Mapping the universe

In its 5 terabytes of data are 217 million individual objects, including 800,000 galaxies and 100,000 quasars. 8220;Nobody8217;s ever done anything like this before,8221; said Bruce Gillespie, administrator of the Astrophysical Research Consortium, made up of 300 astronomers, which helped carry out the 100 million sky-mapping project. 8220;They8217;ll still be looking at this data in 50 years.8221;

Among the survey8217;s notable achievements has been helping to confirm the existence of dark energy, showing the universe is flat, and giving weight to the big-bang theory. But perhaps its greatest achievement has been bringing a sense of order to the seemingly undefined vastness of the universe.

The Sloan survey was the brainchild of Jim Gunn, a 69-year-old astrophysicist at Princeton University and one of the world8217;s leading experts on galaxy formation. Before Sloan, the most authoritative map of the heavens in visible light was the Palomar Observatory Sky Survey, which from 1950 to 1957 mapped the Northern Hemisphere8217;s night sky with its 48-inch Samuel Oschin Schmidt telescope. But by the mid-1980s, it was out of date.

Another motivating factor, Gillespie said, was a seminal discovery in the 1980s: Galaxies, once thought to be the largest structures in the universe, sometimes formed clusters. 8220;This was a hint of larger structures in the universe,8221; Gillespie said.

Gunn began designing a new telescope capable of capturing four times as much light as the Oschin instrument at Palomar Observatory in San Diego County, California. His goal was to map 1 million galaxies. As ambitious as it was, even the new sky survey would contain blind spots. The great river of stars in the Milky Way galaxy cut right through the night sky, obscuring a chunk of the universe stretching out behind it. Further, the telescope8217;s location in the Northern Hemisphere meant that most of the sky in the Southern Hemisphere would remain unexplored. The most daunting part of the survey was determining the distances to objects so that the map could be constructed in 3D.

Astronomer Edwin Hubble discovered nearly 80 years ago that the farther away a star or galaxy, the more its light shifts toward the red end of the spectrum. But to determine the red shifts of millions of objects? Gunn and his collaborators realised that could take hundreds of years. Gunn8217;s team came up with a simple solution. By feeding the image of the sky through aluminum plates with holes drilled to match the placement of stars and galaxies, they could analyse 640 objects at the same time. The team chose about 1.4 million of the most interesting objects within 2 billion light-years from Earth.

The survey found large numbers of galaxies clustered together. The most massive structure discovered is called the 8220;Great Wall8221;, a huge collection of galaxy clusters strung out in a huge filament arcing across the night sky, about 600 million light-years away. The Sloan collaboration is now preparing to launch the next phase8212;a six-year survey looking more deeply at galaxy clustering and the chemistry of the Milky Way.
_John Johnson Jr.,LATWP