As I understand it, we used Type Ia supernovas to determine that there is some sort of repulsive force (which we call dark energy) making things fly away from us at an accelerated rate. The reason we could use this measurement is because a Type Ia always happens when a white dwarf reaches 1.4 solar masses.

According to this new research, it's possible that some white dwarfs can actually reach more than 1.4 solar masses.

As Dr. Plait explains

So what you get is a white dwarf with more than enough mass to explode, but its spin prevents the supernova from occurring. For a while, that is. Various factors slow the star down over time (for example, a magnetic field will accelerate particles in the stellar wind, acting a bit like a parachute dragging on the white dwarf). At some point — and this may take a billion years — the white dwarf slows to the point where centrifugal force can no longer win the fight against gravity. Fusion of the material begins, and BANG! Supernova.

Which would mean that a white dwarf of 1.5 solar masses could be causing a supernova. What implications does this have on the method we used to determine the effect of dark energy?


1 Answer 1


Good question. I have a vague idea about how errors like this are catered for so I'll take a shot at answering your question. I stand to be corrected by anyone who's closer to SNe Ia cosmology. The short answer is that the discovery of dark energy is based on empirical calibrations, so any scatter in the progenitors of the supernovae is already accommodated.

The first thing is that the theory of Type Ia's is actually not all that clear. The overall picture of nuclear detonation in a carbon-oxygen white dwarf is pretty solid but there are some tricky bits to explain. For example, the accretion onto the WD must be within a very narrow range. Too little and the material builds up, surface detonations blast off the material, and the WD doesn't grow. Too much and, again, surface detonations wreck the picture. I think there's still a mismatch in the predictions from theoretical populations and the number of Ia's we actually see. Also, there's a lingering suspicion that there are other progenitor channels. There are already some underluminous Type Ia's and it is thought that some WD mergers will also cause trouble.

So, at the end of the day, the calibrations that are used for the cosmological results are empirical, so they already accommodate statistical scatter in the Type Ia sample without knowing whether or not we're accidentally confusing more than one type of event. The new spin-up/spin-down paper might explain some of the brightness variation in the samples but the spread is implicitly already in the dark energy result.

Refining our understanding of Type Ia's is big business for exactly this reason. As far as I know, the biggest contributor on the error of the pure SNe results is from the lightcurve fits. Improving those would therefore improve the precision of the cosmological result. But, that said, we now know independently that dark energy is out there. Dark energy was discovered before WMAP and those results are now bolstered by baryon acoustic oscillations. Those two results each centre on the standard cosmology, but with much bigger scatter. Fortunately, when we combine the data of SNe Ia, WMAP, and BAO, they all neatly converge on one set of parameters, which is why it is sometime called the "concordance" model.

  • $\begingroup$ "there's a lingering suspicion that there are other progenitor channels": This cannot be emphasized enough. There are all sorts of Ia's - underluminous, overluminous, rapid evolving, lacking Si II, extra intermediate-mass elements, strange light curves, ... Cosmologists throw out a large chunk of the data before processing it - anything "unusual" in any way is simply not counted. $\endgroup$
    – user10851
    Commented Dec 28, 2012 at 22:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.