Our universe may not be expanding at the same speed in all directions, according to a new study, challenging one of our basic ideas about the universe.
The assumption that the universe is isotropic – or the same in all directions – has underpinned the rest of our understanding of where the cosmos came from, and where it may be going. Researchers are almost certain that at least at its beginning, the universe was expanding uniformly.
But that assumption might be wrong, at least in the universe of today, according to a new study using data from Nasa and European Space Agency observatories. Researchers using that information found that the different parts of the universe are actually expanding at different rates, with clusters of galaxies in different parts of the sky behaving differently.
Our usual understanding of the universe suggests that after the Big Bang, the universe began to expand in all directions, with galaxies and galaxy clusters moving apart at the same rate across the cosmos. The new study suggests that might not be the case, with that rate actually varying depending on where we look.
"Based on our cluster observations we may have found differences in how fast the universe is expanding depending on which way we looked," said co-author Gerrit Schellenberger from the Harvard-Smithsonian Center for Astrophysics. "This would contradict one of the most basic underlying assumptions we use in cosmology today."
Researchers have long attempted to find a definitive answer to whether the universe is in fact the same in all directions. They have attempted to measure it using a variety of different methods, including looking at galaxies through infrared and watching exploded stars, but nothing has proven decisive and the results have given indications both ways.
To find their measurements in the new study, researchers used a new technique to try and measure the expansion of the universe. They looked at more than 800 galaxy clusters using the new technique, in an attempt to understand at precisely what rate it they are expanding at.
Firstly, they worked out the amount of X-rays a given galaxy cluster would be emitting, in a way that gives a constant answer independent of changes such as the expansion speed of universe. Taht relied on the relationship between the temperature of the hot gas in a galaxy cluster and the amount of X-rays, or its X-ray luminosity.
Secondly, they worked out X-ray luminosity using a different method that did in fact rely on the universe's expansion speed. Those numbers showed that the expansion speed was not actually uniform, and that the universe is moving away from us at different speeds in different parts.
It is not the first time that researchers have found evidence of the uneven expansion of the universe, but may be the one that shows it more convincingly than any other.
"This is a hugely fascinating result," said Norbert Schartel, a project scientist on ESA's XMM-Newton observatory, which helped contribute towards the discovery. "Previous studies have suggested that the present Universe might not be expanding evenly in all directions, but this result – the first time such a test has been performed with galaxy clusters in X-rays – has a much greater significance, and also reveals a great potential for future investigations."
One explanation for this unusual discovery is that the universe's expansion may be uniform but that some galaxies are being pulled away by something else, such as the gravity of other galaxy clusters. But this may be unlikely given that scientists expect the universe's expansion to be the main deciding factor of that speed.
If that is not the case, the research suggests that the universe is not actually isotropic, and that it is different in different directions. Dark energy, for instance, could be distributed differently throughout the universe and could be causing unusual results.
Researchers have described the expansion of the universe being like a loaf of raisin bread that has been placed in the oven: as it cooks, it expands, and the raisins that represents the galaxies move away from each other. If the bread is evenly mixed, the expansion would be uniform, but the latest results suggest that there may be an overlooked ingredient in the dough.
"This would be like if the yeast in the bread isn't evenly mixed, causing it to expand faster in some places than in others," said co-author Thomas Reiprich, also of the University of Bonn. "It would be remarkable if dark energy were found to have different strengths in different parts of the universe. However, much more evidence would be needed to rule out other explanations and make a convincing case."
The study is published in the latest issue of the journal Astronomy and Astrophysics and can be read online.
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