What happens if two of the greatest enigmas of our Universe, black holes and dark energy, are shown to be actually one and the same thing? Black holes form as remnants of explosions of heavy stars, or as collapsed deposits of matter at the center of galaxies. Dark energy is the hypothetical substance that drives the accelerated expansion of the Universe. Black holes drive the extreme warping of the local space-time, dark energy shapes the geometry of the entire universe. So far, they were thought to be completely independent phenomena, although both originate within the same theoretical framework, Einstein’s General Relativity.

Now some recent work put forward a bold idea: if black holes grow in mass not, or not only, by accreting local material (gas, dust, or even other stars), but by a sort of long-distance cosmological coupling, their mass growth can compensate the number density dilution due to cosmic expansion, in such a way that their energy density remains constant. But a constant energy density is exactly what is required for dark energy to explain the observed acceleration!

This conjectured mass growth might have been just a cute theorist toy to play with if it were not supported by some recent observations of supermassive black holes in elliptical galaxies. According to this work, in fact, such object seem to have grown in mass by a large factor across cosmic history, in a way that cannot easily be reconciled with what we know about their local environment. In other words, we don’t know where this large mass increase comes from. Could it be due to the cosmological coupling?

This mysterious synergy between black holes and dark energy is too intriguing to be ignored, and many scientists already published research trying to support or criticize it. In a very recent paper, we investigated whether it was possible to test the cosmological coupling of black holes by looking at the gravitational wave detection of merging black holes by the LIGO-Virgo-KAGRA collaboration. The signal from the final seconds of the merging allowed the collaboration to build a catalog of a few dozen events, indicating for each one the presumed mass of the black holes. If they have been growing in mass as required by the cosmological coupling hypothesis, then they had to be born with a mass much smaller than observed at merging. But many independent lines of evidence, both observational and theoretical, show that stellar black holes can only form above a certain minimum mass, believed to be around 2 or 3 solar masses. Below this threshold, stars do not form black holes but neutron stars, which should not be undergoing the same kind of cosmological growth.

We found that if the cosmological coupling theory is correct, at least some of the currently observed merging black holes should have been formed below threshold. Therefore, either there is something wrong with our understanding of stellar black hole formation, or with the coupling theory.

As can be expected when two mysteries merge into one, our investigation, as many other works in this area, is only a preliminary recognition. If the black holes are cosmologically coupled, we might expect their properties to be quite different from the ordinary ones (although there is nothing “ordinary” in any kind of black hole!). It is therefore not very difficult to imagine scenarios in which the coupling hypothesis is not ruled out by observations, at the price of some change in our view of black hole formation and evolution. Luckily, our test can be rapidly improved and extended: every month new merging event are observed, and in one year or so we will have twice as many events as today. And each one of them is a potential test for the fascinating idea that black holes drive not only science fiction stories, but the entire cosmic expansion.