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.
The answer is simple and obvious - those dark filaments are the cooled hyper-pasta of the Flying Spaghetti Monster's noodly appendages!
Thanks, my jacket and hat is the pirate one with the book on global warming in the pocket...
Very interesting post, and one I agree with practically everything on. Whoever your leading astronomer was, I'd agree with him, too, absolutely. The LHC may be able to turn up evidence on any supersymmetry in nature. If nature is supersymmetric, then there exists a lightest supersymmetric particle, into which eventually all other supersymmetric particles will tend to decay. The LSP itself can't decay easily, since the only possible route is into normal standard model particles. If there's an LSP, then, that *is* a dark matter. It may not be abundant enough to solve the dark matter problem either - personally I suspect that an LSP does exist and it won't be abundant enough to be the entire solution - but it's certainly part of the answer.
For the record, neutrinos are technically a dark matter, too, but they definitely cannot be "the" dark matter -- they don't cluster anything like enough to give us the observed distribution of galaxy clusters.
Otherwise, I'd agree with most of what you've said. I'm not at all convinced by MOND itself, since it's an ad-hoc, unmotivated modification of one of Newton's laws - and Milgrom himself is very happy to admit that it's nothing other than phenomenology. It cannot be applied to cosmology and even applying it on cluster scales it dies. However, it fits galaxy rotation curves so well that personally I think it's a sign that there's something interesting going on - even if it turns out to be (chiefly) particulate in origin and happens to be describable by the MOND equations. MOND is not the only modification to gravity being actively studied; a relativistic version of MOND can be described as a "scalar/vector/tensor" theory of gravity (a scalar field - in effect similar in a way to the Higg's, actually - a vector field similar in a way to an electromagnetic field, and a tensor field similar to the gravitational field of general relativity). There are other SVT theories kicking around, and enormous numbers of scalar/tensor theories starting with Brans-Dicke theories back in the 60s. There are also things like "Einstein-aether theories" which attempt to keep practically every allowed symmetry, bimetric theories which are related to SVT theories (or vice-versa, depending which you prefer) and so on. This is definitely an active field of research.
Along those lines, it's also worth noting that galaxies do not live in the Minkowski spacetime of special relativity. Instead, if we assume there is no dark matter, they live in some sort of cylindrical spacetime while if we assume there *is* dark matter they live in a complicated mix of cylindrical and spherical. While the gravitational potentials on the outskirts of a galaxy are certainly small, it's not entirely obvious that they're being defined with respect to the right background spacetime -- we almost universally assume that to be Minkowski. It's still controversial whether this has any impact. Probably not, but in principle we definitely are applying gravity wrongly.
As for dark energy, since I've worked myself on novel ways of dealing with it - in my case, chiefly from the fact that cosmology has gone about things totally the wrong way, assuming the universe to be flat and then adding ripples on, whereas the real universe is actually lumpy and we should instead reconstruct the flat background; the complicated nature of GR means that these approaches, which are equivalent in Newtonian theory, bear no resemblance to one another - I'd balk at saying it's "easier to explain away". Certainly the foundations for declaring it are relatively weak... except that observational support for the standard cosmological paradigm, assuming a dark energy and dark matter, whatever they may be, is staggeringly overwhelming. And attempts to do work like I have leads you either to make approximations that aren't quite valid (as I've done) and find nothing that acts like dark energy, or into a problem that's so complicated as to be practically intractable.
Still, you're totally right and everything absolutely must be considered - and it is being considered. My hunch is that "dark matter" is a combination of a lot of things, including that gravity does not behave as GR on large scales (probably not even on galactic scales, more likely not not on cluster scales, and probably not on cosmological scales), that we're applying it wrongly anyway, that there is at least one supersymmetric particle cluttering up the universe, that there may even be sterile neutrinos, and so forth. Problem is that at the minute the data we have is more than good enough to pin down "dark matter" but as soon as we split that in two or three parts the errors bars go shooting through the roof and you constrain practically nothing.
if you happened to be asleep that day in your astrophysics class
Yes, I was! So please explain this in simple measurements I can understand such as standard London double decker bus or bronotosaurus lengths!