The force is with us, according to cosmologists working to understand a mysterious “something” that is causing the universe to expand. His name? Dark Energy. And it turns out it has been present throughout cosmic history.
Astronomers have known that the universe is expanding since the 1920s. This understanding began with Edwin Hubble’s groundbreaking observation of a Type I supernova in the Andromeda galaxy. And astronomy has been on the road for many years, using this extension to measure distances and other parameters in the cosmos. Then, in 1998, something happened. Astronomers discovered that cosmic expansion is accelerating.
The culprit? This completely misunderstood force of dark energy that cannot be seen, but with demonstrable effects. Some explain it as a property of space that causes the universe to expand at an ever-accelerating rate. Others suggest that it is some kind of new energy fluid or field that fits all of space but has an impact on the expansion of the universe. It could also be something that doesn’t fit with our current theories of gravity, and that a new theory of gravity could explain the effects of dark energy.
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There is still no consensus on which of these theories is correct. However, its discovery immediately raised a number of questions, e.g. B. When did the expansion rate accelerate? Will that change too? Was it the same rate throughout the universe at all times?
This chart shows changes in the rate of expansion since the universe was born almost 15 billion years ago. The flatter the curve, the faster the rate of expansion. The curve changed noticeably about 7.5 billion years ago when objects in the Universe began to fly apart at a faster rate. Astronomers suspect the faster expansion rate is due to a force called “dark energy” pulling galaxies apart. Credit: NASA/STSci/Ann Feild
Dark energy, eROSITA and galaxy clusters
To answer these, a group of researchers used something called eROSITA to study a specific subset of galaxy clusters over time. eROSITA is the main X-ray sensitive instrument on board the Spectrum-ROENTGEN-GAMMA (SRG) mission, launched in 2019. (It is currently shut down due to the ongoing conflict between Russia and Ukraine.) One of its tasks is to conduct a full mid-energy X-ray (up to 10 keV) sky survey. The returned data should help study the nature and ubiquity of dark energy by examining up to 100,000 galaxy clusters and the material between them. It also studies eclipsed black holes in galaxies and examines X-ray sources ranging from young stars and supernova remnants to X-ray binaries.
Astronomers I-Non Chieu from the National Cheng Kung University in Taiwan and Matthias Klein, Sebastian Bocquet and Joseph Mohr from the Ludwig-Maximilians-Universität in Munich used the data from the eROSITA Final Equatorial Depth Survey (eFEDS) taken before the shutdown were used to characterize about 500 low-mass galaxies clusters. It is one of the largest samples of its kind and has “seen” it for the last ten billion years. That’s about 3/4 the age of the universe.
Combining data to measure distant galaxy clusters
The team linked the eFEDS data with optical data recorded with the Hyper Suprime Cam instrument on the Subaru Telescope in Hawaii. They used the combined data to characterize the galaxy clusters in eFEDS and measure their masses using weak gravitational lenses. The combination enabled the first cosmological study using galaxy clusters discovered by eROSITA.
Cosmologists have long assumed that dark energy makes up approximately 68% of the universe. This new result increases that number. In essence, the team showed that dark energy accounts for about 76% of the total energy density in the universe.
The analysis shows that the distribution of dark energy is also spatially fairly even and temporally constant. “Our results are also in good agreement with other independent approaches, such as B. previous galaxy cluster studies as well as those using weak gravitational lensing and the cosmic microwave background,” said Bocquet.
galaxy clusters and dark energy
Dark energy is a slippery concept to pin down. Sure, astronomers know it’s there, affecting the universe. And they say it provides a kind of “antigravity” that pushes objects away from each other. This also has the effect of slowing down or even stopping the formation of large objects such as galaxy clusters. So if dark energy has been at work throughout the universe and over time, then it affects where and how things like galaxy clusters formed.
Knowing this actually gives astronomers a tool to assess this force. “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,” Klein said.
In their work, the research team suggests that the number of galaxy clusters across the universe can be used as another way to study the nature of dark energy. The eROSITA study is a mini survey of these clusters. The current work is a “proof of concept” for a method they hope can be applied to larger surveys of galaxy clusters to further study the effects of dark energy across time and space.
For more informations
Cosmology: On the trail of a mysterious force in space
Cosmological constraints of galaxy clusters and groups in the eROSITA Final Equatorial Depth Survey
What is dark energy
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