Black Holes as the Source of Dark Energy

What do black holes have to do with dark energy, the subtle pressure that accelerates the expansion of our universe? New research suggests that the two may be inextricably linked, potentially solving the long-standing mystery of the nature of dark energy.

Multiple Black Hole Models

When a massive star ends its life, it collapses to form a black hole, creating a well in spacetime so deep that even light cannot escape. Although our understanding of black holes has grown over time, there’s still much we don’t know about them, and we rely on mathematical models to learn more and make predictions that we can test. The most common model predicts a spinning black hole containing a singularity — a point of hypothetically infinitely curved spacetime where our equations describing gravity break down — hidden by the black hole’s event horizon.

However, this common black hole model is in tension with the overall expansion of the universe, leading some scientists to propose alternative models. In one such model, black holes do not contain a singularity, but are instead filled with vacuum energy. These vacuum-energy black holes are intriguing because their growth is coupled to the expansion of the universe: as the universe expands, these black holes gain mass.

Do Black Holes Gain Mass as the Universe Expands?

In a research article published today, Duncan Farrah (University of Hawaiʻi) and collaborators tested this black hole model by studying the growth of supermassive black holes over billions of years. The team studied elliptical galaxies since these galaxies’ central supermassive black holes are unlikely to grow much by other means, such as by consuming nearby stars and gas.

By measuring how the masses of black holes at the centers of elliptical galaxies billions of years ago compare to those present today, Farrah and coauthors determined the strength of the coupling between black hole mass and the expansion of the universe. Traditional singularity-containing black holes would have a coupling strength of 0, while vacuum-energy black holes would have a coupling strength of 3. Ultimately, the team found the coupling strength to be around 3.11, and they ruled out the possibility of zero coupling at 99.98% confidence. This finding supports the vacuum-energy black hole model and suggests that black holes do gain mass as the universe expands.

Dark Energy, Illuminated

What does it mean for black hole growth to be linked to the expansion of the universe? Certain physical quantities must be conserved as black holes gain mass, and as a result, the growth of black holes produces pressure that drives the acceleration of the universe’s expansion. In other words, black holes that grow as the universe expands are a source of dark energy, a long sought-after component of our models of the cosmos.

Farrah and collaborators performed additional modeling showing that the expected population of vacuum-energy black holes can account for the density of dark energy previously measured by the Planck satellite — so not only do vacuum-energy black holes generate dark energy, they generate the same amount of dark energy we’ve measured! Hopefully, the next few months to years will bring more research on this topic, expanding our understanding of the fundamental physics of our universe.

Citation

“Observational Evidence for Cosmological Coupling of Black Holes and Its Implications for an Astrophysical Source of Dark Energy,” Duncan Farrah et al 2023 ApJL 944 L31. doi:10.3847/2041-8213/acb704