In the 1920s, Edwin Hubble made a groundbreaking discovery by observing distant galaxies and found that the universe is expanding. However, it wasn’t until 1998 that scientists studying Type Ia supernovae made a further discovery: the universe isn’t just expanding, but it’s actually accelerating its expansion. To explain this acceleration, scientists postulated the existence of dark energy, which behaves like an “anti-gravity” force, causing cosmic expansion to speed up.
The existence of dark energy and cosmic acceleration is surprising and suggests that our current understanding of physics is either incomplete or inaccurate. In 2011, the discoverers of cosmic acceleration were awarded the Nobel Prize in Physics, emphasizing the significance of this discovery. Astrophysicist Joseph Mohr states that the nature of dark energy has now become the next Nobel Prize-winning problem.
A recent study by I-Non Chiu from National Cheng Kung University in Taiwan, in collaboration with astrophysicists Matthias Klein, Sebastian Bocquet, and Joe Mohr from LMU, focused on dark energy using the eROSITA X-ray telescope and examined galaxy clusters. Dark energy’s anti-gravity force prevents the formation of large cosmic objects that would otherwise form due to the attractive force of gravity. It affects the formation of the universe’s largest objects, such as galaxy clusters, which have total masses ranging from 1013 to 1015 solar masses. Klein explains that counting the number of galaxy clusters formed as a function of time or redshift provides insights into the nature of dark energy.
Finding galaxy clusters is a challenging task, as they are exceedingly rare and require extensive sky surveys using the most sensitive telescopes. The eROSITA X-ray space telescope, led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Munich, was launched in 2019 to conduct an all-sky survey for galaxy clusters. The eROSITA Final Equatorial-Depth Survey (eFEDS) was a mini-survey aimed at performance verification before the all-sky survey. It discovered roughly 500 low-mass galaxy clusters, representing one of the most extensive samples of such clusters to date. The survey spans 10 billion years of cosmic evolution.
Energy density of dark energy appears to be uniform in space and constant in time
Chiu and his colleagues utilized an additional dataset, the optical data from the Hyper Suprime-Cam Subaru Strategic Program, on top of the eFEDS data for their study. By characterizing the galaxy clusters in eFEDS and measuring their masses through weak gravitational lensing, the researchers were able to conduct the first cosmological study using galaxy clusters detected by eROSITA.
The study found that dark energy accounts for approximately 76% of the total energy density in the universe, with its energy density appearing to be constant in time and uniform in space. The results were consistent with other independent approaches, including previous galaxy cluster studies, weak gravitational lensing, and the cosmic microwave background. The study has laid a solid foundation for future studies of the full-sky eROSITA sample and other cluster samples. Despite current errors, which are still larger than desired, the sample from eFEDS covers less than 1% of the full sky.