On the trail of a mysterious force in outer space

May 03, 2023

(Nanowerk News) When Edwin Hubble observed distant galaxies in the 1920s, he made the groundbreaking discovery that the universe was expanding. Only in 1998 did scientists observing a Type Ia supernova further discover that the universe was not only expanding but had already begun a phase of accelerating expansion.

“To explain this acceleration, we need sources,” said Joe Mohr, an astrophysicist at LMU. “And we call this source ‘dark energy’, which provides a kind of ‘anti-gravity’ to accelerate cosmic expansion.”

Scientifically, the existence of dark energy and cosmic acceleration is a shock, and it shows that our current understanding of physics is either incomplete or wrong. The significance of accelerated expansion was underscored in 2011 when its discoverers received the Nobel Prize in Physics.

“Meanwhile, the nature of dark energy has become a problem for the next Nobel Prize winner,” said Mohr. X-ray (above) and optical pseudo-color (below) images of the three low-mass clusters identified in the eFEDS survey data. The highest redshift clusters date back to a time when the Universe was about 10 billion years younger than today. The galaxy cluster in this case is definitely much redder than the galaxies in the other two clusters. (Image: eRosita)

Now I-Non Chiu of the National Cheng Kung University in Taiwan, in collaboration with LMU astrophysicists Matthias Klein, Sebastian Bocquet, and Joe Mohr, have published the first study of dark energy using the eROSITA X-ray telescope, focusing on galaxy clusters (Monthly Notices of the Royal Astronomical Society, “Cosmological constraints of galaxy clusters and clusters in the eROSITA late equatorial depth survey”).

The anti-gravity that might be caused by dark energy pushes objects away from each other and suppresses the formation of large cosmic bodies that should have formed due to the gravitational attraction. As such, dark energy influences where and how the largest objects in the universe form – namely clusters of galaxies with a total mass 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 that form in the universe as a function of time — or in the observed world as a function of redshift,” explains Klein.

However, galaxy clusters are extremely rare and difficult to find, requiring surveys of large portions 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 – was launched in 2019 to conduct an all-sky survey for galaxy clusters.

In the eROSITA Final Equatorial-Depth Survey (eFEDS), a mini-survey designed to verify the performance of subsequent whole-sky surveys, approximately 500 galaxy clusters were found. It represents one of the largest samples of low-mass galaxy clusters to date and covers the last 10 billion years of cosmic evolution.

For their study, Chiu and his colleagues used an additional data set on top of the eFEDS data – optical data from Subaru’s Hyper Suprime-Cam Strategic Program, led by the Japanese and Taiwanese astronomical communities, and Princeton University. Former LMU doctoral researcher I-Non Chiu and colleagues at LMU used this data to characterize galaxy clusters in the eFEDS and measure their masses using a weak gravitational lensing process. The combination of the two data sets enabled the first cosmological study using the galaxy clusters detected by eROSITA.

The results show that, through a comparison between the data and theoretical predictions, dark energy makes up about 76% of the total energy density in the universe. In addition, calculations show 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,” said Bocquet.

So far, all pieces of observational evidence, including recent results from eFEDS, suggest that dark energy can be explained by a simple constant, usually referred to as the ‘cosmological constant’.

“Although the current error at the dark energy threshold is still larger than we expected, this study used samples from eFEDS that occupy less than 1% of the full sky,” said Mohr. This first analysis has thus laid a solid foundation for future studies of the full-sky eROSITA sample as well as other cluster samples.

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