Grunge music: a source of affirmation for a generation of disillusioned young people. And an unexpected wellspring of scientific creativity for Earl Bellinger from the Max Planck Institute for Astrophysics. While immersing himself in Soundgarden’s 1994 hit “Black Hole Sun” two years ago, he pondered an intriguing question: Could diminutive black holes from the early universe be concealed within the cores of massive stars?
A recent study by Bellinger and his colleagues proposes that this notion is not as outlandish as it might seem. Astronomers could potentially detect these ensnared black holes by observing the vibrations they induce on the surface of the star. Moreover, if a sufficient number of them exist, they could serve as the enigmatic dark matter responsible for the cohesion of the cosmos.
“It is speculative, but intriguing,” remarks Juan García-Bellido, a theoretical physicist at the Autonomous University of Madrid, who was not part of the research but commented on it. The study, published in The Astrophysical Journal, introduces a novel avenue for understanding the evolution of stars.
Traditional black holes originate from the demise of massive stars, when their dense cores collapse to a point where not even light can escape their gravitational grasp. However, in 1971, renowned physicist Stephen Hawking proposed an alternative genesis. In the dense particle soup immediately following the big bang, certain regions might have possessed sufficient density to collapse and form black holes spanning from minuscule to immense sizes.
If these primordial black holes are abundant and widespread, they could constitute the dark matter that binds the cosmic framework together through gravitational force, comprising an estimated 85% of the universe’s matter. Astronomers have sought them by observing transient brightenings that occur when they pass in front of distant luminous objects, amplifying their light like a cosmic lens. So far, none have been detected. However, if a primordial black hole were sufficiently tiny, with a mass akin to an asteroid and a diameter as minute as a hydrogen atom, such surveys would likely overlook their faint flashes.
Bellinger and his team decided to explore the implications of a cosmos where dark matter consisted entirely of such minuscule black holes. On average, they calculated that one of these black holes should traverse the Solar System at any given moment. Some may occasionally become ensnared within the nebulous clouds that give rise to stars, ultimately settling at the core of a nascent star. “I thought it would be amusing to place a black hole inside a star and observe the consequences,” remarks Bellinger.
The researchers determined that these black holes would descend to the star’s core, where hydrogen fusion generates heat and light. Initially, little would occur, as even a dense stellar core is predominantly empty space. The tiniest black holes would struggle to find matter to consume, leading to exceedingly slow growth. “It could take longer than the age of the universe to devour the star,” Bellinger notes.
However, larger black holes, roughly the mass of Ceres or Pluto, would undergo significant growth within a few hundred million years. Material would spiral onto the black hole, forming a disk that heats up due to friction and emits radiation. Once the black hole approaches the mass of Earth, it would emit substantial radiation, illuminating brightly and stirring the star’s core akin to a boiling pot. “It would transition from being powered by fusion to being powered by the black hole,” explains study co-author Matt Caplan, a theoretical physicist at Illinois State University. They refer to these entities as “Hawking stars.”
To cool down, the outer layers of a Hawking star would expand, creating a red giant—the anticipated fate of the Sun as it ages. However, a red giant harboring a primordial black hole at its core would be slightly cooler than one evolving through conventional means.
According to Bellinger, the European Space Agency’s Gaia satellite has identified approximately 500 such anomalously cool giant stars, known as red stragglers. To determine whether any of these might conceal a black hole, astronomers could analyze the specific frequencies at which these stars vibrate. Because a Hawking star would exhibit internal turbulence rather than merely at its surface like a typical red giant, it would pulsate with a distinctive combination of frequencies.
These oscillations can be discerned through variations in the star’s light intensity and patterns. Bellinger intends to seek funding to investigate known red stragglers and ascertain whether any exhibit the characteristic vibrations of a black hole.
However, astrophysicist Shravan Hanasoge from the Tata Institute of Fundamental Research highlights a crucial omission from the team’s study—how frequently these primordial black holes are anticipated to become trapped within a star. “The entire premise hinges on that calculation,” he asserts, suggesting that this calculation should have been a primary consideration from the outset.
Bellinger concurs, noting that his team deliberated on the matter but found too many uncertainties to offer a definitive conclusion. Nevertheless, he aims to make headway in addressing this question in the near future.
And if, one day, he were to identify a red straggler exhibiting the surface characteristics indicative of a primordial black hole? “That would be fantastic,” he exclaims. “I jestingly remarked to some colleagues that it would be the most unconventional Nobel Prize ever, discovering dark matter because of inspiration drawn from ‘Black Hole Sun.’”
