The energetic universe as seen with the eROSITA X-ray telescope

In the 1920s, Edwin Hubble made a groundbreaking discovery while observing distant galaxies: the universe is expanding. However, it wasn’t until 1998 that the observation of Type Ia supernovae led to the further discovery that the universe is not just expanding, but also experiencing accelerated expansion. Astrophysicist Joe Mohr from LMU explains that to account for this acceleration, we need to identify a source, which is referred to as “dark energy” and appears to provide a sort of “anti-gravity” that speeds up cosmic expansion.

The existence of dark energy and cosmic acceleration are scientifically surprising, indicating that our current understanding of physics may be incomplete or incorrect. The importance of the accelerating expansion was underlined in 2011 when its discoverers were awarded the Nobel Prize in Physics. Mohr notes that the nature of dark energy has become a potential Nobel Prize-winning problem.

Now, I-Non Chiu from National Cheng Kung University in Taiwan has worked in collaboration with LMU astrophysicists Matthias Klein, Sebastian Bocquet, and Joe Mohr to publish the first study of dark energy using the eROSITA X-ray telescope, which focuses on galaxy clusters.

The anti-gravity caused by dark energy pushes objects away from each other and hinders the formation of large cosmic objects that would otherwise come together due to the attractive force of gravity. As a result, dark energy has an impact on the formation of the largest objects in the universe, namely galaxy clusters with total masses ranging from 1013 to 1015 solar masses. Klein explains that we can learn a great deal about the nature of dark energy by tracking the number of galaxy clusters formed in the universe over time, or as a function of redshift in the observational world.

Galaxy clusters are a rare and challenging find, requiring extensive sky surveys using the most advanced telescopes available. The eROSITA X-ray space telescope, led by the Max Planck Institute for Extraterrestrial Physics in Munich, was launched in 2019 to conduct an all-sky survey to detect these elusive objects. In the eROSITA Final Equatorial-Depth Survey (eFEDS), which was designed to verify the telescope’s performance, approximately 500 low-mass galaxy clusters were identified, representing one of the largest samples to date and spanning 10 billion years of cosmic evolution.

To enhance their study, Chiu and his colleagues utilized additional data from the Hyper Suprime-Cam Subaru Strategic Program, led by the astronomical communities of Japan, Taiwan, and Princeton University. By analyzing this data in combination with the eFEDS data, Chiu and his LMU colleagues were able to characterize the galaxy clusters and measure their masses using weak gravitational lensing. This powerful approach enabled the first cosmological study utilizing galaxy clusters detected by eROSITA.

Their findings indicate that dark energy accounts for approximately 76% of the total energy density of the universe. Furthermore, the calculations suggest that the energy density of dark energy is uniform in space and constant in time. According to Bocquet, “Our results agree well with other independent approaches, such as previous galaxy cluster studies, weak gravitational lensing, and the cosmic microwave background.” All observational evidence, including the latest results from eFEDS, suggests that dark energy can be described by a simple constant, commonly known as the ‘cosmological constant.’