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What lights up the lamps for humanity to measure the universe

Annually, about 1,000 Type Ia supernovae erupt in the sky These stellar explosions light up and then dim in a pattern so repeatable that they are used as “standard candles”—uniformly bright objects that come so that astronomers can infer the distance to one of them based on their appearance.

Our understanding of the universe is based on these standard candles. Consider two of the greatest mysteries in cosmology: What is the expansion rate of the universe?? And why is that expansion speed up? Attempts to understand both of these problems rely primarily on distance measurements made with type Ia supernovae.

However, researchers still don’t fully understand what causes these strange uniform outbursts – an uncertainty that worries theorists. If there are multiple ways in which they can occur, minor inconsistencies in how they appear could derail our measurements of the universe.

Over the past decade, support has accumulated for a particular story of what makes a Type Ia supernova—one that traces each explosion to a pair of faint stars known as white dwarfs. Now, for the first time, researchers have successfully reproduced a Type Ia explosion in computer simulations of a double white dwarf scenario, giving the theory an important boost. But the simulations also produced some surprises, showing how much more we still have to learn about the motives behind some of the most important explosions in the universe.

Detonate a Dwarf

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

In 1993, astronomer Mark Phillips conspiracy How the luminosity of a type Ia supernova changes over time. Importantly, nearly all Type Ia supernovae follow this curve, known as the Phillips relationship. This consistency—along with the extreme luminosity of these explosions, visible billions of light years away—makes them the most powerful standard candles astronomers have. But what is the reason for their consistency?

One hint comes from the uncertain element nickel. When a Type Ia supernova appeared in the sky, astronomers detected radioactive nickel-56 spilling out. And they know that nickel-56 originates from white dwarfs—dim, deflated stars that retain only a dense, Earth-sized core of carbon and oxygen covered with a layer of helium. However, these white dwarfs are inert; Supernova is anything but. The puzzle is how to go from one state to another. “There is still no clear question ‘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 make it explode?”

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

Courtesy of Rudiger Pakmor

Until about 10 years ago, the prevailing theory was that a white dwarf sucked in gas from a nearby star until the dwarf reached its critical mass. Its core would then become hot and dense enough to trigger a nuclear reaction and explode into a supernova.

Then in 2011, this theory was overturned. SN 2011feThe closest type Ia found in decades, was discovered so early in its explosion that astronomers had a chance to find a companion star. No one is seen.

The researchers turned their attention to a new theory, the so-called scenario D6—an acronym for the “dynamically controlled double degenerate double explosion,” coined by Ken Shen, an astrophysicist at the University of California, Berkeley. Scenario D6 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 first dwarf’s helium shell. . Molten helium sends shock waves deep into the dwarf’s core. Then it exploded.

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