule applies: the further away an object is from us, the faster it will be moving away. Therefore, from its speed we can deduce its distance, regardless of the direction in which the object is moving away. Or at least that’s what we thought until now. “Actually,” emphasizes Migkas, “our brightness measurements seem to disagree with the distance calculation above.”

The amount of light that reaches Earth from a distant object decreases as its distance increases. Therefore, knowing the original luminosity of any celestial body and its distance, we can know how bright the image we capture with the telescope should be. And that’s precisely where researchers have found discrepancies. Differences that are very difficult to reconcile with the isotropy hypothesis. In fact, the brightness of some galaxy clusters has turned out to be much weaker than expected, making their distance from Earth greater than that calculated from their speed. In other clusters, to top it all off, the exact opposite is true.

“There are only three possible explanations for this,” says Migkas. Firstly, the X-ray radiation we have measured may decrease in intensity on its way from the galaxy clusters to Earth. This could be due to clouds of gas and dust (which would slow it down) not yet discovered inside or outside the Milky Way. However, in the preliminary tests we found a discrepancy between our measurement and the theory, and not only in X-rays, but also in other wavelengths. It is extremely unlikely that any type of matter nebula will absorb completely different types of radiation in the same way. We were not sure of this until after several months.

The second possibility is the so-called “mass flows”. They are groups of clusters of neighboring galaxies that move continuously in a certain direction, for example, because of some structures in space that generate a strong gravitational pull. Those structures, therefore, would attract galaxy clusters to themselves, altering their speed (and also their derived distance). “This effect,” Migkas clarifies, “would also mean that many calculations about the properties of the local Universe would be accurate and should be repeated.”

But the third possibility is the most serious: What if the Universe were not isotropic? What if, metaphorically speaking, the “yeast” in our raisin cake was so unevenly distributed that it would bulge at some points while others would barely grow dough? Such “anisotropy” could, for example, be a consequence of the properties of the mysterious “dark energy”, which acts as an additional driving force in the expansion of the Universe. In fact, there is still no theory that makes the behavior of dark energy consistent with observations. “If we are successful in developing such a theory,” says Migkas, “we could greatly accelerate the search for the exact nature of this form of energy.”

The study carried out by Migkas and his team is based on data from more than 800 galaxy clusters, 300 of which were directly analyzed by the authors, while the rest comes from previous studies. Only the X-ray analysis was so thorough and demanding that it took several months of work. Now, the study’s authors hope that the new satellite-based X-ray telescopes eROSITA and Euclid from the European Space Agency will be able to record several thousand additional galaxy clusters in the coming years.

Only then will we be clear whether the isotropy hypothesis, and all that it entails, should be abandoned. .

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