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Dark-matter filament spotted

06 Jul 2012
Dark bridge

Physicists claim to have reliably detected a mammoth filament of dark matter stretching between two galaxy clusters, for the first time. If the detection is bona fide, it could be one of the best confirmations yet of the “standard model” of the universe’s evolution, the so-called lambda cold-dark-matter (ΛCDM) model.

The ΛCDM model posits that, in the early universe, dark matter was spread out in a web of filaments. Over time, this cosmic web would have helped all the normal “baryonic” matter to clump together, particularly in the regions where its filaments intersected. Today, we see the result of this clumping at the filament intersections: galaxy clusters and, on a smaller scale, individual galaxies and stars.

Universal webbing

The ΛCDM model seems to explain most aspects of the universe, from the large-scale structure through to that lasting remnant of the Big Bang – the cosmic microwave background. Yet, if the universe did evolve according to the model, the dark-matter filaments ought still to exist, strung between galaxy clusters like ancient cobwebs. Unfortunately, like a cobweb, dark matter is difficult to make out, despite being thought to make up roughly a quarter of the universe’s total mass–energy. It does not interact strongly with light and is, therefore, invisible.

Astronomers have been searching for it nonetheless. In the 1980s they managed to map some of the baryonic (visible) matter running along the filaments, thereby showing that the cosmic web does indeed exist. Later, the possibility of detecting dark matter in the filaments opened up too, with use of a technique known as “weak gravitational lensing”. In this technique, astronomers examine the light from a backdrop of many distant galaxies, and determine how much of it is distorted because of the gravity of matter in the foreground. From the late 1990s, observations seemed to suggest that there was more gravitational distortion in the regions between certain galaxy clusters than could be accounted for by baryonic matter alone: this, the astronomers claimed, must have been the dark-matter filaments.

But it was not to be. As the lensing studies required observations over a very large field of view, the astronomers had been forced to coat their telescopes’ focal planes with not one but several CCD detectors. In subsequent tests, it was discovered that a slight misalignment between these detectors could have caused false distortion signals.

Advanced arrays

Now, however, Jörg Dietrich at the University Observatory Munich, Germany, thinks that physicists need not be fooled any longer. “Our understanding of the behaviour of those optical systems, the cameras with the multi-chip arrays, and the ways to correct for them, have advanced immensely over the past 10 years,” he says. Together with colleagues in the US and Europe, Dietrich has found evidence for a dark-matter filament between two neighbouring galaxy clusters – Abell 222 and Abell 223.

Dietrich and colleagues picked these galaxy clusters because, lying at a redshift of 0.21 and separated on the sky by just 0.23°, they are relatively close together, and are therefore likely to be bound by a thick dark-matter filament. Indeed, the lensing signal found by the researchers was strong: a maximum of just 9% of it could be accounted for by X-ray emission from hot gas. Add another 5% or so for stars and 5% for colder, inconspicuous matter, says Dietrich, and one is left with the normal mass estimate for dark matter as a proportion of all matter: roughly 80%.

“I find this important because it provides the – so far – clearest confirmation of a key prediction of our current cosmology paradigm, where most of the matter in the universe is constituted by unseen dark matter,” says theoretical astrophysicist Håkon Dahle of the University of Oslo.

Dahle admits that the observation does not necessarily rule out alternatives to dark matter – a minority of theorists believe that they can explain these sorts of phenomena with tweaked theories of gravity – modified Newtonian dynamics, or MOND, is a popular example. “There are, and will still be, potential alternatives to dark matter – but [the latest observation] adds to an extensive set of observations that are fully consistent with our current cosmological paradigm,” he says.

Confident case

Yet the fact that astrophysicists have wrongly claimed sightings of dark-matter filaments before raises a question: could this latest observation also be an instrumental artefact?

“That is a fair question,” says Dietrich. He believes that his group’s observations will stand the test of further scrutiny because, unlike the previous observations, they were not recorded near the gaps between the CCD detectors. Moreover, he says, the dark matter they observed hugged the same outline as the X-ray emission and light from galaxies, which is what the ΛCDM model predicts. “I am very confident in this case,” he adds.

The results are published online in Nature.

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