Revealed at last: Universe's intergalactic dark matter skeleton

Higgs, Schmiggs... When that infinitesimal speck was sucking up all the journalistic oxygen on Independence Day, another momentous scientific discovery was also being announced: the first observation of filaments of dark matter, the stuff that forms the "skeleton" of our universe.

Invisible, inexplicable dark matter makes up the vast majority of the mass of our universe. All the matter that we can see – stars, galaxies, planets, haggis, Michele Bachman – total only between 4 and 5 per cent of our universe's mass.

The rest? There's dark matter and dark energy, but exactly what those dark enigmas are ... well, as Geoffrey Rush's Philip Henslowe was wont to say in Shakespeare in Love, "It's a mystery."

Sky boffins have been able to detect galactic-sized blobs of dark matter by observing light being bent by their enormous gravitational pull. The exact same star-filled galaxy, for example, can appear twice in the sky as light from it bends on either side of a dark matter formation.

What has not been observed – until now, that is – are the thin filaments of dark matter that have been thought to connect the massive dark-matter nodes, and which give the universe, both visible and invisible, its structure.

"Dark matter really governs structure formation," the lead author of the paper which reveals the discovery, Jörg Dietrich of University Observatory Munich, told the Boston Herald. "The galaxy clusters and the filaments are mostly made up of dark matter. The normal matter just follows the distribution of dark matter."

As noted in an announcement of the paper on Nature.com – with, by the way, the lovely title of "Dark matter's tendrils revealed" – Dietrich and his team were able to track down a particularly massive filament bridging the galaxy clusters Abell 222 and Abell 223.

And when we say "massive", we're not just whistling the proverbial Dixie. The filament that the team detected is about 18 megaparsecs long – if you happened to be asleep that day in your astrophysics class, know that a megaparsec is equal to about 3.09x10²² meters – and has a mass they calculate to be somewhere between 6.5x10¹³ and 9.8x10¹³ times that of our Sun.

What's more, most of the mass of the filament happens to be on a direct line of sight to Earth. With the filament in that orientation and of that immense mass, Dietrich and his team were able to detect its gravitational lensing of the light provided by 40,000 individual background galaxies.

The team then used observations of the filament's constituent materials, made by the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton) spacecraft, to determine that not more than 9 per cent of the filament could be composed of hot gas, and about 10 per cent could be accounted for by such garden-variety matter as stars and galaxies. The remainder, the team concluded, must be dark matter.

Cosmologists believe that visible matter somehow follows the paths laid out in a "cosmic grid" of intersecting dark-matter filaments. The mechanism for how this occurs, however, remains a mystery – but now that a method has been demonstrated for mapping at least the most massive of those filaments, progress can be made towards understanding just how our universe came to be structured the way it is.

The Higgs boson at one end of the cosmic scale, and super-massive dark-matter filaments at the other – it's been a boffo week for boffins. ®

Bootnote

Speaking of that other boffinary discovery announced this week: A Higgs boson walks into a Catholic church. The priest says, "We don't allow Higgs bosons in here." Puzzled, the Higgs boson replies, "But without me, how can you have mass?"

OK....

Lets see what they say.....link at post #6:

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Direct measurement of a dark-matter ‘filament’ confirms its existence in a galaxy supercluster.

Zeeya Merali

04 July 2012

Dark-matter filaments, such as the one bridging the galaxy clusters Abell 222 and Abell 223, are predicted to contain more than half of all matter in the Universe.

Jörg Dietrich, University of Michigan/University Observatory Munich

A ‘finger’ of the Universe’s dark-matter skeleton, which ultimately dictates where galaxies form, has been observed for the first time. Researchers have directly detected a slim bridge of dark matter joining two clusters of galaxies, using a technique that could eventually help astrophysicists to understand the structure of the Universe and identify what makes up the mysterious invisible substance known as dark matter.

According to the standard model of cosmology, visible stars and galaxies trace a pattern across the sky known as the cosmic web, which was originally etched out by dark matter — the substance thought to account for almost 80% of the Universe’s matter. Soon after the Big Bang, regions that were slightly denser than others pulled in dark matter, which clumped together and eventually collapsed into flat ‘pancakes’. “Where these pancakes intersect, you get long strands of dark matter, or filaments,” explains Jörg Dietrich, a cosmologist at the University Observatory Munich in Germany. Clusters of galaxies then formed at the nodes of the cosmic web, where these filaments crossed.

The presence of dark matter is usually inferred by the way its strong gravity bends light travelling from distant galaxies that lie behind it — distorting their apparent shapes as seen by telescopes on Earth. But it is difficult to observe this 'gravitational lensing' by dark matter in filaments because they contain relatively little mass.

Dietrich and his colleagues got around this problem by studying a particularly massive filament, 18 megaparsecs long, that bridges the galaxy clusters Abell 222 and Abell 223. Luckily, this dark bridge is oriented so that most of its mass lies along the line of sight to Earth, enhancing the lensing effect, explains Dietrich. The team examined the distortion of more than 40,000 background galaxies, and calculated that the mass in the filament is between 6.5 × 10¹³ and 9.8 × 10¹³ times the mass of the Sun. Their results are reported in Nature today¹.

Mass equation

By examining X-rays from plasma in the filament, observed by the XMM-Newton spacecraft², the team calculated that no more than 9% of the filament's mass could be made up of hot gas. The team's computer simulations suggest that roughly another 10% of the mass could be due to visible stars and galaxies. The bulk, therefore, must be dark matter, says Dietrich.

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Boffins' first glimpse of the structural framework of our universe

Bootnote

Direct measurement of a dark-matter ‘filament’ confirms its existence in a galaxy supercluster.

Mass equation