A number of researchers, including Ohio State University astrophysicist David Weinberg, realized they might find some answers by looking at the way galaxies are distributed across the universe. The large-scale structure seen today has its origins in the turbulent infancy of the universe. Back then the cosmos was a hot, dense sea of particles, laced with density variations that arose during the very first fraction of a second after the Big Bang. These density variations produced pressure waves that crashed into each other over and over. Eventually, about 400,000 years after the Big Bang, the growing and cooling universe could no longer support these roiling waves. But their imprint remained, frozen in a pattern of high- and low-density distributions of matter that would ultimately develop into vast collections of galaxies and relatively empty cosmic voids.
Dwarf galaxy remnants streak the sky around our own Milky Way.
Image courtesy of V. Belokurov and SDSS
By merging this concept of the early universe with specific mathematical models of the effects of dark energy, scientists were able to predict a characteristic scale—a typical distance between concentrations of galaxies—that should be evident in the structure of the universe. “Wherever you see a concentration of galaxies today, you should find a slight excess of galaxies—about 1 percent—in a ring a billion light-years across surrounding that concentration,” say's Weinberg, who is the project scientist for SDSS-III.
That is just what the Sloan telescope found, zeroing in on a population of unusually bright red galaxies that were identified by Daniel Eisenstein of the University of Arizona. The results strongly confirm that dark energy is no illusion; there really is an unseen force pushing the universe apart. Future Sloan surveys should help unravel some of that mystery. By accumulating even larger catalogs of how galaxies are distributed through the universe, the Sloan telescope will measure the changing expansion speed of the universe, epoch by epoch, putting various theories of dark energy to the test. “I’m excited,” says Eisenstein, who took over last year as director of the SDSS-III project. “It’s a very robust way to probe dark energy.”
The Milky Way Has a Posse
Astronomers have known since the 1920s that our galaxy, the Milky Way, is surrounded by smaller collections of stars, essentially dwarf galaxies. The largest and brightest of these, the Large and Small Magellanic Clouds, are easily seen with the naked eye in the Southern Hemisphere. By the 1990s another 10 such companions had been found; these are much fainter and are visible only through powerful telescopes. But modern cosmological theories say there should be more of these galactic dwarfs—a lot more. The dark matter that outweighs visible matter by five to one should be fragmented, like the blobs in a lava lamp, into clumps with a range of sizes. Under the influence of gravity, each clump would pull regular matter into its center, forming stars and galaxies. The smallest clumps would create tiny galaxies, and there should be dozens or hundreds of them around. So where are they?
The Sloan Digital Sky Survey has bridged some of the gap between theory and observation by finding 15 more dwarf galaxies surrounding the Milky Way. Because the survey covers only one quarter of the sky and must look past various obstacles, both local and cosmic, it probably missed another 60 to 80 similar dwarf galaxies, according to Gerry Gilmore of the University of Cambridge. Thus the Sloan results bolster current ideas about dark matter, much as they confirmed the reality of dark energy. In the new picture, visible matter—the kind we are built of—is actually the exotic and rare stuff, making up just 4 percent of the universe.
Sloan’s discovery of all those minigalaxies changes our perception of how the cosmos evolved, Gilmore notes. Dwarf galaxies are too small to suck in much star-making gas. Whatever stars did form inside them came together long ago. The first stars in the universe were huge, fast-burning, and short-lived. They exploded and died, spewing gas that helped spawn a second generation of smaller, much longer-lived stars—many of which should still be around. “The stars in these dwarf galaxies,” Gilmore says, “have chemical properties that suggest they are indeed from that second set of stars. So you can study, star by star, the very oldest surviving objects in the universe.”
Dwarf galaxies also offer another way to study dark matter, because these galaxies are “almost pure dark-matter blobs, with just a few stars in them,” Gilmore adds. “The smallest one has perhaps a thousand stars but has a total mass equivalent to a million stars like the sun.” One key finding: At 300 light-years across, the dark components are bigger than most theorists expected. “Current theory suggests that dark matter should form much smaller blobs than that,” Gilmore says, “so this might suggest an important feature of the particles we should be looking at” to figure out what dark matter actually is.
The Milky Way Is a Galactic Cannibal
“When you look at a picture of a spiral galaxy like the Milky Way,” says Heidi Newberg of Rensselaer Polytechnic Institute in New York, “the most obvious thing is the spiral arms.” From her perspective, though, the real interest lies in a much larger but sparser spherical cloud of stars, known as the spheroid, surrounding such galaxies. Some stars in the spheroid are the remains of galactic cannibalism, having come from dwarf galaxies that fell into the spiral galaxy, were ripped apart by powerful tidal forces, and were incorporated into the larger galaxy’s structure.
