One of the pillars of cosmology is that the universe is isotropic, that is, the same in any direction and place we look. But a new study suggests something else, and that may compel astronomers to review their understanding of the cosmos. According to the research, the rate of expansion of the universe can vary from one place to another.
The universe expands continuously from the Big Bang, at an ever-increasing rate, and researchers use something called the Hubble Constant to determine speed. With equations based on Einstein's general theory of relativity, they concluded that this expansion is isotropic, as explained by Konstantinos Migkas, the study's lead author.
It turns out that observations of radiation called the cosmic microwave background (CMB), which is something reminiscent of the Big Bang, give astronomers and cosmologists this notion that the expansion is isotropic. Thus, they conclude that this property found in the early universe is also valid for the current cosmos, almost 14 billion years later. But that conclusion may not be so correct.
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One of the factors that make things more complicated is the fact that dark energy, which seems to “spin the gears” of this expansion, is something important to understand the evolution of the universe in the last 4 billion years – but we cannot see it or measure it. Without properly understanding dark energy, astrophysicists may end up missing some details. "Therefore, to assume that it is isotropic is almost a leap of faith for now," wrote Migkas.
The new research studied 842 galaxy clusters, the largest gravitational structures in the universe, using data collected by space telescopes. They calculated the temperature of each cluster by analyzing the X-ray emissions from the huge hot gas fields within them. With this information, they estimated the luminosity inherent in X-rays, without having to consider cosmological variables, such as the rate of expansion of the universe.
Then, the researchers calculated the luminosity of the X-rays for each cluster with an approach that requires taking into account the expansion of the universe. This revealed expansion rates that do not match everywhere these clusters are located. "We were able to identify a region that seems to expand more slowly than the rest of the universe, and one that seems to expand faster!" Said Migkas.
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According to Thomas Reiprich, co-author of the study, the team saw that “clusters with the same properties, with similar temperatures, seemed less bright than expected in one direction of the sky and brighter than expected in another direction. The difference was quite significant, around 30%. These differences are not random, but they have a clear pattern, depending on the direction in which we observe in the sky. "
However, nothing can yet be stated with any certainty. This result may have other explanations, such as some of the galaxy clusters being pulled gravitationally by other clusters, giving the illusion of a different expansion rate. But the new study investigates groups up to 5 billion light years away, and we still cannot say whether over such great distances these gravitational "pulls" can alter the forces of expansion in this way.
In any case, the research team warns that if this difference in the expansion rate is real, we may discover unexpected things about the universe. For example, perhaps the dark energy itself varies from place to place throughout the cosmos. But before rethinking the current cosmological model, new observations will be needed to prove the results of the new study.