Selections from 2015: Two Kinds of Type Ia Supernovae

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift

Published April 2015


Main takeaway:

A team of scientists led by Peter Milne (University of Arizona) used ultraviolet observations from the Swift spacecraft to determine that type Ia supernovae, stellar explosions previously thought to all belong in the same class, actually fall into two subgroups: those that are slightly redder in NUV wavelengths and those that are slightly bluer.

Supernovae fraction

Plot of the percentage of supernovae that are NUV-blue (rather than NUV-red), as a function of redshift. NUV-blue supernovae dominate at higher redshifts. [Milne et al. 2015]

Why it’s interesting:

It turns out that the fraction of supernovae in each of these two groups is redshift-dependent. At low redshifts (i.e., nearby), the population of type Ia supernovae is dominated by NUV-red supernovae. At high redshifts (i.e., far away), the population is dominated by NUV-blue supernovae. Since cosmological distances are measured using Type Ia supernovae as standard candles, the fact that we’ve been modeling these supernovae all the same way (rather than treating them as two separate subclasses) means we may have been systematically misinterpreting distances.

What this means for the universe’s expansion:

This seemingly simple discovery carries hefty repercussions — in fact, our estimates of the expansion rate of the universe may be incorrect! The authors believe that if we correct for this error, we’ll find that the universe is not expanding as quickly as we thought.


Peter A. Milne et al 2015 ApJ 803 20. doi:10.1088/0004-637X/803/1/20