Edwin Hubble’s observations of distant galaxies in the 1920s led to the revolutionary conclusion that our universe is expanding. But only in 1998, scientists studying Type Ia supernovae made a striking discovery. They discovered that the universe is not only growing, but also accelerating its expansion.

“We need a source to explain this acceleration,” says Joe Mohr, an astrophysicist at LMU. “And we call this source ‘dark energy,’ which provides a kind of ‘antigravity’ to accelerate the expansion of space.”

From a scientific point of view, the existence of dark energy and cosmic acceleration is a surprise, showing that our current understanding of physics is either incomplete or wrong. The importance of accelerated expansion was highlighted in 2011 when its discoverers won the Nobel Prize in Physics.

“Meanwhile, the nature of dark energy became the next Nobel Prize-winning problem,” says Mohr.

Working in collaboration with LMU astrophysicists Matthias Klein, Sebastian Bockke and Joe Mohr, Yi-Nong Chiu of the National Cheng Kung University in Taiwan has published the first dark energy study with the eROSITA X-ray telescope focused on galaxy clusters.

The antigravity, possibly caused by dark energy, pushes objects apart and prevents the formation of large cosmic objects that would normally occur due to gravity. Thus, dark energy influences where and how the largest objects in the universe are formed, namely galaxy clusters with a total mass of 1013 to 1015 solar masses.

“We can learn a lot about the nature of dark energy by counting the number of galaxy clusters that form as a function of redshift in the Universe or the observable world as a function of time,” explains Klein.

However, galaxy clusters are extremely rare and difficult to find, requiring surveying large areas of the sky with the world’s most sensitive telescopes. To that end, in 2019 the X-ray space telescope eROSITA, a project led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Munich, was launched to scan the entire sky in search of galaxy clusters.

Nearly 500 galaxy clusters were found in the eROSITA Final Equatorial Depth Survey (eFEDS), a mini-survey designed to test the performance of the next all-sky survey. It is one of the largest low-mass galaxy clusters to date and spans the last 10 billion years of cosmic evolution.

For their study, Chiu and colleagues used an additional dataset, in addition to the eFEDS data, optical data from Subaru’s Japanese and Taiwanese astronomical societies and the strategic Hyper Suprime-Cam program led by Princeton University.

Former LMU postdoctoral fellow Yi-Nong Chiu and colleagues at LMU used the data to characterize clusters of galaxies in eFEDS and measure their masses using a weak gravitational lensing process. The combination of the two datasets enabled the first cosmological survey using galaxy clusters detected by eROSITA.

Their results show that when we compare the data with theoretical predictions, dark energy accounts for about 76% of the total energy density in the universe. Additionally, calculations showed that the energy density of dark energy appears to be uniform in space and constant in time.

“Our results are also in good agreement with previous studies of galaxy clusters and other independent approaches, such as those using weak gravitational lensing and the cosmic microwave background,” says Bokke. Currently, all observational data, including the latest eFEDS results, suggest that dark energy can be described by a simple constant, often called the “cosmological constant.”

“Although current dark energy constraints are still greater than we would like, this study uses an eFEDS sample that ultimately covers less than 1% of the entire sky,” says Mohr. Thus, this initial analysis provided a solid foundation for future studies of the eROSITA full sky sample as well as other cluster samples. Source