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New KiDS result: Universe 10 per cent more homogeneous than assumed

New results from the Kilo-Degree Survey (KiDS) show that the Universe is almost 10 percent more homogeneous than previously thought. The new KiDS map was created using the partly Dutch OmegaCAM on ESO's VLT Survey Telescope on Cerro Paranal in Northern Chile. An international team of astronomers from, among others, Leiden University has described the KiDS-1000 results in five articles, the last three of which appeared online on 31 July. They were submitted for publication in the journal Astronomy & Astrophysics.

The new KiDS map covers about 1000 square degrees, which is 5% of the extragalactic sky. For the analysis, 31 million galaxies were used. Some of these galaxies are more than 10 billion light years away. Their light was emitted when our Universe was only half its current age.

Distribution of matter

KiDS uses the galaxies to map the distribution of matter in the Universe. This is done using weak gravitational lenses, in which the light from distant galaxies is slightly distorted by the gravity effect of large amounts of matter, such as clusters of galaxies. This effect is used to determine the 'lumpiness' of the distribution of galaxies. It concerns all matter in the Universe, of which more than 90 percent consists of invisible dark matter plus invisible thin gas.


The gravity of matter in the Universe causes a less homogeneous distribution; areas with a little more mass than average attract matter from their surroundings and provide more contrast. But the expansion of the Universe counteracts this growth. Both of these processes are driven by gravity and are therefore very important for testing the cosmological standard model (L-CDM), which fairly accurately predicts how density variations increase with the age of the Universe. However, the new KiDS results show a discrepancy: the Universe is almost 10 percent more homogeneous than the standard model predicts.

The KiDS map projected to scale on the night sky above the Old Observatory Leiden. The map shows the uneven distribution of matter in the universe based on data from weak gravitational lenses. The smallest details cover about 30 million light years. Credit: B.Giblin, K.Kuijken and the KiDS team.

Cracks in the model

Leiden professor and KiDS project leader Koen Kuijken calls the result intriguing. 'We have a very good physical model of the Universe, which describes the observations well but appeals to very remarkable and mysterious physics, especially that of dark matter and dark energy. You have to test such a model in as many ways as possible, and that's exactly what we are doing.'

The KiDS results may indicate small inaccuracies in the standard model, just as another discrepancy in the expansion rate, the so-called Hubble constant. Kuijken: 'The question is whether these can be solved with a small adjustment, for example with a more complex behaviour of dark matter than the simple "cold dark matter", which shows no significant reactions.' 

Whether this will eventually lead to a fundamentally different theory, and whether, for example, we should revise Einstein's general theory of relativity, Kuijken cannot say. 'For the time being, I consciously keep myself far from possible theoretical interpretations, and focus on the measurements and carrying them out as accurately as possible. It remains exciting.'

To be continued

In one to two years a final KiDS map will appear, 30% larger than the current one. This will include all KiDS observations. Two other projects, one American and one Japanese, are also working on similar analyses from other observatories. From 2022 onwards, the VLT Survey Telescope will pass on the baton to even more powerful telescopes: the Rubin telescope, which will be more than 60 times more powerful than the VLT, and the Euclid satellite, which will produce images from outside the atmosphere that are much sharper than pictures taken from the ground. Many of the KiDS team members are also involved in these projects.

KiDS is an international project led by astronomers in The Netherlands, Scotland, England, Germany and Italy. The Principal Investigator of KiDS is Koen Kuijken of the Leiden Observatory. More information about KiDS and the team members: https://kids.strw.leidenuniv.nl

The articles are available on : https://kids.strw.leidenuniv.nl/papers.php

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