5 July 2012

Scientists see dark matter web between galaxies

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The fundamental ‘cosmic web’ of dark matter throughout the universe has finally been observed from Earth, confirming theories of how the universe was shaped.
Dark matter

Galaxy clusters Abell 222 and Abell 223 connected by dark matter filament. Yellow contours and blue shading are indicative of matter density. Credit: Jörg Dietrich, University of Michigan/University Observatory Munich

LONDON: The fundamental ‘cosmic web’ of dark matter throughout the universe has finally been observed from Earth, confirming theories of how the universe was shaped.

“This result is a resounding confirmation of a key prediction of structure formation in the universe,” said Jörg Dietrich from University Observatory Munich, Germany, and lead author of the study published in Nature today.

“Not only did we for the first time see a dark matter filament directly, we also confirmed that it’s total mass [dark matter plus normal matter] and the amount of hot gas are in agreement with predictions.”

Dark matter

Large cosmic structures, such as galaxies, exhibit gravitational affects that cannot be justified by the amount of normal matter present in the universe. A principle exists to describe this behavior and is known as the cold-dark-matter model, and it is the foundation of modern cosmology.

Within the model, vast quantities of dark matter, moving much slower than the speed of light (cold), must exist to explain the gravitational affects seen on normal matter.

As the name implies, dark matter cannot be observed in the traditional sense as it does not emit or absorb light at any significant level. However, its presence can be detected.

Gravitational lensing

“The technique we used is gravitational lensing,” said Dietrich. This is where light from, for example, a distant galaxy, is deflected and bent by huge gravitational fields generated by other similarly large objects. “It was widely believed that with current telescopes we would not be able to image dark matter filament with gravitational lensing.”

Dietrich and his team devised a method to boost the lensing signal sufficiently so that the dark matter filament between two clusters of galaxies could be observed with an 8m ground-based telescope.

“The key ingredient that made this result possible, is that we decided to study a very peculiar system of two massive galaxy clusters,” he said. These are called Abell 222 and Abell 223.

“Gravitational lensing is now allowing us to see parts of the universe that were previously invisible,” said astrophysicist Joanna Dunkley, from the University of Oxford, England, who was not involved in the study. “We already have plenty of indirect evidence that galaxy clusters trace a cosmic web of dark matter, but seeing this directly is an important step forward.”

The cosmic web

She added: “Our standard picture of cosmology tells us that filaments of invisible matter thread through the universe, and this bridge of dark matter connecting two clusters is exactly what we would expect.”

The research confirms that galaxy clusters form at the intersections of these vast filaments of dark matter.

Keen to develop this ground-breaking research further, Dietrich said, “We now want to study the interplay of dark matter density and galaxy population to get a better understanding of the mechanism that transforms galaxies from blue, star-forming spiral galaxies – which are the most common galaxies in filament – to red elliptical galaxies, which are the majority of galaxies in galaxy clusters.”

“The dark matter web makes up about a quarter of the universe,” Dunkley explained. “Being able to measure its morphology should help tell us more about how galaxies and clusters were formed, and may even give us a clue about what the dark matter itself is.” This in itself is a question that has been fascinating scientists for decades.

This research states that the underlying dark matter structure of the universe could comprise over half of all matter in existence. So developing it further is a must if scientists are to understand the elementary nature of the universe.

“We need to observe the lensing signal of more filaments to understand how special the one we found is,” Dietrich said. This will involve both in-depth statistical analysis of thousands of galaxy clusters, and a satellite mission to increase the possibilities of directly observing dark matter filaments.

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