Our universe began with a puzzle. For 100 million years after the big bang, it expanded. Then something strange happened — this expansion suddenly accelerated and has continued to accelerate ever since.
Today cosmologists think some kind of pressure must have forced this acceleration, all powered by huge amounts of energy from an unknown source. Cosmologists call it dark energy. But why this accelerating expansion occurred, and why it happened at that time, is one of the great unsolved mysteries in science.
Now Duncan Farrah and the University of Hawaii in Honolulu and colleagues think they know the answer. They say the acceleration is the result of a previously unknown interaction between black holes and spacetime. When spacetime expands, they say, this interaction makes black holes more massive and this extra mass accelerates the expansion of the universe, creating the accelerated expansion we see today.
First some background. The new idea has its origins in the work of theoretical physicists who have recently shown that black holes cannot be independent of the spacetime in which they sit. Instead, spacetime and black holes must be coupled in such a way that a change in the properties of one immediately influences the properties of the other.
So how might this manifest itself? One possibility is that any stretching of spacetime as it expands makes black holes more massive. An analogous effect is the way the same stretching causes light from the early universe to become red-shifted as it travels through space and time to be observed today.
Farrah and co reasoned that if this coupling does occur, then black holes in the early universe would be less massive than those in the more recent past. So they looked for evidence by studying supermassive black holes at the center of galaxies.
It turns out that supermassive black holes in the closer, more recent universe are up to 20 times more massive than those in the more distant, early universe (relative to the mass of the stars around them). “We find evidence for cosmologically coupled mass growth among these black holes,” they say.
This growth cannot be explained by the black holes swallowing nearby stars — there aren’t enough of them. Nor cannot it be explained by the merger of supermassive black holes as galaxies collide, since this would not change the mass ratio of nearby stars.
Instead, Farrah and co say this is evidence that black hole mass must be coupled to spacetime and must increase as the universe expands. Indeed, the change in mass over time is consistent with this explanation.
But this increase in mass itself exerts a pressure on spacetime. Farrah and co say this the pressure, or dark energy, causes the expansion of the universe to accelerate. Indeed, their calculations suggest this pressure is the order of magnitude necessary to explain the observed expansion rate.
It also explains why the accelerated expansion began only after the universe was 100 million years old, a time that cosmologists refer to by its redshift, denoted z. In this case z ∼ 0.7.
The answer is because black holes form when stars die and so can only have begun to influence the expansion after the first stars had formed. That was 100 million years after the big bang.
“We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z ∼ 0.7” say the team.
The same idea could explain another of cosmology’s great mysteries— why the structure of the universe that we can see seems to be influenced by the gravitational pull of stuff we cannot see, so-called dark matter.
One hypothesized explanation for this is that dark matter takes the form of massive compact halo objects, or MACHOs, that float through interstellar space but do not emit much radiation and so are hard to observe.
Farrah and co’s theory applies to black holes at every scale, from a those few times the mass of our sun to those that are many millions of times bigger. They point out that the smallest black holes form a population that is consistent with properties MACHOs.
“If these BHs are distributed in galactic halos, they will form a population of Massive Compact Halo Objects,” say Farrah and co. In other words, their theory also explains the origin of dark matter.
The team go on to make several predictions that could make or break their theory. For example, they say the effect of this black hole-spacetime coupling should have an observable influence on the cosmic microwave background, the echo of the big bang that astronomers have been observing with increasing precision for decades. Farrah and co also predict how the coupling effect should influence the properties of the mysterious gamma ray bursts that astronomers observe from various parts of the universe.
And they say that the coupling between black holes and spacetime should influence the rate at which small black holes merge. “This can lead to significant increases in merger rate,” they say. The mergers of small black holes have recently become observable thanks to the detection of gravitational waves.
These predictions should be readily testable in the near future. If Farrah and co are correct, then observational conformation of their idea should begin trickling in over the next few months and years.
There will also be inevitable disputes. But make no mistake—an explanation for the origin of dark energy and dark matter will be a major breakthrough in astronomy and one that solves one of the outstanding mysteries of our time.
Ref: Observational Evidence For Cosmological Coupling Of Black Holes And Its Implications For An Astrophysical Source Of Dark Energy : arxiv.org/abs/2302.07878