Are black holes the source of dark energy?

Featured Image Credit: NASA

Featured Image description: An artist’s impression of a black hole. A black sphere surrounded by a large bright orange disc, attracting swirls of dust.

Black holes are the source of dark energy, suggests a paper that has taken astronomers by storm.

Dark energy is the name given to the mysterious “missing” 70% of the mass in our universe. According to observations, our universe is expanding, at an increasing rate. In other words, the distance between things in the universe: stars, galaxies, planets is increasing. This expansion, however, doesn’t exactly correspond to the amount of mass astronomers can detect. There must be some invisible, “dark” mass, and this is what we call dark energy. The source and nature of dark energy are considered to be some of the biggest puzzles physicists are working on today.

On the outset, it is hard to guess what this would have to do with black holes. Black holes are thought to be at the centre of each galaxy, the echoes of large stars that collapsed under their own gravity, some with a gravitational pull equivalent to millions of suns. Black holes “accrete” all matter that comes near it, and in this way increase in size. First predicted in Einstein’s theory of relativity, black holes were initially thought to be a mistake in the mathematics. For a long time, their existence was hotly contested by physicists, a debate that was brought to rest for good upon the first photograph of a black hole, taken as recently as 2019.

The expansion of the universe is also predicted in the theory of relativity, something that Einstein initially called his “biggest blunder”. This expansive force is needed, though, to keep the universe in balance: were only gravity present, the universe would collapse in on itself under its gravity. However, what observations suggest is that this dark energy does more than compensate for the gravity, it is in fact far stronger.

The theory that black holes, where gravity is the strongest in the universe, could be the source of the main force opposing it has theorists polarised. The paper was a product of collaboration between 17 researchers across 9 countries, including the UK.

The theory itself is a conclusion from observations of black holes across time: by viewing very distant black holes, we have some clue as to what all black holes looked like in the early universe, close to when they first formed. What the researchers found was that in the early universe, black holes were far smaller relative to their host galaxies than they are today.  The researchers focused their efforts on studying black holes that had little gas and dust surrounding them, as they would not expect the size of this sort of black hole to increase very much over time. Yet they still found that the size of the black holes increased significantly over time. This suggests that accreting mass alone was not responsible for the growth of the black hole, because if there were enough mass to feed the black hole, this matter would also have coalesced into many new stars in the surrounding galaxy.

The research team, led by Duncan Farrah at the University of Hawaii, then looked at how the size of the black holes related to the estimated size of the galaxy over time. Since the theory is that black holes are made of dark energy, one can predict that a black hole would increase in size by the same factor that the universe increases over a given time period. This is exactly what was observed in elliptical galaxies by the researchers. This squares well with a quantum mechanical idea of “vacuum energy”, which, despite being concentrated in one place, would have effects reaching out into the distant universe. This would reformulate a black hole as a ball of vacuum energy, which provides a source of dark energy and, in one fell swoop, removes the biggest issue theorists have with black holes: the singularity at their centre. If this theory is true, it would also mean that nothing new needs to be added to existing models to make them work, and points yet again to Einstein being right all along, over 100 years after the initial publication of his gravitational theory.

If the theory holds, this is going to revolutionise the whole of cosmology.

This evidence may appear strong, but the physics community is not wholly convinced. Dr Chris Pearson, a co-author of the study, stated “If the theory holds, this is going to revolutionise the whole of cosmology.” Pearson is a researcher at the Rutherford Appleton Laboratory, located just outside of Oxford.  Other cosmologists are taking a more measured approach, waiting for “a lot more evidence” before fully committing to the idea. Theoretical physicists feel that this does not quite solve the mystery, with concerns about how the proposed “ball of dark energy” could remain stable (i.e not explode, collapse, or otherwise become something else). It is clear that there is more work to be done, but this could be the beginning of something very exciting.

That said, it is an attractive idea, because it is satisfying if true. There is a nice symmetry to the idea that black holes, once thought of to be a mistake, would be the key to the resolution of Einstein’s “biggest blunder”. Regardless of the outcome of this avenue of research, one thing is certain: physicists like symmetries.