Are tiny black holes hiding within giant stars?

Phenomenon could account for universe’s mysterious dark matter

a dark spot on the Sun’s surface

This month, an enormous dark and cool spot, known as a coronal hole, opened up on the Sun’s surface—almost as if it were being swallowed by a black hole.NASA

Grunge music: a source of validation for a generation of disaffected youth. And a surprising source of scientific inspiration for Earl Bellinger of the Max Planck Institute for Astrophysics. While listening to Soundgarden’s 1994 hit Black Hole Sun 2 years ago, he contemplated a curious question: Might itty-bitty black holes from the dawn of time be lurking in the hearts of giant stars?

A new study by Bellinger and colleagues suggests the idea is not so far-fetched. Astronomers could detect these trapped black holes by the vibrations they cause on the star’s surface. And if there’s enough of them out there, they could function as the mysterious dark matter that holds the universe together.

“It is speculative, but interesting,” says Juan García-Bellido, a theoretical physicist at the Autonomous University of Madrid who was not involved in the work, published today in The Astrophysical Journal. “It opens a new channel for the evolution of stars.”

Ordinary black holes are born in the deaths of gigantic stars, when their massive cores collapse and become so dense that even light can’t escape their gravitational pull. But in 1971, famed physicist Stephen Hawking proposed another possible origin. In the thick soup of particles present moments after the big bang, certain spots might have been dense enough to collapse and create black holes ranging in size from the microscopic to the incredibly huge.

If numerous and pervasive enough, these primordial black holes could function as the dark matter that knits the cosmic web together with its gravity and is thought to make up 85% of the matter in the universe. Astronomers have searched for them by looking for flashes that would arise when they pass in front of a distant, bright object and magnify its light like a lens. None have been spotted so far. But if a primordial black hole was tiny enough, with a mass roughly that of an asteroid and a diameter as small as a hydrogen atom, the flashes would be too dim to be picked up in such surveys.

Bellinger and his colleagues decided to consider the consequences of a universe in which dark matter was made entirely of such teensy black holes. On average, they found, one should be zipping through the Solar System at any given time. Some ought to occasionally get trapped within the gas clouds that give birth to stars, ending up in the center of a newly formed star. “I thought it would be kind of funny to put a black hole inside of a star and just see what happens,” Bellinger says.

The researchers found that the black holes would sink to the star’s core where hydrogen atoms undergo fusion to produce heat and light. At first, very little would happen. Even a dense stellar core is mostly empty space. The most microscopic of the black holes would have a hard time finding matter to consume and its growth would be extremely slow, Bellinger says. “It could take longer than the lifetime of the universe to eat the star.”

But larger ones, roughly as massive as the asteroid Ceres or the dwarf planet Pluto, would get bigger on timescales of only a few hundred million years. Material would spiral onto the black hole, forming a disk that would heat up through friction and emit radiation. Once the black hole was about as massive as Earth, it would produce significant amounts of radiation, shining brightly and churning up the star’s core like pot of boiling water. “It will become a black hole–powered object rather than fusion-powered object,” says study co-author Matt Caplan, a theoretical physicist at Illinois State University. He and his colleagues have dubbed these entities “Hawking stars.”

A middleweight black hole hides at center of giant star cluster – Astronomy  Now

To cool off, the exterior layers of a Hawking star would puff out, forming a red giant—the expected fate of the Sun as it gets older. But a red giant with a primordial black hole in its center would be slightly cooler than one that reaches that stage through normal means.

The European Space Agency’s Gaia satellite has spotted about 500 such anomalously cool giant stars, known as red stragglers, Bellinger says. To figure out whether these might actually be hiding a black hole, he says, astronomers could tune in to the particular frequencies at which the stars vibrate. Because a Hawking star would churn throughout its interior, rather than just in the topmost layers like an ordinary red giant, it would be expected to thrum with a particular combination of frequencies.

Such waves can be detected in the way the star’s light pulses and throbs. Bellinger is applying for funding to study the known red stragglers and see whether any display the characteristic vibrations of a black hole.

Still, the team appears to have avoided one crucial question—how often these primordial black holes would be expected to get stuck within a star, says astrophysicist Shravan Hanasoge of the Tata Institute of Fundamental Research. “The whole thing hinges on that calculation,” he says, which “in my opinion should have been performed at the very outset.”

Bellinger agrees and says his team considered the matter, but ultimately found there were too many unknowns to provide a definitive answer. He hopes to make progress on pinning it down in the near future.

And if he were to one day find a red straggler with the right surface features to indicate a primordial black hole? “That would be great,” he says. “I joked with some people that it would be the dumbest ever Nobel Prize, to discover dark matter because you’re inspired by Black Hole Sun.”

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