The observation of black holes with unexpectedly high masses could be partly explained by an effect related to the expansion of the universe, astronomers in the US have proposed. The team, led by Kevin Croker at the University of Hawai’i at Mānoa, used comparisons between simulated black hole mergers, and gravitational waves detected by the LIGO–Virgo collaboration, to show how ignoring the expansion of the universe may be limiting our understanding of black-hole physics.
Since 2015, the LIGO-Virgo collaboration has made 90 detections of gravitational waves, mainly originating from mergers of two black holes. These observations are a triumph of experimental physics and have revealed a mystery regarding the black-hole masses that have been seen so far. Current theories suggest that black holes involved in such mergers should be roughly between 8–35 times the mass of the Sun. Furthermore, instabilities in the cores of large stars should leave a gap in black hole masses between roughly 50–120 solar masses.
In contrast, gravitational waves detected by the LIGO-Virgo collaboration point to the existence of black holes within this theoretical gap. Several explanations have emerged to maintain the consistency between theory and observation. These relate to factors including stellar mass loss, metallicity (heavy elements in a star), and the explosion mechanics of stars. So far, however, no one theory has explained the observations.
Introducing “cosmological coupling”
As Croker’s team point out, current theories share the assumption that black holes inhabit a non-expanding universe. This idea greatly simplifies calculations and had not been thought to have any important impact on predictions about black hole physics. Yet in their study, Croker and colleagues suggest for the first time that black hole masses could be growing on cosmological timescales. They reckon the growth rate is linked with the expansion of the universe by an effect they call “cosmological coupling”.
LIGO gravitational-wave signal backs up Hawking’s area theorem
If this occurs, the researchers predict that it would enable black holes within binary systems to expand into the mass range thought to be forbidden by existing theories. To test this idea, Croker’s team simulated the birth, evolution, and death of millions of large stars in binary systems. They then calculated the gravitational waves that would appear when the resulting black holes finally spiralled into each other, following billions of years of growth through cosmological coupling. The astronomers’ predictions agreed reasonably well with real LIGO–Virgo data, without requiring any changes to our current understanding of the stellar lifecycle.
Croker and colleagues acknowledge that their results are still far from solving the mystery posed by the observed diversity of merging black hole masses. On top of this, current observational techniques are not good enough to determine the strength of this cosmological coupling. But with future improvements to the sensitivities of gravitational wave observatories, the researchers soon hope to measure the relationship between black hole growth and universal expansion for the first time.
The research is described in The Astrophysical Journal Letters.
