Astronomers Determine Color of the Universe

The irregularities of individual computer screens make it tough to convey the new cosmological hue with complete accuracy, but below is the average color of all the light of the universe. It is, in other words, the color you would see if you could somehow view all the visible light in the universe together.

"The color is quite close to the standard shade of pale turqoise, although it's a few percent greener," says Karl Glazebrook, an assistant professor of astronomy in the Krieger School of Arts and Sciences at Hopkins. (The RGB values are 0.269, 0.388, 0.342.)

Glazebrook and Ivan Baldry, a postdoctoral fellow at Hopkins, are the authors of a presentation at this week's meeting of the American Astronomical Society that includes the new discovery.

Clues for hues
Although both authors joke about promoting "color of the universe T-shirts and coffee mugs" and other humorous implications, their determination of the color is really a byproduct of a serious attempt to use the light from thousands of galaxies to assess scientists' theories of the history of star formation and stellar population dynamics.

The scientists worked with data from the Australian 2dF Galaxy Redshift Survey, a survey of over 200,000 galaxies at a distance of 2 billion to 3 billion light years from Earth. The Anglo-Australian Observatory is conducting the survey.

Using the visible portion of the spectrum, Glazebrook and Baldry combined the data on the 2dF galaxies to produce a chart they call the "cosmic spectrum." For any given wavelength of visible light, the chart reveals the intensity -- the total amount of that light -- emitted by all the galaxies emitted by what Glazebrook and Baldry call the "local universe."

The cosmic spectrum initially took the standard scientific form of a graph, but researchers then transformed it into an array of colors, replacing each wavelength into the color the human eye sees at that wavelength, and varying the intensity of the color in proportion to that wavelength's intensity in the universe.

"This would be what we'd get if we took all the light in the universe and passed it through a prism to break the light into its component wavelengths and produce a rainbow," Baldry says. "We believe that the 2dF survey is large enough, reaching out several billion light years, to make this a truly representative sample."

Included in this rainbow is information on the prevalence of various elements in the universe, discernible by the dark and bright bands the elements leave at characteristic wavelengths of light particular to each element.

Glazebrook, Baldry, members of the 2dF team and other astronomers analyzed the results to check four different models of the rates of star formation through the history of the universe. By looking at each model's predictions for star formation during various time periods, astronomers could make some predictions regarding the cosmic spectrum the model should produce, and check the real cosmic spectrum to see how well it matched.

Fun With Opthalmology
For what Glazebrook calls "a bit of fun," he and Baldry determined how this universal light would be perceived by the human eye if it wasn't broken into its component parts. They used methods established by ophthalmologists to calculate the eye's response to particular wavelengths of light.

"From one perspective, it's surprising that it turns out to be greenish, because there are no green stars," says Glazebrook. "But it's the large numbers of old red stars and young blue stars in the universe that gives us the green."

Borrowing a term from the arts, the universe probably started with a "blue period" early in its history dominated by young blue stars, has moved into a middle "green period," and will eventually enter a final "red period" where decreased star formation allows older, redder stars to dominate the universe.

This research was funded in part by a grant from the David and Lucile Packard Foundation. The 2dF survey is funded by Australian and British governments.