Light the lights that make mankind measure the universe

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Every year, around 1,000 Type Ia supernovas explode in the sky. These starbursts light up and then disappear in highly repetitive patterns—used as “standard candles”—objects so uniformly bright that astronomers can determine their distance by looking at each other.

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

But researchers do not fully understand what triggers these unusual uniform explosions – uncertainty worries the theorists. If there are many ways they can happen, small inconsistencies in how they appear can disrupt our cosmic parameters.

Over the past decade, support has grown for a different story about what causes Type Ia supernovae—a story that links each explosion to faint stars called white dwarfs. Now, for the first time, researchers have successfully recreated the AI ​​explosion in computer simulations of two white dwarfs, making the theory even more important. But the simulations also produced some surprises, revealing how much we still have to learn about the engine behind the most important explosions in the universe.

Blow up the pony

For an object to serve as a standard candle, astronomers need to know its brightness, or light. To work out the distance, they compare the object to how bright (or dim) it appears in the sky.

In the year In 1993, astronomer Mark Phillips calculated how the brightness of type E supernovae changes over time. Crucially, almost all type Ia supernovae follow this curve, known as the Phillips relation. This consistency—along with the intense luminosity of these explosions millions of light-years away—makes them the most powerful standard candles available to astronomers. But what is the reason for their persistence?

A clue comes from the elusive nickel element. When a Type IA supernova appears in the sky, astronomers know that it is a flood of radioactive nickel-56. And nickel-56 originates from a white dwarf – a dim, dense star with a dense, Earth-sized core of carbon and oxygen, covered by a layer of helium. Yet these white dwarfs are restless; Supernovas are something else. The puzzle is how to get from one state to another. “Nothing is clean yet,” said Lars Bildsten, a researcher and director of Kavli Theoretical Physics in Santa Barbara, California, who focuses on Type E supernovae. “How do you get it to explode?”

In computer simulations by Rüdiger’s Packmore team, the accompanying white dwarf sometimes explodes. Researchers don’t know if this happens in nature.

Courtesy of Rudiger Packmore

Until about 10 years ago, the theory was that a white dwarf would blow gas from a nearby star until the dwarf reached a critical mass. The core is hot and dense enough to trigger a runaway nuclear reaction and explode into a supernova.

Then in the year In 2011, the theory was debunked. SN 2011fe, the closest Type Ia found in decades, appeared early in the outburst, giving astronomers a chance to look for a companion star. None were seen.

Researchers have turned their attention to a new theory, the so-called D6 scenario—a tongue-twisting acronym for “dynamically driven double-degenerate double explosion,” coined by Ken Shen, an astronomer at the University of California, Berkeley. The D6 scenario proposes that a white dwarf captures another white dwarf and steals helium, a process that releases so much heat that it causes nuclear fusion in the first dwarf’s helium shell. Fusing helium sends a shock wave into the dwarf’s core. Then it explodes.

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