The source of ghostly dark energy could be supermassive black holes
One of the studied galaxies NGC 524 in the constellation Pisces. Credit: ESA/Hubble.
Despite enormous efforts, scientists are still grappling with two annoyingly mysterious substances in the universe. Dark matter and even darker energy. They are specters that scare us from our sleep, but at the same time we absolutely need them, because they explain cosmological phenomena that we would otherwise not know how to deal with.
Duncan Farrah. Kredit: University of Hawai’i.
Very briefly, dark energy was “invented” to save the day after it was discovered that the expansion of the universe was accelerating. It is dark energy that should drive this acceleration. Dark energy is generally considered an even more ghostly matter than dark matter, which is at least solidly thought to exist. Still, he may be glancing at better times. Duncan Farrah from the American University of Hawai’i and a large team of colleagues have observed supermassive black holes in the centers of galaxies and are convinced that it is there that we will find the desired source of dark energy.
Their analyzes of supermassive black holes revealed that they are growing faster than they should. In a nutshell, this means that the source of the dark energy phenomenon could be supermassive black holes in the context of Einsteinian gravity. It is a surprising result even for the study authors themselves.
Logo. Kredit: University of Hawai’i.
As another member of the team, Dave Clements of the UK’s Imperial College London, revealed, he and his colleagues started by tracking the growth of supermassive black holes and may have stumbled upon one of the most sought-after answers in cosmology. Meanwhile, dark energy has haunted physicists for more than 20 years.
Farrah and co. studied the 9 billion year evolution of supermassive black holes. They focused on giant elliptical galaxies, immense galactic monsters that date back to very ancient times and are just dormant in today’s universe. Virtually no new star formation takes place in them, and there is almost no material left in them that could be “eaten” by their supermassive black hole.
When researchers compared giant elliptical galaxies from the very distant, or young, universe with these galaxies in the nearby universe, their supermassive black holes were found to be much larger than they would have been if they had grown by accreting surrounding matter and merging with other black holes. They are about 7-20 times larger today than they were 9 billion years ago. Their growth corresponds to the idea that black holes contain vacuum energy.
The analyzes showed that the size of supermassive black holes is consistent with the size of the universe. According to Farrah’s team, it turns out that the amount of dark energy in the universe is equivalent to the vacuum energy of black holes. This is the first observational evidence that black holes actually contain vacuum energy and that they are linked to the expansion of the universe. Entwining manifests itself in the way that the mass of black holes increases as the universe expands. Researchers call this cosmological entanglement (cosmological coupling).
If this is confirmed by further research, it will be a revolution in the understanding of dark energy, black holes, a revolution in the entire cosmology. As Farrah succinctly adds, she and her colleagues did not add anything new to the universe. They simply concluded that supermassive black holes in the context of Einstein’s general relativity are dark energy.
Video: Explainer: First evidence black holes are source of dark energy
Literature
Imperial College London 15. 2. 2023.
Astrophysical Journal 943: 133.
