Einstein Is Still Teaching Us
In 1936 Albert Einstein described a phenomenon that he believed was inevitable in principle but so rare in practice that we would never actually see it. According to his general theory of relativity, gravity warps space and bends light rays that pass near a massive object. If a distant galaxy were lined up right behind one more close by, this warping would bend and magnify the faraway galaxy’s image, a phenomenon now called gravitational lensing.
Einstein was right about the inevitable part but wrong about its rarity. Since the discovery of the first gravitational lens in 1979, astronomers have in fact seen innumerable examples. Sloan survey scientists are now using the magnifying power of gravitational lensing as yet another way to probe the construction of galaxies and the distribution of otherwise-undetectable dark matter.
From their studies of the dynamics of galaxies and galaxy clusters, researchers have inferred that pretty much every galaxy is surrounded by a huge, massive halo of dark matter. The mass and size of these halos are still known only crudely, however. That is where the Sloan survey and gravitational lensing come in. The lensing effect is strong if the distant galaxy is right behind the close one, much less if the distant galaxy is significantly off-axis. There are so many distant galaxies that every nearby galaxy is seen against a backdrop of thousands of others, and the image of each background galaxy is distorted just a little bit. Measuring these thousands of distortions and piecing all the information together could provide a detailed map of the mass of the nearby galaxy—a map based purely on the pull of gravity, not on light.
This kind of research is fantastically difficult. “The deflections are tiny. People were pretty sure we couldn’t detect them” with the Sloan telescope, says Rachel Mandelbaum, an astrophysicist at the Institute for Advanced Study (where, as it happens, Einstein was working when he made his half-correct prediction). But the Sloan has seen the distortions after all. Although the warping of distant galaxies proved too subtle to see one by one, Mandelbaum found that she could analyze the shapes of millions of background galaxies and detect a statistical departure from what such galaxies would normally look like. The pattern of stretching is now being used to probe the dark-matter clouds that are causing the distortion.
There are other ways to infer the presence of invisible mass, such as by looking at how cannibalized dwarf galaxies are shredded, as Heidi Newberg does, or by measuring the rotation speeds of stars within galaxies. The trouble with these methods is that they can detect dark matter only if it is close to the visible part of a galaxy, out to about 30,000 light-years. With lensing, Mandelbaum says, “we can probe to about a thousand times farther out.” And indeed, she is finding that the dark-matter halos appear huge, consistent with the once-bizarre but increasingly convincing picture that we live in a universe made mostly of dark energy and dark matter.
The Fun Is Just Beginning
In addition to probing distant quasars and massive clusters of galaxies, the Sloan surveys are making major discoveries closer to home. Astronomers can now learn about the chemical and orbital properties of asteroids just by identifying their color, guided by the tens of thousands of new asteroids revealed in the survey. Other Sloan researchers have identified a new class of white dwarfs, the cores left over after sun-size stars die, and have sighted elusive brown dwarfs, objects too big to be planets but not quite massive enough to ignite fusion reactions and become stars.
While much of last August’s gathering in Chicago celebrated what the Sloan has already achieved, astronomers were also treated to a preview of what lies ahead. Sloan III, says incoming director Eisenstein, is expected to last six years and will include four major projects. Two of them—a more extensive survey of luminous galaxies, intended to tease out more information about galaxy clustering on large scales, and a more sensitive search for the cannibalized remnants of dwarf galaxies—will extend recent findings from the second Sloan survey.
The other two projects are brand- new. One will peer toward the Milky Way’s core, studying the composition of the stars there. These stars, which include the first to form in our galaxy, should still display much of their original mix of elements. “It’s a way of tracking the history of the Milky Way, from youth to the present day,” Eisenstein says. The final project is a search for planets around other stars, exploiting the Sloan telescope’s exceptional ability to take in large patches of sky all at once. “Most ground-based telescopes look at one star at a time. We’ll be looking at 60,” Eisenstein adds.
That is the plan, anyway. Like previous Sloan projects, these four will undoubtedly add to our knowledge of the universe. But there will also undoubtedly be shocks, like the sighting of ancient quasars and the discovery of cannibal galaxies—and that is what really motivates the people who run the hardest-working telescope in the world. “We went in with this idea that we’d make a 2-D map of the universe and 3-D maps of galaxies,” Eisenstein says. “Now we’re making discoveries in almost every branch of astronomy.”
See for Yourself
The Sloan Digital Sky Survey has scanned, in incredible detail, a huge portion of the sky into an electronic database, and it is not just professional astronomers who get to play with all that data. Anyone with an Internet connection can explore the deep sky from his or her desktop—and even participate in an active research program.
The ultimate repository of Sloan data is the project’s own SkyServer. Visitors can browse the sky at will or pick spectacular objects of interest from lists created by the survey staff.
The University of Chicago department of astronomy has put together several videos based on the Sloan survey results, including 3-D flythroughs of the galactic map.
Everyone is invited to help astronomers classify galaxies found by the Sloan telescope. After a free registration and 15 questions to test your abilities, you are directed to the Galaxy Analysis page, where you categorize images that are tough to identify by computer.