An initial study of dark energy with the eROSITA X-ray telescope, focused on galaxy clusters, indicates that the density of this mysterious force is evenly distributed in space and time.
When Edwin Hubble observed distant galaxies in the 1920s, he made the groundbreaking discovery that the universe is expanding. However, it wasn’t until 1998 that scientists observing Type Ia supernovae discovered that the universe is not just expanding, but has begun an accelerating expansion phase. “To explain this acceleration, we need a source,” says Joseph Mohr, an astrophysicist at LMU (Ludwig-Maximilians-Universität) in Munich. “And we refer to this source as ‘dark energy,’ which provides a kind of ‘anti-gravity’ to speed up cosmic expansion.” Scientifically, the existence of dark energy and cosmic acceleration come as a surprise, indicating that our current understanding of physics is incomplete or incorrect. The importance of the accelerating expansion was underscored in 2011 when its discoverers received the Nobel Prize in Physics. “Meanwhile, the nature of dark energy has become the next Nobel Prize-winning problem,” says Mohr.
Now I-Non Chiu of National Cheng Kung University in Taiwan, in collaboration with LMU astrophysicists Matthias Klein, Sebastian Bocquet and Mohr himself, has published the new research on this cosmological enigma in the Monthly Notices of the Royal Astronomical Society. .
Antigravity 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 pull 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 1013 to 1015 solar masses. “We can learn a lot about the nature of dark energy by counting the number of galaxy clusters formed in the universe as a function of time, or in the observational world as a function of redshift,” explains Klein.
However, galaxy clusters are extremely rare and hard to find, requiring surveys of a large part of the sky using the world’s most sensitive telescopes. To this end, the eROSITA X-ray space telescope, a project led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Munich, 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 to verify the performance of the subsequent all-sky survey, around 500 galaxy clusters were found. This represents one of the largest samples of low-mass galaxy clusters to date and spans the last 10 billion years of cosmic evolution.
For their study, Chiu and his colleagues used an additional dataset in addition to 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. Former LMU doctoral researcher I-Non Chiu and his LMU colleagues used these data to characterize the galaxy clusters in eFEDS and measure their masses using the weak gravitational lensing process. Combining the two data sets enabled the first cosmological study using galaxy clusters detected by eROSITA.
Their results show that, through comparison between data and theoretical predictions, dark energy accounts for about 76% of the total energy density of the universe. Furthermore, 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 studies of galaxy clusters, as well as those using weak gravitational lensing and the cosmic microwave background,” says Bocquet. “So far, all the observational evidence, including the latest eFEDS results, suggests that dark energy can be described by a simple constant, often referred to as the ‘cosmological constant.'”