Prophets of gloom might be right. The universe is gradually growing darker. That’s the conclusion of astronomers who have conducted a detailed analysis of the colors of some 37,000 nearby galaxies.

A galaxy’s spectrum is a good indicator of the rate of past star birth because most of the light from newly minted stars is blue. In contrast, older stars are redder. Galaxies with an overall reddish tinge therefore contain mostly old stars.

The new analysis, based on galaxies studied at many wavelengths by the Sloan Digital Sky Survey, reveals that the rate of star formation today is only one-thirtieth of what it was 6 billion years ago. With that pace of dimming, an observer 5 billion years from now would see a universe about twice as dark as today. Alan F. Heavens of the University of Edinburgh in Scotland and his colleagues describe their findings in the August Monthly Notices of the Royal Astronomical Society.

Other studies had already found similar results, but they relied on observations of galaxies so distant that the light that telescopes record today shows what the galaxies looked like billions of years ago. In contrast, Heavens’ team looked only at nearby galaxies. The study is the “first time that star-formation history has been determined from the fossil record of the present-day spectra of galaxies,” says Heavens.


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