More Issues In Cosmology: The Universe Appears To Be More Homogeneous Than Expected

NASA, ESA, H. Teplitz, M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

Our model of cosmology can’t seem to catch a break lately. Measurements of the expansion rate of the universe continue to be impossible to reconcile with each other and now new observations are shaking the model even more. The universe appears to be 10 percent more homogenous than previously expected.

By homogeneous, scientists mean that at a large scale the universe is more or less the same. Basically, if you were to zoom out far enough, it would be difficult to tell the difference between empty space and clusters of galaxies.

These kinds of observations require a huge amount of data. The Kilo-Degree Survey (KiDS) collaboration studied 1,000 degrees of extragalactic sky (roughly 5 percent of it), observing 31 million galaxies, from the local universe all the way to 10 billion-light years away.

In the standard model of cosmology, there is a battle between matter (both regular and dark) and dark energy. Matter has gravity and pulls things together, making the distribution of galaxies “clumpier”. Dark energy on the other hand is believed to be responsible for the accelerated expansion of the universe, so is pushing things away, countering to some extent the clumpiness.

A zoom-in on a part of the KiDS map, showing a patch of the universe approximately 1.5 x 1 billion light-years across. In this false-color image high-density regions (galaxy clumps) of the universe are shown in yellow, and low-density regions (empty space) in pink. They look about the same. Credit: B.Giblin, K.Kuijken and the KiDS team.

Dark matter and dark energy remain hypothetical, but there is good reason to believe they exist as they can explain a lot of what we see in the universe. But perhaps they are slightly different from our best theories so far. By using their expected properties, the universe should be less clumpy than the KiDS data show, researchers reported in Astronomy & Astrophysics.

"The standard model of Cosmology relies on rather mysterious physics that we call dark matter and dark energy. Scientists have to test this remarkable model in as many ways as possible, and that is exactly what we are doing," Dr Marika Asgari, from the University of Edinburgh, who co-led the analysis, said in a statement.

While the standard model has successfully predicted many features we observed in the universe, the advancement of our observational and analytical capabilities are pushing at the seams of the theory.

"As an observing cosmologist, you try to remain impartial and make the measurements as accurate as possible without theoretical prejudices. One thing is clear, we live in exciting times!" Professor Hendrik Hildebrandt from Ruhr University Bochum added.

There is a 1 in 1,000 chance that the team observed a particularly unusual part of the universe. So more observations are necessary to strengthen these results. The full KiDS result will be published in 2022 and will be 30 percent larger, but more projects, like the Vera Rubin telescope and the Euclid satellite, will expand on this important work.  

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