It’s been said that in an infinite universe, anything can happen. Now, our universe may or may not count as infinite, but there’s definitely some weird stuff going on out there – and, as it turns out, a black hole sitting right in the middle of a star doesn’t even rate that highly in the unbelievability stakes.
Indeed, “Stars harboring a black hole at their center can live surprisingly long,” says Earl Bellinger, now an Assistant Professor at Yale University, who led a recent study into whether the scenario was feasible. “Our sun could even have a black hole as massive as the planet Mercury at its center without us noticing.”
It sounds impossible, we know – after all, black holes are pretty much defined by their ability to gobble up any and everything that gets too close. We’ve all seen 2009’s Star Trek; we know what happens when you plop a black hole inside an astronomical body, and spoiler alert: it doesn’t end well for the body.
Except, according to Bellinger and his colleagues, it wouldn’t just work – it could potentially clear up one of the most stubborn mysteries of the universe: where the heck all the dark matter has been hiding.
“The dark matter problem has now become serious,” Bellinger and his team write in their paper. “Numerous lines of evidence […] indicate that most of the matter in the Universe is invisible. Yet despite nearly a century of research, the origin of this matter remains unknown, and no compelling evidence has emerged for a solution.”
Ever since the 70s, though, there’s been one potential explanation that keeps resurfacing: primordial black holes. First proposed all the way back in 1966, these babies have never been proven to actually exist – but if they did, the hypothesis is that they formed within the first microseconds after the Big Bang, when the universe was still just a thick, dense sludge of particles.
Why should we care whether these ancient cosmic quicksands exist? Well, the suggestion is that – if there were enough of them out there, created at the right instant and hovering around the right sizes – then they might actually be what functions as the dark matter of the universe.
There is of course a problem with this idea: so far, there’s even less evidence of primordial black holes existing than of dark matter. Without some kind of previously undiscovered breakthrough, the consequences of this admittedly beguiling hypothesis would remain unknown.
Luckily, though, Bellinger and his colleagues had precisely such a realization. What if, they suggested, we just ignored all that and did it anyway?
The thought experiment
“Scientist[s] sometimes ask crazy questions in order to learn more,” said Selma de Mink, director of the stellar department at the Max Planck Institute for Astrophysics and one of the co-authors of the paper. “We don't even know whether such primordial black holes exist, but we can still do an interesting thought experiment.”
So: if we assume that dark matter is indeed composed of these tiny primordial black holes, what would happen? Well, the first thing the team realized is that there would be a heck of a lot more of them out there than we thought: they “would be far more numerous and far more densely spaced than stars,” the paper notes, “raising the possibility of their capture by stars or star-forming clouds.”
What happened next would depend on how big the primordial black hole was: a very small one – the size of an atom, say – wouldn’t do much at all, regardless of its being literally right in the middle of a star. In that case, “it could take longer than the lifetime of the universe to eat the star,” Bellinger told Science.
But a black hole the size of an asteroid or a small moon would get big, fast – though, to be fair, we’re talking astronomical timescales here, so “fast” still means “hundreds of millions of years”. The result would be something essentially indistinguishable from a normal star, and yet fundamentally different: “It will become a black hole-powered object rather than fusion-powered object,” study co-author Matt Caplan, a theoretical physicist at Illinois State University, told Science.
Of course, this immediately throws up another question: if these “Hawking stars”, as the team have dubbed them, are so similar to the regular kind, how would we ever know the difference?
The answer, it turns out, is exactly as beautifully bizarre as the rest of the study. You just listen to them.
The music of the void
“The main difference between such a Hawking star and a normal star would be near the core, which would become convective due to the accretion onto the black hole,” explains the Max Planck Society in a statement on the study.
“However, it could be detectable using the relatively new field of asteroseismology, where astronomers are using acoustic oscillations to probe the interior of a star.”
Failing that, the researchers could scan the skies for strange red giants – ones that are cooler than they ought to be. That low temperature might be a sign of a hidden black hole at the center of the star, rather than a normal stellar core, and here’s the really exciting part: we already know of around 500 of them.
Which is why Bellinger’s next step is to get funding to scour those “red stragglers”, as they’re known, and see whether any of them do indeed show signs of a black hole core. “We aim to perform a detailed asteroseismic characterization of stars being powered by [primordial black holes],” the team writes. “If they present a unique signature, then these objects could potentially be discovered through the data archives of the CoRoT, Kepler, and TESS missions.”
In best Star Trek fashion, it relies on a lot of hope and unknowns – but it might just work.
“There are good reasons to think that Hawking stars would be common in globular clusters and ultra-faint dwarf galaxies,” Caplan said.
“This means that Hawking stars could be a tool for testing both the existence of primordial black holes, and their possible role as dark matter.”
The study is published in The Astrophysical Journal.