Revealed at last: Universe's intergalactic dark matter skeleton
Boffins' first glimpse of the structural framework of our universe
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 – totals 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. ®
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?"
We're here all week, folks.