What makes a supernova go boom? I've just been at a meeting on ways to observe such stellar explosions at the Royal Society in London, where there was a session discussing this question.
Type Ia supernovae result from the explosion of white dwarf stars and are now celebrated for their role in revealing that the expansion of the universe is accelerating. Textbooks say that they can form in one of two ways: The first (known as the single-degenerate model) is when a white dwarf -- itself an extremely dense star and thus gravitationally hungry -- rips matter from a binary companion until its own mass exceeds a critical limit (the Chandrasekhar mass, about 1.38 times the mass of the sun), triggering nuclear fusion and an explosion. Since they all explode when they hit the same mass, they explode with the same energy and peak luminosity, which means they can be used as "standard candles" for measuring distance -- that is, just by looking at how bright (or faded) they are, you can calculate how far away they must be. The second theorized way that fusion could be triggered is through the merger of two white dwarfs that have a combined mass that is greater than the Chandrasekhar limit (the double-degenerate model).
But that standard story contains a few gaping plot holes, as Marten van Kerkwjik of the University of Toronto pointed out at the meeting. First, there don't seem to be enough white dwarfs in close binaries to explain the number of Type Ia supernovae seen. Also, even in theory, explosions triggered in these ways do not naturally produce the mix of elements seen in observations -- unless you tweak the theory to make it fit. Oh, and the standard candle thing? They aren't so much "standard" as "standardizable" as van Kerkwjik also noted (that is technically, they aren't identical -- a consideration that was well understood and accounted for the in the dark energy discovery). All of these factors, van Kerwjik says, "cast doubt on the standard picture."
In 2010, van Kerkwjik and colleagues suggested an alternative theory: The merger of two carbon-oxygen white dwarfs can lead to Type Ia supernovae, even if their combined mass is less that the Chandrasekhar mass limit (arXiv:1006.4391v3). Simulations show that a merger of two white dwarfs, each with a mass of around 60 per cent of the sun could lead to an explosion that provides a better match with observations. (Follow up simulations by van Kerkwjik and others here.)
It's going to be hard to prove that van Kerkwjik's idea is correct with actual observations, however, since his predictions tend be "negative." For instance, if you look at a supernova remnant and *fail* to find evidence of a companion from which matter was accreted, then that's consistent with his alternative model -- but doesn't stand as proof for it. Astronomers have been searching for such evidence -- see for example, "An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5", Schaefer &Pagnotta, Nature 481, 164-166 (12 January 2012), which seems to at least rule out the single-degenerate models for that particular supernova.
The take-home message seemed to be that Type Ia supernovae are even more of a mixed bag than previously thought. Now variety may be the spice of life, but these supernovae have been lauded as standard candles because they all seem to be doing the same thing. The suggestions that things aren't quite that simple don't seem to affect the conclusion that the expansion of the universe is accelerating -- at least Brian Schmidt, who won a share of the Nobel for the discovery and who was sitting alongside van Kerkwjik on the discussion panel didn't seem to be sweating. But since so much of our understanding of the past and future of the universe is tied to these entities, it might be a good idea to work out what's going on with them.