What Lit the Lamps That Let Humanity Measure the Universe
Every year, around 1,000 Type Ia supernovas erupt in the sky. These stellar explosions brighten and then fade away in a pattern so repeatable that they’re used as “standard candles”—objects so uniformly bright that astronomers can deduce the distance to one of them by its appearance.
Our understanding of the cosmos is based on these standard candles. Consider two of the biggest mysteries in cosmology: What is the expansion rate of the universe? And why is that expansion rate accelerating? Efforts to understand both of these issues rely critically on distance measurements made using Type Ia supernovas.
Yet researchers don’t fully understand what triggers these strangely uniform explosions—an uncertainty that worries theorists. If there are multiple ways that they can happen, tiny inconsistencies in how they appear could be corrupting our cosmic measurements.
Over the past decade, support has accrued for a particular story about what sets off Type Ia supernovas—a story that traces each explosion to a pair of dim stars called white dwarfs. Now, for the first time, researchers have successfully re-created a Type Ia explosion in computer simulations of the double white dwarf scenario, giving the theory a critical boost. But the simulations also produced some surprises, revealing how much more we have to learn about the engine behind some of the most important explosions in the universe.
Detonating a Dwarf
For an object to serve as a standard candle, astronomers must know its inherent brightness, or luminosity. They can compare that to how bright (or dim) the object appears in the sky to work out its distance.
In 1993, the astronomer Mark Phillips plotted how the luminosity of Type Ia supernovas changes over time. Crucially, nearly all Type Ia supernovas follow this curve, known as the Phillips relationship. This consistency—along with the extreme luminosity of these explosions, which are visible billions of light-years away—makes them the most powerful standard candles that astronomers have. But what’s the reason for their consistency?
A hint comes from the unlikely element nickel. When a Type Ia supernova appears in the sky, astronomers detect radioactive nickel-56 flooding out. And they know that nickel-56 originates in white dwarfs—dim, fizzled-out stars that retain only a dense, Earth-size core of carbon and oxygen, enshrouded by a layer of helium. Yet these white dwarfs are inert; supernovas are anything but. The puzzle is how to get from one state to the other. “There still isn’t a clean ‘How do you do this?’” said Lars Bildsten, an astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, who specializes in Type Ia supernovas. “How do you get it to explode?”
Until around 10 years ago, the prevailing theory held that a white dwarf siphoned gas from a nearby star until the dwarf reached a critical mass. Its core would then become hot and dense enough to spark a runaway nuclear reaction and detonate into a supernova.
Then in 2011, the theory was overthrown. SN 2011fe, the closest Type Ia found in decades, was spotted so early in its explosion that astronomers had the chance to look for a companion star. None was seen.
Researchers shifted their interest to a new theory, the so-called D6 scenario—an acronym standing for the tongue twister “dynamically driven double-degenerate double detonation,” coined by Ken Shen, an astrophysicist at the University of California, Berkeley. The D6 scenario proposes that a white dwarf traps another white dwarf and steals its helium, a process that releases so much heat that it triggers nuclear fusion in the first dwarf’s helium shell. The fusing helium sends a shock wave deep into the dwarf’s core. It then detonates.