Earlier this week, a study made headlines claiming that the mysterious “dark energy” cosmologists believe is accelerating the expansion of the universe could arise from supermassive black holes at the heart of galaxies. If true, the connection would connect two of the most thought-provoking concepts in physics – black holes and dark energy – and suggest that the source of the latter has been under the noses of theorists for decades. However, some leading theorists are deeply skeptical of the idea.
“What they’re proposing makes no sense to me,” said Robert Wald, a theoretical physicist at the University of Chicago who specializes in Einstein’s theory of general relativity, the standard understanding of gravity. Other theorists were more receptive to the radical claim—even if it ends up being wrong. “I’m personally excited about it,” says astrophysicist Niayesh Afshordi at the Perimeter Institute for Theoretical Physics.
At first blush, it seems that black holes and dark energy have nothing to do with each other. According to general relativity, a black hole is a pure gravitational field so strong that its own energy sustains its existence. Such peculiar beasts are thought to emerge when massive stars collapse into an infinitesimal point, leaving behind only their gravitational fields. Supermassive black holes, millions or billions of times the mass of our Sun, are thought to lurk in the hearts of galaxies.
In contrast, dark energy is a mysterious phenomenon that literally stretches space and accelerates the expansion of the universe. Theorists believe that dark energy may represent a new type of field in space, somewhat like an electric field, or it may be a fundamental property of empty space itself.
So how could the two be connected? Quantum mechanics suggests that the vacuum of empty space should contain a type of energy known as vacuum energy. This is believed to be spread throughout the universe and exert a force that opposes gravity, making it a prime candidate for the identity of dark energy. In 1966, the Soviet physicist Erast Gliner showed that Einstein’s equations could also produce objects that to outside observers look and behave exactly like a black hole – but are actually giant balls of vacuum energy.
If such objects were to exist, it would mean that instead of being evenly spread throughout space, dark energy is actually confined to specific places: the interior of black holes. Even bound in these particular knots, dark energy would still exert its space-expanding effect on the universe.
A consequence of this idea – that supermassive black holes are the source of dark energy – is that they will be linked to the constant stretching of space and their mass should change as the universe expands, says astrophysicist Duncan Farrah at the University of Hawaii in Manoa. “If the volume of the universe doubles, the mass of the black hole also doubles,” he adds.
To test this possibility, Farrah and colleagues studied elliptical galaxies, which contain black holes with millions or billions of times the mass of the Sun at their centers. They focused on galaxies with little gas or dust floating around between their stars, which would provide a reservoir of material for the central black hole to feed on. Such black holes would not be expected to change much during cosmic history.
But by analyzing the properties of ellipticals over about nine billion years, the team saw that black holes in the early universe were much smaller relative to their host galaxies than those in the modern universe, indicating that they had grown by a factor of seven to 10 times as much, Farrah and colleagues reported this month in Astrophysical Journal.
The fact that the black holes swelled while the galaxies did not is the key, says Farrah. If the black holes had grown by eating nearby gas and dust, that material should also have generated many new stars in parts of the galaxy far from the black hole. But if black holes were made from dark energy, they would respond to changes in the universe’s size in exactly the way that scientists observe in the centers of elliptical galaxies, Farrah’s team also reported this week in Astrophysical Journal Letters.
Wald is not convinced. He questions how a ball of pure dark energy can be stable. He also says the numbers don’t seem to add up: Dark energy is known to make up 70 percent of the mass energy of the universe, while black holes are only a fraction of ordinary matter, which makes up less than 5 percent of the universe. “I don’t see how it is in any way conceivable that such objects could be relevant to the observed dark energy,” he says.
Others have a wait-and-see attitude. “At the moment this is an interesting possibility,” says cosmologist Geraint Lewis of the University of Sydney, but “there needs to be a lot more evidence on the table if this is even a very likely source of dark energy.”
Afshordi agrees. If black holes and dark energy are connected in this way, it will probably have other visible consequences in the universe, he says. At the moment, however, he is unsure what that will be. Determining exactly how galaxies evolve over time is a difficult matter, he adds, and there may be other mechanisms for growing black holes that the team has not considered.
Nevertheless, Afshordi supports the work of rethinking basic assumptions about the universe. “Most new theoretical ideas are rejected by skepticism,” he says. “But if we reject all the new ideas, there will be nothing left.”