Cosmic Web’s dark matter strands revealed for the first time

Dark matter has been shown to be concentrated in threads within a spectacular cluster of galaxies popular with amateur astronomers. These filaments explain why the galaxies occupy their locations. The finding shows that while our models of the universe face challenges, they continue to predict many things quite well.

Galaxies are not randomly distributed through space, but are in small groups, like our own Milky Way, and in much larger clusters. These also follow patterns, and according to the concordance cosmological model, they occur where vast filaments that form the cosmic web come together.

Tests of this theory have found evidence supporting this theory when it comes to the common baryonic matter that makes up stars. However, the model predicts that these filaments will consist largely of dark matter, the presence of which is much more difficult to confirm. After all, we cannot see dark matter – that’s in the name – and we don’t even know what it is. Mapping its spread poses obvious challenges.

These are not insurmountable, however, as a team from South Korea’s Yonsei University claims to have overcome them to the point of showing that these cosmic web filaments also have a dark matter component. Rather than inferring the presence of dark matter from the distribution of light sources, the team looked for distortions in the light of more distant objects, which indicated distortions in spacetime.

Such a bending of the fabric of the universe can, as far as we know, only be accomplished by enormous gravitational forces. We can see this happening from massive objects that we can see, and the gravity of dark matter should do the same.

The Yonsei team pointed the Subaru Telescope at the Coma cluster, which despite its 320 million light-year distance is so bright that the brightest galaxies attract backyard telescopes to observe the Coma Berenices constellation. They found that the cluster influences light from more distant sources much more than the galaxies within it – even including associated dark matter – could explain.

There is a certain mass concentration between the galaxies, and if the models are correct, this should lie along so-called intracluster filaments (ICFs). A statement from the Subaru Telescope describes the ICFs as the “terminal ends of dark matter filaments” that extend for millions of light years.

There is also a history to the choice of the Coma Cluster. In 1933, Fritz Zwicky reported that the speed at which the galaxies in the Coma Cluster were moving was so great that it required a staggering amount of mass to hold them together. Zwicky showed that there was more mass than the visible component of these galaxies could yield, indicating that there must be something more, which was eventually called dark matter. Although the existence of dark matter had been proposed before and was not widely accepted until decades later based on galactic rotation rates, Zwicky is now considered a pioneer in the field.

The authors mapped the distribution of mass within the cluster by determining where the distortions in more distant light are most extreme, a process known as ‘weak lensing’. They compared the locations of peaks in the mass with predictions about where these filaments should lie.

What the authors call the northern ICF matched expectations well, although the article announcing the result notes that the western ICF is “at the lower end of the distribution.”

The ICFs are also much more densely packed with mass than their surroundings – 67 and 33 times the background density, respectively – the authors report. Together, the findings indicate that a substantial portion of the Coma Cluster’s mass resides in ICFs that pass through it.

In addition to providing further evidence for the existence of dark matter, something accepted by the vast majority of astronomers, the work indicates that our understanding of its distribution is on the right track.

On the other hand, the authors suggest that cluster masses are likely underestimated because these ICFs are not visible if we just count galaxies and measure their brightness.

The research has been published in Nature Astronomy.

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