Mapping the Universe

 

 

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The largest sky survey ever has unveiled dark energy and dark matter. These galactic phenomena make up 96 percent of the universe, and are leading scientists to question long-held beliefs such as Einstein’s theory of relativity.

 

 

Los Alamos, a world leader in computing since the beginning of the digital revolution is mounting a major initiative to help unravel the mysteries of dark matter and dark energy, producing the largest sky survey ever.

For most of the 20th century, everything that cosmologists observed in the heavens confirmed the laws of physics we know on Earth. But that’s about to change.

Starting with early observations of galaxy clusters, evidence has accumulated to suggest that the matter in planets, stars, and interstellar gas—ordinary matter made of neutrons, protons, and electrons—is but a small fraction of the matter in the universe. Most of it appears to be cold and dark, to have no electric charge (making it unable to emit or absorb light), and to collide so infrequently with other matter that it never heats up or cools down. This dark matter is invisible to us, but Newton's laws tell us that it must exist to provide the gravitational force that keeps the fastest stars confined within our own galaxy and the fastest galaxies bound into giant clusters.

In 1998, the search for distant supernovae (exploding stars) revealed that the overall expansion of the universe, which began about 14 billion years ago, is not slowing down as it should under the braking power of gravity.

Instead, the expansion appears to be accelerating under the influence of a mysterious force, dubbed dark energy. Over the past eight years, the evidence for dark energy has finally turned the world of physics on its ear. Computer simulations of the dark matter universe, when compared with the latest maps of luminous matter and the latest maps of the cosmic microwave background (the radiation left over from the early universe), indicate that the universe contains an astonishing 74 percent dark energy, 22 percent dark matter, and only 4 percent ordinary matter.

"This is a very stimulating time for physics. Fully 96 percent of the universe seems to be composed of stuff we've never seen directly on Earth!" says Emil Mottola, Los Alamos theorist, who has his own model of dark energy.

Los Alamos National Laboratory is collaborating in a multi-billion-dollar exploration of the deep universe. If they can discover the true nature of dark energy, they will find out whether Einstein's theory of general relativity, the description of the expanding universe that has held for 75 years, needs to be changed in some fundamental way.

Success will depend on coordinating theory, computation, and the many types of observation planned for the future. Lab Director Mike Anastasio has named the problem a Grand Challenge for Los Alamos and the Laboratory is assisting with the Sloan Digital Sky Survey at the Sloan telescope in southern New Mexico.

Dedicated to mapping the universe, the Sloan survey has imaged more than 200 million celestial objects, and its researchers have seen back in time to when the universe was about 5 billion years old and only two-thirds of its present size. The Sloan 2.5-meter-diameter digital telescope records continuously through the night, recording images of a narrow strip of sky the width of the moon as the Earth turns on its axis. The strips are then laid side by side to give a contiguous view of the quarter of the sky visible to the telescope.

Powerful computing capabilities, developed to simulate the performance of nuclear weapons in the U.S. stockpile, can be applied to the problem of simulating the cosmos. "We are also developing a unique statistical approach to minimize uncertainties in the predictions drawn from computer simulations," says Los Alamos scientist Katrin Heitmann. "It allows us to get more accurate results with many fewer simulations and to interpolate to new models of what the universe might look like."

Los Alamos is importing tens of terabytes of imaging and spectroscopic data—the entire Sloan Digital Sky Survey—with the goal of investigating gravitational lenses. A strong lens is a large mass concentration (presumably dominated by a dark matter halo) somewhere along the line of sight between the observer and a distant source that produces multiple images of the same object. Los Alamos scientist Przemek Wozniak is developing a high-speed program specially designed to search for the characteristic signature of multiple images of quasars lensed by galaxy cores to solve the evolution of the universe.

These are heady times in physics, with the biggest questions of all—what is the universe made of and how did it get here—being asked once again.