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A new study of one of the universe's fundamental constants casts doubt on a popular theory of dark energy, scientists say.

Dark energy is the name given to whatever is causing the expansion of the universe to accelerate. One theory predicts that an unchanging entity pervading space called the cosmological constant, originally suggested by Albert Einstein, is behind dark energy. But a popular alternative, called rolling scalar fields, suggests that whatever's causing dark energy isn't a constant, but has changed through time.

If that were true, though, it should have caused the values of other fundamental constants of nature to change, too. And a new measurement of one such constant, the ratio between the mass of the proton and mass of the electron, shows that this constant has remained remarkably steady over time.

The proton and the electron are two fundamental particles that make up the atoms inside stars, galaxies and people. Recently, a team of astronomers used the German Effelsberg 100-meter radio telescope to observe alcohol molecules in a distant galaxy, and found that the protons and electrons in those molecules' atoms weigh about the same as the ones right here on Earth.

Because the galaxy they studied lies 7 billion light-years away, its light has taken that long (7 billion years) to travel to Earth, and thus we are seeing it as it was half the universe's lifetime ago. (The universe is thought to be about 13.7 billion years old.) The observations strongly suggest that this fundamental constant has remained mostly unchanged over the past 7 billion years. [The Big Bang to Now in 10 Easy Steps ]

Building on that finding, which was detailed in the Dec. 14 issue of the journal Science, University of Arizona astronomer Rodger Thompson calculated how much the proton-to-electron mass ratio would change over time if the rolling scalar fields theory were true. He found that the predictions did not match the data.

That result supports the idea of Einstein's cosmological constant. "Basically, it says alternatives to Einstein's theory are really running out of room here," Thompson said Jan. 9 during a press conference at the 221st meeting of the American Astronomical Society (AAS) in Long Beach, Calif., where he presented his work. "This puts some very serious constraints on these rolling scalar field cosmologies."

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