Ever since the Big Bang, the universe has been expanding. In the 1990s cosmologists observing a special type of star explosion called a type Ia supernova concluded that the rate of the universe's expansion was actually accelerating, as though some unseen force were pushing the universe outward.

Now a team of scientists have detected a variation in the brightness of type Ia supernovae, one that suggests that the acceleration may not be as much as previously thought. If this is true, then this unseen force – now called dark energy – may not be quite as abundant as scientists once thought.

A type Ia supernova occurs when a system of two stars, one of which is a white dwarf star, explodes spectacularly. Up until now, it was believed that all type Ia supernova are equally bright at their peak luminosity, making them useful for measuring distances across the universe.

In the 1990s, astronomers Brian Schmidt, Saul Perlmutter, and Adam Riess independently determined that distant type Ia supernova were actually fainter than would be expected if the universe were expanding at a uniform rate. Thus, they concluded that the expansion of the universe was accelerating, pushed along by mysterious dark energy. Their work won them the 2011 Nobel Prize in Physics.

But now the team, led by University of Arizona astronomer Peter A. Milne, has found that there are actually two subclasses of type Ia supernova. Using data from NASA's Swift spacecraft and the Hubble Space Telescope, Dr. Milne and his team found variations in the light emitted by different supernovae, particularly in the ultraviolet wavelengths, with some emitting light that is shifted toward the red end of the spectrum and others with light that shifts toward the blue end.

A University of Arizona press release likened it to buying several 100-watt light bulbs at the hardware store only to discover they vary in brightness.

"The idea behind this reasoning," Milne explained in the press release, "is that type Ia supernovae happen to be the same brightness — they all end up pretty similar when they explode. Once people knew why, they started using them as mileposts for the far side of the universe."

Milne continued: "The faraway supernovae should be like the ones nearby because they look like them, but because they're fainter than expected, it led people to conclude they're farther away than expected, and this in turn has led to the conclusion that the universe is expanding faster than it did in the past."

Milne and his colleagues still believe that the universe's expansion is accelerating, just not as much. This means that scientists calculations for the amount of dark energy, currently thought to comprise 68.3 percent of the mass-energy of the observable universe, will also need to be revised.

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"We're proposing that our data suggest there might be less dark energy than textbook knowledge, but we can't put a number on it," Milne said in the press release. "Until our paper, the two populations of supernovae were treated as the same population. To get that final answer, you need to do all that work again, separately for the red and for the blue population."

"The changing fractions of type Ia supernova NUV–optical subclasses with redshift" appears in the current issue of Astrophysical Journal.