What lit the lamps that allow humanity to measure the universe

around every year 1,000 Type Ia supernovae erupted in the sky. These starbursts light up and then fade in such a repeatable pattern that they are used as “standard candles”—objects so uniformly bright that astronomers can infer the distance to one by their appearance.

Our understanding of the universe 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 this rate of expansion accelerating?? Efforts to understand these two issues depend critically on distance measurements made with Type Ia supernovae.

However, researchers don’t fully understand what causes these oddly uniform explosions—an uncertainty that worries theorists. If there are several ways in which they could occur, small inconsistencies in how they appear could mess with our cosmological measurements.

Over the past decade, support has built up for a particular story about what triggers Type Ia supernovae—one that follows each explosion into a pair of faint stars called white dwarfs. Now, for the first time, researchers have succeeded in recreating a Type Ia explosion in a computer simulation of the double white dwarf scenario, giving the theory a crucial boost. But the simulation also produced some surprises, revealing just how much we have to learn about the engine behind some of the most important explosions in the universe.

Elf bombing

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 darker) the object is in the sky to calculate the distance.

In 1993, astronomer Mark Phillips conspiracy How the luminosity of Type Ia supernovae changes over time. Crucially, almost all Type Ia supernovae follow this curve, known as the Phillips relation. This consistency—along with the intense brightness of these explosions, which can be seen billions of light-years away—make them the most powerful standard candles astronomers have. But what is the reason for its consistency?

A hint comes from the unlikely element nickel. When a Type Ia supernova appears in the sky, astronomers detect a stream of radioactive nickel-56. They know that nickel-56 originates in white dwarfs — dim, dazzling stars that retain only a dense Earth-sized core of carbon and oxygen, covered by a layer of helium. However, these white dwarfs are inert. Supernovae are just that. The puzzle is how to get from one state to another. “There’s still a clean question,” he said, “How do you do this?” ” Lars Bildesten, an astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, who specializes in type I supernovae. “How do you make it explode?”

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

Courtesy of Rudiger Packmore

Until about 10 years ago, the prevailing theory held that a white dwarf sucked in 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 explode in a supernova.

Then in 2011, the theory was toppled. SN 2011fethe closest Type Ia found in decades, was spotted so early in its explosion that astronomers had the opportunity to search for a companion star. Nothing was seen.

The researchers turned their attention to a new theory, the so-called D6 scenario—an acronym that stands for “Double Dynamically Driven Decay Double Detonation” tongue detonator, coined by him Ken Shin, an astrophysicist at the University of California, Berkeley. The D6 scenario proposes that a white dwarf trap another white dwarf and steal its helium, a process that releases so much heat that it triggers nuclear fusion in the dwarf’s first helium shell. The fused helium sends a shock wave deep into the dwarf’s core. Then it explodes.

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