A study of more than 200,000 galaxies shows the universe is producing half as much energy as it did two billion years ago, slowly dying in a long, drawn-out decline, astronomers say.

An international team of researchers used several of the world's most powerful telescopes to study the huge number of galaxies at wavelengths from infrared to far ultraviolet to measure how much energy they were putting out.

What they found agreed with previous calculations that the universe's light in these wavelengths is slowly going out.

"Newer galaxies are simply putting out less energy than galaxies did in the past," says astronomer Mehmet Alpasian, with NASA's Ames Research Center in Mountain View, Calif.

With older stars fading and going out faster than new stars are being created, eventually, the universe will end as a cold and very dark place, he said.

"At some point, all matter will eventually decay," he explains. "We're observing the lights slowly shutting down."

That eventual complete shutdown is a long way off, though, probably hundreds of trillions of years in the future.

The researchers' study, known as the Galaxy and Mass Assembly survey or GAMA, was presented at the general meeting of the International Astronomical Union held in Hawaii.

The study has also been submitted to the Monthly Notices of the Royal Astronomical Society.

The wide spread of wavelengths scanned for the study allowed the astronomers to look at stars of all ages, young and old, and at a huge variety of galaxies, some of them previously obscured by intergalactic dust.

The finding confirmed previous assumptions and allowed a more precise look at the universe's long decline, researchers say.

"We know that star formation peaked a few billion years ago and has been declining since," says GAMA team member Stephen Wilkins of the University of Sussex in England. "This is just a new way of measuring that decline."

The total energy output of the galaxies surveyed showed a significant decline over long time spans, the researchers say; from 2.25 billion years ago to 75 billion years ago, the energy output of the universe apparently fell by about 40 percent.

It's an indication that the universe does, in fact, have a lifespan – admittedly long, but finite, says study principal investigator Simon Driver with the International Center for Radio Astronomy Research in Western Australia.

"The universe will decline from here on in, like an old age that lasts forever," he says. "The universe has basically sat down on the sofa, pulled up a blanket and is about to nod off for an eternal doze."