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What Lit the Lamps That Let Humanity Measure the Universe

Every year, around 1,000 Type Ia supernovae erupt in the sky. These starbursts brighten and then fade in a pattern so repetitive that they are used as “standard candles”—objects so uniformly bright that astronomers can tell the distance to one by its appearance.

Our understanding of the cosmos is based on these standard candles. Consider two of the greatest mysteries in cosmology: What is the rate at which the universe is expanding? And why is that rate of expansion accelerating? Efforts to understand both of these questions critically rely on distance measurements made using Type Ia supernovae.

Still, researchers don’t fully understand what drives these strangely uniform explosions — an uncertainty that worries theorists. If there are multiple ways they can happen, small inconsistencies in what they look like could mess up our cosmic measurements.

Over the past decade, support has grown for a particular story about what drives Type Ia supernovae—a story that traces each explosion to a pair of dim stars called white dwarfs. Now, for the first time, researchers have successfully recreated a Type Ia explosion in computer simulations of a double white dwarf scenario, giving the theory a critical boost. But the simulations have also thrown up some surprises, revealing how much we still have to learn about the engine behind some of the most important explosions in the universe.

Detonates the dwarf

For an object to serve as a standard candle, astronomers must know its inherent brightness, or luminosity. They can compare this to how bright (or dim) the object appears in the sky to calculate its distance.

In 1993, astronomer Mark Phillips plotted how the luminosity of a Type Ia supernova changes over time. Most importantly, almost all Type Ia supernovae follow this curve, known as the Phillips ratio. 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 astronomers have. But what is the reason for their consistency?

A hint comes from the unlikely element nickel. When a Type Ia supernova appears in the sky, astronomers detect an outpouring of radioactive nickel-56. And they know that nickel-56 comes from white dwarfs — dim, vaporous stars that retain only a dense Earth-sized core of carbon and oxygen, wrapped in a layer of helium. However, these white dwarfs are inert; supernovae are anything but. The puzzle is how to get from one state to another. “There’s still no clear ‘How do you do it?'” said Lars Bildsten, an astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, who specializes in Type Ia supernovae. “How do you make it explode?”

In computer simulations by Ruediger Pakmor’s team, the white dwarf companion also sometimes explodes. Researchers don’t know if this happens in nature.

Courtesy of Ruediger Packmore

Until about 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 cause a runaway nuclear reaction and detonate in a supernova.

Then in 2011, that theory was debunked. SN 2011fe, the closest Type Ia found in decades, was spotted so early in the explosion that astronomers had the opportunity to search for a companion star. None were seen.

The researchers turned their interest to a new theory, the so-called D6 scenario — an acronym for the speaker “dynamically driven double-degenerate double detonation,” coined by Ken Shen, an astrophysicist at the University of California, Berkeley. Scenario D6 proposes that a white dwarf captures another white dwarf and steals its helium, a process that releases so much heat that it triggers nuclear fusion in the first white dwarf’s helium shell. The melting helium sends a shock wave deep into the dwarf’s core. Then it detonates.

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