The universe is more homogeneous than expected

Recent results of the Kilo-Degree Survey have shown that matter in the universe is about ten percent more evenly distributed than predicted by the standard model of cosmology. An international team, led by astronomers from the Netherlands, Scotland, Great Britain, and Germany - with the participation of the Ruhr-Universit├Ąt Bochum - describes the results in five articles, three of which were published on a preprint server on 31 July 2020. They are also submitted for publication in the journal Astronomy and Astrophysics.

The data were recorded with the European Space Organisation's Very Large Telescope Survey Telescope (VST) on Cerro Paranal in northern Chile. The resulting map covers five percent of the extragalactic sky and includes 31 million galaxies, all of which were included in the analysis. They are up to ten billion light years away, and their light was emitted when the universe was only about a quarter of the age it is today.

Using the galaxies, the research consortium produced a map of the distribution of matter in the universe. To do this, the scientists used the so-called weak gravitational lensing effect: light from distant galaxies is deflected and distorted on its way to Earth by the gravitational effect of large accumulations of matter such as galaxy clusters. Based on this effect, the clumping tendency of matter can be determined - namely visible matter, gases and invisible dark matter, which makes up about 85 percent of the total matter in the universe.

The researchers make the distribution of matter visible in such maps. The grey square shows the size of a single map taken, with the moon in it for size comparison. The total map consists of more than 1,000 images. Areas of high density matter are shown in yellow, areas of low density in pink.

Over time, the gravitation of matter causes the universe to become less and less homogeneous. Areas with slightly more mass than average attract matter from their surroundings. Thus the differences in distribution become greater and greater. At the same time, the expansion of the universe counteracts this effect. Both processes are driven by gravity and are therefore suitable for putting the standard model of cosmology to the test. The equations predict precisely how much the density of matter will change over time.

However, the data from the Kilo-Degree Survey reveal a discrepancy: the universe is ten percent more homogeneous than it would appear to be according to the Standard Model. "The Standard Model of Cosmology has described all the cosmological observations we have made for the past 20 years. However, it is somewhat unsatisfactory that we have to accept mysterious substances such as dark matter and dark energy. That's why we're trying to test this model as best we can," says Prof. Dr. Hendrik Hildebrandt, head of the Observational Cosmology group at the Ruhr University Bochum.

The current analysis could indicate that the Standard Model is cracking. It is not the first discrepancy, even the so-called Hubble constant, which represents the expansion rate of the universe, does not match the model's predictions. "These discrepancies could of course be caused by systematic measurement errors," admits Prof. Dr. Catherine Heymans (University of Edinburgh), who together with Hendrik Hildebrandt heads the German Centre for Cosmological Lensing at the RUB, where she also holds a guest professorship. "But the measurements are becoming more and more precise, so that this is becoming increasingly unlikely," Heymans continues.

The researchers are not yet able to assess whether the Standard Model will eventually have to be replaced by a completely new theory, for example by replacing Einstein's General Theory of Relativity. "There are many theories that try to explain the measurements with new physics," says Hendrik Hildebrandt. "As an observing cosmologist, you try to remain impartial and make the measurements as accurate as possible without theoretical prejudice. One thing is clear, we live in exciting times!"
In one or two years the final map of the Kilo-Degree Survey will be available, with all the observations made in the project. It will be another 30 percent larger than the current map.

Meanwhile, two other projects, one in the US and one in Japan, are working on similar analyses based on observational data. From 2022, even better measuring technology will be available: the Ruby telescope, which is 60 times more powerful than the VST, and the Euclid satellite, which will take much sharper images outside the atmosphere.

The Kilo-Degree Survey is an international project led by astronomers in the Netherlands, Scotland, England and Germany. The project coordinator is Prof. Dr. Koen Kuijken from the Leiden Observatory in the Netherlands. Other partners come from Italy, Australia, Poland, the USA, and China.

adapted from Julia Weiler, RUB