On the trail of a mysterious force in space

Measurements show that the universe is expanding faster and faster. Where this accelerated expansion comes from, however, is a mystery. A first investigation of the dark energy with the X-ray telescope eROSITA by ORIGINS scientists now provides indications that this is evenly distributed in space and time.

X-ray images (top) and pseudocolour optical images (bottom) of three low-mass galaxy clusters identified in the eFEDS survey data. The galaxy clusters with the highest redshift date from a time when the Universe was about 10 billion years younger than it is today. In this case, the cluster galaxies are significantly redder than the galaxies in the other two clusters. Image: eROSITA

When Edwin Hubble observed distant galaxies in the 1920s, he made the groundbreaking discovery: the universe is expanding. It was not until 1998, however, that scientists observing Type Ia supernovae further discovered that the universe is not just expanding but has begun a phase of accelerating expansion. “To explain this acceleration, we need a source,” says Professor Joseph Mohr, ORIGINS astrophysicist at LMU. “And we refer to this source as ‘dark energy,’ which provides a sort of ‘anti-gravity’ to speed up cosmic expansion.” Scientifically, the existence of dark energy and cosmic acceleration are a surprise, and this indicates that our current understanding of physics is either incomplete or incorrect. The significance of the accelerating expansion was underscored in 2011 when its discoverers received the Nobel Prize in Physics. Now I-Non Chiu, who did his doctorate at the ORIGINS predecessor cluster "Universe", Matthias Klein (MPE), Sebastian Bocquet (LMU/ORIGINS) and Joe Mohr, have published a first investigation of dark energy using the X-ray telescope eROSITA, with the focus on the galaxy clusters in the universe.

The anti-gravity possibly caused by dark energy pushes objects away from each other and suppresses the formation of large cosmic objects that would otherwise form due to the attractive force of gravity. As such, dark energy affects where and how the largest objects in the universe form – namely, galaxy clusters with total masses ranging from 10^13 to 10^15 solar masses. However, galaxy clusters are extremely rare and hard to find, requiring surveys of a large portion of the sky using the most sensitive telescopes in the world. To this end, the eROSITA X-ray space telescope – a project led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Munich – was launched in 2019 to carry out an all-sky survey to search for galaxy clusters. In the eROSITA Final Equatorial-Depth Survey (eFEDS), a mini-survey designed for performance verification of the subsequent all-sky survey, about 500 galaxy clusters were found. This represents one of the largest samples of low-mass galaxy clusters to date and spans the past 10 billion years of cosmic evolution.

Energy density of dark energy appears to be uniform in space and constant in time

For their study, Chiu and his colleagues used an additional dataset on top of the eFEDS data – optical data from the Hyper Suprime-Cam Subaru Strategic Program, which is led by the astronomical communities of Japan and Taiwan, and Princeton University. The former LMU doctoral researcher I-Non Chiu and his LMU colleagues used this data to characterize the galaxy clusters in eFEDS and measure their masses using the process of weak gravitational lensing. The combination of the two datasets enabled the first cosmological study using galaxy clusters detected by eROSITA.

Their results show that, through comparison between the data and theoretical predictions, dark energy makes up around 76% of total energy density in the universe. Moreover, the calculations indicated that the energy density of dark energy appears to be uniform in space and constant in time. “Our results also agree well with other independent approaches, such as previous galaxy cluster studies as well as those using weak gravitational lensing and the cosmic microwave background,” says Bocquet. So far, all pieces of observational evidence, including the latest results from eFEDS, suggest that dark energy can be described by a simple constant, usually referred to as the ‘cosmological constant.’

“Although the current errors on the dark energy constraints are still larger than we would wish, this research employs a sample from eFEDS that after all occupies an area less than 1 percent of the full sky,” says Mohr. This first analysis has thus laid a solid foundation for future studies of the full-sky eROSITA sample as well as other cluster samples.

LMU press release

BBC Podcast with Joseph Mohr starting from minute 48.

I-Non Chiu, Matthias Klein, Joseph Mohr, Sebastian Bocquet (2023) “Cosmological constraints from galaxy clusters and groups in the eROSITA final equatorial depth survey., MNRAS 522, 2, 1601–1642

Prof. Joseph Mohr
Ludwig-Maximilians-Universität München
email: Joseph.mohr(at)