DOE's Office of Science Labs Make Progress in Understanding Dark Energy

Is there a mysterious source of energy driving the expansion of the universe, or is our understanding of gravity incorrect? DOE lab scientists use data from observations and images of the night sky to find out.

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M. Blanton and SDSSIII.

Zoom-in to a piece of the Milky Way night sky.

Since the 1920s, scientists have known that the galaxies—long thought fixed and unmoving—were actually receding deeper into the cosmos. In fact, we now know from observations of Type 1a supernovae that distant objects are speeding faster and faster away from us as the universe expands. That's because, not only is the universe expanding, it's expanding at an increasing rate. Scientists at Fermi National Accelerator Laboratory (Fermilab), Brookhaven National laboratory (BNL), SLAC National Accelerator Laboratory, and Lawrence Berkeley National Laboratory (Berkeley Lab) are eager to find out why. Recent observations by SLAC scientists and the Sloan Digital Sky Survey may provide an answer.

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Andrey Kravtsov, The University of Chicago, and Anatoly Klypin, New Mexico State University

A history of the universe's expansion

To date, scientists have offered two explanations for the accelerating expansion of the universe. Either some type of mysterious energy—dark energy, dark because unknown and unseen—drives the expansion, or Einstein's General Theory of Relativity, which describes how gravity works, is incorrect on cosmic scales; that is, our understanding of gravity is incomplete and gravity behaves differently from what we expect. Most theorists trust Einstein's theory of gravity and believe that dark energy drives the expansion. Its nature, however, remains unknown, somehow pushing objects apart while remaining unseen. From current observations, scientists estimate that ~70% of the mass-energy of the universe is dark energy.

But what is it and where does it come from? With these sorts of questions in mind, scientists studied the patterns of large galaxy clusters across the sky. These patterns provide information on the interplay between dark energy and gravity.

Once a statistically significant number of galaxy clusters, their masses, and their distances from Earth had been measured, the data was compared with predictions made by various dark-energy theories. The data supported the hypothesis that a form of dark energy drives the expansion of the universe. Specifically, it gave additional credence to the vacuum-energy model, which describes how dark energy could come from the very vacuum of space. Still, these results are not enough to call the vacuum-energy model complete. Much more data are needed. And some scientists continue to suspect that Einstein's theory of gravity may need to be modified.

To help find out the nature of dark energy and to decide between competing explanations, Fermilab and 23 institutions in the U.S., Brazil, Spain, Germany, and the U.K. are building a powerful camera to attach to the Blanco telescope in the Chilean Andes. This camera will allow scientists to collect data on galaxy clusters further away than any we have been able to observe in the past.

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The 570-megapixel Dark Energy Camera (DECam)—about 50 to 100 times the resolution of your typical home digital camera—will have some impressive specifications. It will have 74 highly sensitive detectors called charge-coupled devices (CCDs), which will be cooled to -100 degrees Celsius to minimize background noise. The CCDs will record the long wavelengths of light that come from extremely distant galaxies and supernovae (exploding stars). DECam will also have the world's largest shutter and filter, each about five feet tall. With this camera, scientists will be able to see 300 million galaxies, some of which are 10 million times fainter than what the naked eye can see.

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M. Blanton and SDSS-III

These images, derived from the SDSSIII largest image of the sky ever made, exemplify information which we can obtain from the full image on a wide range of scales, from one particular galaxy to the entire night sky. Above: Zooming into galaxy Messier 33 to a particular nebula, NGC 604. Below: Maps of the southern and northern hemispheres of the Milky Way.

Images of the sky offer much information for scientists to analyze. By comparing images from one point in time to another, we observe how the universe changes. The more images we collect, the better we understand these changes. Recently, the Sloan Digital Sky Survey (SDSS) collaboration released the largest digital image of the night sky ever made. It is composed of millions of images taken over the last twelve years. It is so large that 500,000 HD TVs would be needed to view the entire color image at its full resolution.

Analyzing the data from this image and those soon to be taken by DECam will help scientists discover which dark energy theory (if any) is correct or whether Einstein's theory of gravity is incomplete. For more information regarding the largest image of the sky ever made, see Berkeley Lab's press release.

Fermilab, SLAC, BNL, and Berkeley Lab are national laboratories funded by the Department of Energy's Office of Science. For more information on the research conducted at these laboratories, visit Fermilab's websiteExternal link, SLAC's websiteExternal link, Brookhaven's websiteExternal link, and Berkeley Lab's websiteExternal link. For more information about the Office of Science, visit science.energy.gov.

This article was written by Abigail Pillitteri, a writer for the Office of Science.

Last modified: 8/21/2015 2:02:24 PM