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The Universe is Green

Like a cosmic traffic light, the color of the universe is changing from green to red, but imperceptibly slowly. This is because the stars that supply the light are changing color as they age.

To us, the heavens are colorful. The sky is blue, the sun appears yellow, Mars looks red. But two Johns Hopkins University astronomers are interested in colors far beyond our solar system, on the galactic scale. Through a complex computation, they have found that the average color the galaxies emit is not one you would associate with the heavens.

Astronomer Karl Glazebrook explains what you'd see if you were able to observe a blend of all starlight from every direction. "If you had a very big eye and it was very sensitive and you could observe whole universe at once, you would see the color green," he says.

To determine this, Mr. Glazebrook and Ivan Baldry used data from a survey of more than 200,000 galaxies at a distance of two to three billion light years from Earth. They say this is a representative sample of all the galaxies in the universe.

The spectrum of wavelengths those galaxies emit initially was depicted as a graph, but Mr. Baldry and his colleague transformed it into a spectrum of colors that the eye would see for each wavelength at the same relative intensities that exist in the universe. "That's going from violet, blue up to red," he says. "So it's the colors of the rainbow."

The Johns Hopkins team had a serious scientific purpose for obtaining this spectrum. They wanted to determine the ages of the galaxies within the survey. The colors give the answer. Young, hot stars are blue. Middle aged ones appear greener. Old, cooler ones are red.

But for fun, Karl Glazebrook says they averaged the numerical values of the wavelengths of this color spectrum and turned it into a hue our eyes could recognize. "So if you mix in red with this blue, the universe actually becomes greener," he says.

But not just any green. Not the color of grass or of an olive. Ivan Baldry says it is much paler. "If you take the spectrum of the universe, the color you'd see is pale turquoise, or slightly greener," he says.

But pale turquoise was not always nature's decorating scheme. The average color has shifted over the eons as the stars in galaxies have aged and changed colors. Near the beginning of time, when stars began to form, all were young and blue, so the scientists say blue was the universal average color.

As stars aged and the rate of new star formation declined, the overall galactic color output moved toward green. Mr. Baldry says this trend will eventually lead to a dark red universal color in countless billions of years. "Now the rate of formation has gone down significantly," he says. "Those young, hot blue stars have less of an impact on the color of the universe. So the proportion of red stars is much higher."

And the proportion of red stars will increase with time. Karl Glazebrook says this is the result of a continuous decline in the raw material for star building. "It's basically because as the supply of gas in space is being depleted by star formation, there's less material to form stars," he says. "So the star formation rate declines."

This could become the ultimate fuel shortage and a dark universe, one in which the color is not cheerful, but black.

Gladly we are eons from that prospect. What today's pale turquoise color of the universe tells us is the cosmos is not yet old, nor ready to dim, but is still a vigorous middle age. " says Mr. Baldry. "Middle aged universe. That would be a good way to put it."