The universe is nearly 10 percent more homogeneous than expected

There is something strange about our universe.

The latest KiDS map made with the OmegaCAM on ESO’s VLT Survey Telescope at Cerro Paranal in Northern Chile- shows that the universe is nearly 10 percent more homogeneous than the Standard Model of Cosmology (Λ-CDM) predicts.

The standard model of Cosmology relies on rather mysterious physics that we call dark matter and dark energy.

KiDS uses the galaxies to map the distribution of matter in the Universe. To create this new map, KiDS used 31 million galaxies that lie up to 10 billion light-years away, which means their light was emitted when our Universe was less than half its present age.

The galaxies were mapped through weak gravitational lensing, where the light from distant galaxies is slightly deflected by the gravitational effect of large amounts of matter, such as galaxy clusters. That effect is used to determine the “clumpiness” of the distribution of the galaxies.

Over time, the gravity of the matter in the Universe makes it less and less homogeneous; areas with a little more mass than average attract matter from their surroundings, so increasing the contrast. Meanwhile, the expansion of the Universe counteracts this growth. Both of these processes are gravity-driven and are therefore of great importance for testing the Standard Model of Cosmology (Λ-CDM), which reasonably accurately predicts how density variations increase with the age of the universe.

A zoom-in on a part of the KiDS map
A zoom-in on a part of the KiDS map, showing a patch of the Universe approximately 1.5 x 1 billion lightyears across. In this false-colour image high-density regions of the Universe are shown in yellow, and low-density regions in pink. The grey square shows the size of an individual KiDS image, with a full moon for scale. Over 1000 images make up the KiDS map. Credit: B.Giblin, K.Kuijken and the KiDS team.

The results show a discrepancy: the universe is nearly 10 percent more homogeneous than the standard model predicts.

Dr. Marika Asgari, from the University of Edinburgh, who co-led the analysis, calls the result “intriguing.” “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.”

The KiDS results may indicate small cracks in the standard model, just like another discrepancy in the expansion rate of the Universe, the so-called Hubble constant.

Dr. Tilman Tröster (University of Edinburgh): “The question is whether these can be solved with a small adjustment, for example, with a somewhat more complex behavior of dark matter than the simple hypothesis of totally inert ‘cold dark matter.'”

Leiden professor and KiDS lead Koen Kuijken cannot say whether this will eventually lead to a fundamentally different theory, for example, replacing Einstein’s General Theory of Relativity with a new one. For now, I consciously try to stay away from possible theoretical interpretations, and focus on the measurements and how to make them as accurate as possible.”

Prof. Hendrik Hildebrandt (Ruhr University Bochum) adds: “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!”

However, scientists will need more data to be sure about the results.

Dr. Benjamin Joachimi (University College London) said“There is a 1 in a 1000 chance that our findings could be caused by us just having looked at a particularly unusual part of the Universe.”

Scientists have described the KiDS-1000 result in five articles, the last three of which appeared online today. They have been submitted for publication in the journal Astronomy & Astrophysics.

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