Which lit the lamps that enabled humanity to measure the universe
Every year approx 1,000 Type Ia supernovae erupt in the sky. These stellar bursts brighten and then fade away in a pattern so repeatable they’re used as “standard candles” — objects so uniformly bright that astronomers can tell the distance to one of them by their appearance.
Our understanding of the cosmos is based on these standard candles. Consider two of cosmology’s greatest mysteries: What is the expansion rate of the universe? And why is this rate of expansion accelerating? Efforts to understand both of these problems rely crucially on distance measurements made with Type Ia supernovas.
But researchers don’t quite understand what’s triggering these oddly uniform explosions — an uncertainty that worries theorists. If there are multiple ways they can occur, tiny inconsistencies in their appearance could skew our cosmic measurements.
In the past decade, a particular story about what triggers Type Ia supernovae has gained support – a story that attributes each explosion to a pair of faint stars called white dwarfs. Now, for the first time, researchers have successfully recreated a Type Ia explosion in computer simulations of the double white dwarf scenario, giving the theory a major boost. But the simulations also produced some surprises and showed how much more we need to learn about the engine behind some of the most important explosions in the universe.
Detonate a dwarf
For an object to serve as a standard candlestick, astronomers must know its inherent brightness, or luminosity. You can compare this to how bright (or dark) the object appears in the sky to find its distance.
In 1993, astronomer Mark Phillips recorded how the luminosity of Type Ia supernovae changes over time. Crucially, almost all Type Ia supernovae follow this curve, known as the Phillips relationship. This consistency — coupled with the extreme luminosity of these explosions, visible billions of light-years away — makes them some of the most powerful standard candles astronomers have. But what is the reason for their consistency?
One clue comes from the unlikely element nickel. When a Type Ia supernova appears in the sky, astronomers discover that radioactive nickel-56 is emanating from it. And they know that Nickel-56 comes from white dwarfs—dim, deflagrated stars containing only a dense, Earth-sized core of carbon and oxygen surrounded by a layer of helium. But these white dwarfs are sluggish; Supernovae are anything but. The puzzle is how to get from one state to the other. “There’s still no clear ‘how do you do that?'” 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?”
Up until about 10 years ago, the prevailing theory was that a white dwarf sucked gas from a nearby star until the dwarf reached critical mass. Its core would then get hot and dense enough to trigger a runaway nuclear reaction and detonate in a supernova.
Then in 2011 the theory was overthrown. SN 2011fe, the next Type Ia found in decades, was caught early in its explosion that astronomers had a chance to start looking for a companion star. None was seen.
The researchers shifted their interest to a new theory called the D6 scenario — an acronym that stands 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 captures 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 envelope. The melting helium sends a shockwave deep into the dwarf’s core. It then detonates.