Using the eROSITA space telescope, scientists have discovered that dark energy is uniformly distributed in space and time.
When Edwin Hubble observed distant galaxies in the 1920s, he made the groundbreaking discovery that the Universe was expanding. However, in 1988, further studies indicated that it had also begun a phase of accelerating expansion.
“To explain this acceleration, we need a source,” explained Joe Mohr, an astrophysicist at Ludwig-Maximilians University (LMU). “We refer to this source as ‘dark energy,’ which provides an ‘anti-gravity’ to speed up cosmic expansion.”
The existence of dark energy and cosmic acceleration is a surprise. This indicates that our current understanding of physics is either incomplete or incorrect.
Now, I-Non Chiu from National Cheng Kung University in Taiwan, in collaboration with LMU astrophysicists Matthias Klein, Sebastian Bocquet, and Joe Mohr, has published a first-of-its-kind study. To do this, they used the eROSITA X-ray telescope, which focuses on galaxy clusters.
The work, titled ‘Cosmological constraints from galaxy clusters and groups in the eROSITA final equatorial depth survey,’ is published in Monthly Notices of the Royal Astronomical Society.
What impact does dark energy have on 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 gravity.
Therefore, it affects where and how the largest objects in the Universe form, including galaxy clusters with total masses ranging from 1013 to 1015 solar masses.
Klein said: “We can learn a great deal 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.”
Galaxy clusters are extremely rare
One issue the team needed to overcome was the rarity of galaxy clusters.
To discover them, surveys of a large portion of the sky using the most sensitive telescopes in the world are required. 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.
Chiu and his colleagues used an additional dataset on top of the eFEDS data – optical data from the Hyper Suprime-Cam Subaru Strategic Program. This is led by the astronomical communities of Japan and Taiwan, and Princeton University.
The researchers used this data to characterise 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 to study dark energy.
A surprising discovery
Through comparison between the data and theoretical predictions, the results show that 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.
Bocquet commented: “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.”
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 these constraints are still larger than we would wish, this research employs a sample from eFEDS that occupies an area less than 1% of the full sky,” Mohr concluded.
This first analysis has therefore laid a solid foundation for future studies of the full-sky eROSITA sample, as well as other cluster samples.