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Mystery: How will it all end?

HALFWAY across the universe, a star lies dead. You write it off as routine, the sort of thing that happens a million times in this crummy neighbourhood. Only slowly do you realise how this case could shake cosmology to its core.

Something like this unfolded in March 2017 when, on a routine patrol of the night sky, at the University of Arizona came across something new. At first glance, it was just another type Ia supernova, the fiery end of an over-bloated white dwarf star.

White dwarfs pack the sun鈥檚 mass into just the volume of Earth. They often come in pairs, with one star feeding off the other, sucking material from it. Eat too much, and this vampire star can exceed the critical density at which carbon atoms in its core fuse into heavier elements. 鈥淚t鈥檚 basically a runaway thermonuclear bomb,鈥� says at the University of Southampton, UK. Within seconds, the star becomes billions of tonnes of radioactive shrapnel. As this decays over weeks and months, it gives off heat and radiation visible halfway across the universe.

Extensive observations of SN 2017cbv, however, suggested that the exploding star鈥檚 mysterious companion was not another white dwarf, but a larger star. This matters because we assumed type Ia supernovae all had the same trigger, and therefore a uniform brightness, meaning how they look to us depends only on their distance.

That assumption lay behind one of the most perplexing recent results in cosmology: the discovery in 1998 of a bunch of far-off type Ia supernovae that were consistently dimmer than expected. The conclusion was that these were further away than we had thought, pulled away from us by a shadowy force that is causing the universe to expand at an ever-increasing rate.

No one knows what the 鈥渄ark energy鈥� responsible might be. 鈥淚 hate to use the word antigravity, but it鈥檚 a good way of thinking about it,鈥� says Sullivan. Dark energy鈥檚 pull determines the universe鈥檚 size, its longevity, and even the manner of its eventual demise. If it were to grow sufficiently strong, it could eventually overcome the gravitational attraction holding matter together, resulting in a 鈥渂ig rip鈥� that would spell the end for anything interesting in the universe. A universe without enough dark energy to keep it stretched apart, meanwhile, could collapse back in on itself in a 鈥渂ig crunch鈥�.

Anomalous supernovae such as SN 2017cbv have caused raised eyebrows. 鈥淭here are a lot of things about type Ia supernovae that are still mysterious,鈥� says at the University of Notre Dame in Indiana 鈥� for example, that they have at least two possible triggers. 鈥淚f you got those mixed up, you could be misled,鈥� says Sullivan.

Could that, in turn, mean we have been led down a dark alley with dark energy? Perhaps. 鈥淚f there are two populations, and they鈥檙e mixed, we can get a systematic error,鈥� says Garnavich. 鈥淭here鈥檚 a chance that we鈥檙e not understanding the physics well enough.鈥�

Others are less concerned, saying the dominant uncertainty is not in our theoretical understanding of the explosions, but in our inability to measure them accurately enough. The best way to clear up the case would be to witness more stellar deaths as they happen, rather than a few days later, as is generally the case. 鈥淚t would be fantastic if we found a smoking gun,鈥� says Sullivan. That would blow things wide open.

This article appeared in print under the headline 鈥淥bject: Supernova SN 2017CBV鈥�