Black hole sun, for real?

Ooh, it's been a long time since there was an interesting new story about the possible role of micro black holes in the universe.  But here is one for my Christmas gift, in Science magazine:

Are tiny black holes hiding within giant stars? 

Some extracts:

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.”

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.

The only odd thing about this article is that it fails to talk about Hawking Radiation, which I thought would be a pretty significant issue affecting the life of a black hole which has been around since the dawn of time, and with a size of hydrogen atom.

I used to (many years ago) post a lot about micro black holes, out of concern that the LHC might accidentally make one with unclear consequences for the Earth.   If I recall correctly, there was considerable uncertainty as to the final fate of micro black holes - whether they would evaporate away to leave a remnant of some kind, or not.   

I should start scrolling Arxiv again, or my own old blog entries, to refresh my memory as to what people had thought about primordial black holes being able to account for dark matter.  (I'm pretty sure this is not an entirely new suggestion.)