The answer to a cosmological mystery might be lurking inside black holes.
Black holes seem to gain mass even when there’s nothing for them to feed on, as if they weren’t bizarre enough already. And that weird habit may shed new light on dark energy, the mysterious force that pushes our universe to expand faster and faster – apparently defying gravity, which should be putting the brakes on our expanding universe.
University of Hawai’i physicist Duncan Farrah and his colleagues published their results in two recent papers, one in The Astrophysical Journal and one in The Astrophysical Journal Letters.
Measuring a black hole
Farrah and his colleagues measured the mass of black holes from a type of galaxy called a giant elliptical galaxy. These galaxies have typically finished making new stars and run out of material, which also means there’s nothing for the supermassive black holes at their centers to feed on. You’d expect those black holes to be done gaining mass if that’s the case. But when Farrah and his colleagues compared distant giant elliptical galaxies — viewed as they looked around billion years ago, when they were relatively young, to more nearby ones, viewed as they looked much more recently in their older years — they found that supermassive black holes in the galaxies kept growing long after they should have run out of stars and gas to feed on.
But Farrah and his colleagues also noticed that the black holes grew more massive at about the same rate that the universe expanded over time, which we know thanks to measurements of distant objects like certain types of supernovae. And that suggests that black holes may play a role in producing dark energy.
Dark energy factories
Our current model of black holes says that their centers collapse into something called a singularity: infinitely dense matter crammed into an infinitely tiny point in space. The thing is, singularities are mathematically impossible. The heart of a black hole, as we understand it, is a point where physics, too, collapses under the black hole’s tremendous gravity.
Farrah and his colleagues suggest that when the physics inside a black hole gets weird, one thing that happens is that its mass gets linked, or coupled, to the expansion of the whole universe. When physics gets weird, it gets really weird. Physics has no chill.
“You can think of a coupled black hole like a rubber band, being stretched along with the universe as it expands,” says University of Hawai’i theoretical physicist Kevin Croker in a recent statement. “As it stretches, its energy increases. Einstein’s E+MC2 tells you that mass and energy are proportional, so the black hole mass increases, too.”
In other words, as the universe expands, coupled black holes gain more energy, which at the heart of the black hole is the same thing as gaining more mass. But that new mass creates a pressure that pushes the universe to expand even more — which is why the universe is expanding faster all the time.
Several physicists have suggested, over the years, that instead of a singularity, the heart of a black hole might contain something called vacuum energy, which some physicists say is one form of dark energy.
The European Space Agency’s Planck Satellite (decommissioned in 2013) helped astronomers estimate how much dark energy the universe has to contain in order to explain the way its expansion keeps speeding up. And using data collected, in part, by JWST, other astronomers have built models of how many massive stars have formed and collapsed into black holes since the universe began. Farrah and his colleagues say the vacuum energy in those black holes is about the same as the amount of dark energy that, according to the Planck data, should exist in the universe.
“Future tests of cosmological coupling in black holes are essential to confirm or refute our proposal,” write Farrah and his colleagues in their recent paper.
In other words, Farrah and his colleagues aren’t claiming to have found the answer to the cosmological mystery of dark energy, or to be right about what’s inside a black hole. Instead, they’ve presented something that seems like a plausible idea, which physicists and astronomers can test with new computer simulations and new data from telescopes and projects like the Dark Energy Survey and the Dark Energy Spectroscopic Instrument.
