Color differences could recalibrate cosmic acceleration rate

Two flavors of Type 1a supernovas complicate distance estimates

the M101 galaxy

STAR BRIGHT  Color differences between some nearby supernovas, such as the one shown here (arrow) in this image of the M101 galaxy about 22 million light-years away, and ones farther away could lead to overestimates of how much the cosmic expansion is accelerating.

Swift/NASA

The expansion of the universe might not be accelerating quite as fast as researchers thought. Type 1a supernovas, exploding stars used as yardsticks to measure distances to other galaxies, come in two flavors, new research indicates. That complication could lead to overestimates of how remote the most far-flung supernovas are.

About two-thirds of type 1a supernovas in nearby galaxies are a bit redder than the others. In more distant galaxies, however, the red supernovas are in the minority, researchers report in the April 10 Astrophysical Journal. The color difference might cause researchers to underestimate the brightness of the farthest explosions — and therefore how far away they are, Peter Milne, an astronomer at the University of Arizona in Tucson, and colleagues suggest.

Type 1a supernovas are useful distance markers because researchers can deduce the intrinsic brightness of the explosion. Unfortunately, the previously unknown color difference has led astronomers to miscalculate how much gas and dust sits between some far-off supernovas and Earth, affecting estimates of how bright the detonations are. An underestimated brightness makes a supernova appear farther away than it actually is, which leads to an overestimate of how much the universe has expanded since the star exploded.

The missed distinction between supernova families doesn’t mean there’s no dark energy, the enigmatic force that’s expediting the cosmic expansion (SN: 11/30/13, p. 8). There just might be a little less of it. Milne’s team can’t say for certain how much less until they examine more supernovas.

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