The most detailed 3-D map of the universe ever made, stretching back over billions of years, provides the best evidence yet that mysterious "dark matter" serves as the unseen scaffolding on which everything we can see is hung, astronomers reported Sunday.
The findings are based on years' worth of study, using data from space observatories as well as ground telescopes. But as impressive as those findings are, they still don't tell us exactly what dark matter is made of.
Astronomers from the Cosmic Evolution Survey, also known as COSMOS, revealed their results in a paper published online by the journal Nature and presented here at the winter meeting of the American Astronomical Society.
Just collecting the data required nearly 1,000 hours of observations using the Hubble Space Telescope's best camera, representing 10 percent of the past two years' available observing time.
"It's the largest project that's ever been done by the space telescope," Nick Scoville of the California Institute of Technology, principal investigator for the international research team, told journalists. Additional observations were made by the European Space Agency's XMM-Newton X-ray space telescope, NASA's Spitzer Space Telescope and ground-based telescopes in Hawaii, Chile and New Mexico — involving more than 80 scientists in all.
Locating the missing mass
The dark matter study was aimed at shining new light on one of the deepest mysteries facing astronomers today: Exactly where is the universe's missing mass?
Decades' worth of observations have found that all the matter we can see in surrounding galaxies doesn't account for the gravitational effects of those galaxies. In fact, there appears to be six times more dark matter out there than the ordinary matter we can see.
Is dark matter distributed in the same way as ordinary matter? Computer simulations and studies of individual galaxies, including our own Milky Way, have indicated that it is — and over the past few years, astronomers have been fleshing out their large-scale map of the universe's dark matter. Those maps show galaxies as knots of light caught up within in a "cosmic web" of unseen matter.
The findings announced Sunday provided a "first glimpse of the cosmic web" in true-to-life, three-dimensional detail, said Caltech's Richard Ellis, another member of the COSMOS team.
The lead author of the Nature paper, Caltech astronomer Richard Massey, said the COSMOS study provides the best confirmation that dark matter determines "the underlying structure of space." Galaxies as well as primordial globs of gas and dust form "within this dark-matter scaffolding," he said.
Massey said the findings were consistent with the current mainstream view among astronomers — in which ordinary matter accounts for just 4 percent of the universe, dark matter accounts for another 23 percent, and a mysterious repulsive force known as dark energy takes in the other 73 percent.
From smooth to clumpy
The findings shore up another mainstream theory as well: the idea that the universe started out with a smooth distribution of matter, both ordinary and dark, and became increasingly clumpy as billons of years rolled by.
That view was borne out by the COSMOS results. Astronomers looked farther back in their 3-D map — back as far as 6.5 billion years, when the universe was half its current age — and found that the dark matter was much more evenly distributed then.
Ellis said "it's very reassuring that we see the growth of the dark matter" into a clumpy web structure over time, because it bears out current prevailing theory.
Jason Rhodes of NASA's Jet Propulsion Laboratory, yet another COSMOS team member, said the 3-D map "showed the evolution of the dark matter distribution" over time, and may provide fresh hints for understanding the cosmic tug of war between gravity's attraction and dark energy's repulsion. The clumpiness of matter in the present-day universe may well be the result of that tug of war.
How to see the unseen
Rhodes said the COSMOS survey was also "an invaluable proof of concept" for the method that was used to chart dark matter.
Even though the matter itself couldn't be seen, astronomers detected its effect by analyzing the gravitational effect of that matter on light rays from more distant light sources. As light rays from faraway objects passed by, the unseen matter acted like a gravitational lens, bending and distorting those rays in characteristic ways.
Observations from other instruments, particularly the XMM-Newton space telescope, provided information about how far the light rays had traveled. The COSMIC team combined the data about the distortions with the data about the distances, then produced a 3-D map by building up separate layers. The process is similar to the way a 3-D map of the human body can be built up in a medical CT scan.
The technique has been used in the past to assemble data from ground-based telescopes into dark matter maps. Tony Tyson, an astronomer at the University of California at Davis, participated in an earlier study that took advantage of the technique, but not the COSMOS survey.
"What is different with this work is the higher resolution from space," he said.
The COSMOS findings by no means close the book on the dark matter mystery. First of all, even though the 3-D map is the biggest and most detailed yet, it takes in only a relatively small piece of the sky. Tyson said a full-sky survey would be required to get the full picture.
Eric Linder, an astronomer at the Berkeley Lab at the University of California at Berkeley, agreed: "It's definitely a big step there, but it's still a small part of the sky."
Although the distribution of dark matter mostly parallels the distribution of ordinary matter, there are discrepancies that show up in COSMOS' map. "For the dark-matter skeleton of mass in the universe, flesh sometimes occurs without supporting bones, and bones without surrounding flesh," Linder wrote in a Nature commentary.
Ellis said the COSMOS team was being "cautious" about reading too much into those discrepancies, but if more detailed maps continue to show places where dark matter exists without ordinary matter, or vice versa, that would require some tweaks in the theory.
The biggest question has to do with what exactly dark matter is made of, and the COSMOS findings have nothing to say on that score. There are several contenders for that role, including neutrinos, which were recently found to possess a tiny smidgen of mass; cold bits of ordinary matter, which have been dubbed massive compact halo objects, or MACHOs; and as-yet-undiscovered breeds of exotic matter called weakly interacting massive particles, or WIMPs.
Future experiments could help resolve that question. On Earth, physicists just might create the exotic constituents of dark matter in the next-generation particle accelerator known as the Large Hadron Collider. In space, more data could come from Hubble's Wide Field Planetary Camera 3, due for installation in 2008; or from a proposed space mission called the Supernova/Acceleration Probe, or SNAP.
"It's a tremendously exciting time," Ellis said.
In a previous version of this report, JPL's Jason Rhodes was misidentified.