How can one possibly establish the non-existence of certain objects within an Immeasurably vast Universe? This question continues to perplex researchers who examine gravitational waves—ripples in spacetime generated when two massive celestial bodies, such as black holes, merge. For decades, theorists maintained that, counterintuitively, stars within a specific range of very large masses simply couldn’t undergo collapse to form black holes. However, astronomers studying gravitational waves have thus far failed to find any evidence of this so-called “mass gap”—until now.
“What they are observing aligns strongly with what we predicted,” remarks Stanford Woosley, a theoretical astrophysicist at the University of California, Santa Cruz (UCSC), who roughly forecast the observed mass range using theoretical models in the early 2000s. “Personally, I’m very pleased to see this.”
When a massive star exhausts its nuclear fuel, it collapses into an infinitesimally small point, leaving behind only profoundly powerful gravitational fields—a black hole. Yet, as far back as the 1960s, physicists first hypothesized that stars of exceptional mass might meet their end through a different route—exploding with such immense force that they are completely blown apart, scattering their matter everywhere and leaving behind absolutely nothing, not even a black hole.
This outcome occurs because the largest stars can avert internal collapse due to the tremendous pressure generated by the light within them. If a star heats up sufficiently, the particles constituting this light, known as photons, begin to spontaneously convert into electron-positron pairs, which diminishes the pressure sustaining the star. Eventually, gravity briefly overcomes this pressure, compressing the stellar core until it ignites in a catastrophic explosion, resulting in a brilliant supernova.
Astronomers believe this phenomenon, termed pair-instability, affects stars in the “forbidden range” following their demise. “They don’t leave behind a black hole,” explains Maya Fishbach, an astrophysicist at the University of Toronto and a co-author of the new paper. “Instead, the entire star just explodes, resulting in nothing remaining.” This prediction created a decades-long puzzle: precisely how massive must stars be to self-destruct completely after death?
Since 2015, scientists have possessed the means to tackle this problem through direct observation. Instruments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), with stations located in Louisiana and Washington State, and the Virgo interferometer in Italy, have been capable of detecting the incredibly faint gravitational waves produced when two black holes merge. Over the past decade, LIGO and Virgo have recorded hundreds of these black hole mergers, determining the masses and spins of the coalescing objects from the resulting ripples. Therefore, in theory, researchers merely need to check if a significant gap exists within the spectrum of black hole masses.
However, there is a crucial subtlety. As indicated by LIGO and Virgo observations, two smaller black holes can merge to build one larger one, and the resulting heavier, “second-generation” black holes formed from these mergers could potentially populate this alleged upper mass gap. (A lower mass gap might also exist, stemming from stars too small to form black holes.) Simply examining the masses of all observed black holes does not reveal an obvious discontinuity.
To circumvent this issue, Fishbach and her collaborators undertook a deeper analysis. They scrutinized 153 mergers involving heavy black holes. Critically, they disregarded the mass of the heavier black hole in each merging pair, focusing instead on the mass of the lighter component, as it was more likely to be a first-generation black hole formed from stellar collapse. Indeed, they identified a range of masses—commencing around 44 solar masses—where the stars fail to produce black holes, as detailed in their paper published in the journal Nature.
Although the researchers did find several black holes weighing about 90 solar masses, the measured spin rates for these black holes were faster than anticipated, suggesting they originated from a prior merger of two smaller black holes.
“The narrative presented by the authors is robust and aligns with everything we currently know,” comments Ryan Foley, an astronomer at UC Santa Cruz. Joana Krün, a particle astrophysicist from Durham University who was not involved in the new research, states that the mass range reported in this paper “is an incredibly fascinating prediction at the nexus of particle physics, nuclear physics, stellar evolution, and gravitational-wave astronomy.”
Fishbach intends to leverage future observations to map the boundaries of the mass gap with greater precision, which will illuminate more about the fundamental physics occurring inside stars. Yet, she also acknowledges that it is impossible to state with absolute certainty how strictly forbidden this “forbidden range” truly is. “Can we definitively claim that massive stars absolutely never form these black holes?” she questions. “The Universe is an extraordinarily large place, and truly peculiar stars might exist out there.”
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