Dark energy may have been hiding in the cores of black holes all along

Massive black holes could be the source of dark energy and the accelerating expansion of the universe, according to observations of ancient, dormant galaxies with black holes at their centre.

The laws of physics suggest that gravity should cause the universe to contract, but a mysterious force, which physicists call dark energy, seems to be counteracting this and making the universe expand at an accelerating rate.

One possible explanation is that the source of this dark energy is black holes, but there hasn’t been good experimental evidence to conclusively support this idea.

Chris Pearson at the Rutherford Appleton Laboratory in Harwell, UK, and his colleagues compared groups of galaxies with black holes at their centre: a young, distant group and a closer, older group that have stopped growing. They calculated that the black holes grew in mass by seven to 20 times, which can’t be fully explained by the absorption of stellar material or mergers with other galaxies.

Instead, Pearson and his team tried to account for the growth by proposing that it is related to the universe’s accelerating expansion. “When we model that into what we see, we can actually explain the observations,” says Pearson. “We can see that, in addition to these astrophysical processes for black hole growth, we can explain away this discrepancy in the mass growth by adding in the fact that they may contain dark energy and they’re coupled to the expansion of the universe.”

The model they used involved an interpretation of Albert Einstein’s general theory of relativity that says that black holes contain vacuum energy, a kind of energy that exists in space everywhere due to quantum particles popping in and out of existence. “When we did the sums, we found that these black holes might actually be able to explain the entirety of what’s required to balance the universe with this dark energy,” says Pearson.

If black holes really are the source of the universe’s dark energy and its expansion, it would also solve another outstanding cosmic conundrum: what happens at the centre of black holes, so-called singularities, where the laws of physics break down.

Black holes with cores of dark energy avoid the need for singularities, but there weren’t any easy ways to test this. “These have just been theories until now. Now, you’ve got the observational evidence that supports black holes having dark energy cores,” says Pearson.

The researchers extrapolated from the sample of galaxy growth they had observed by using estimated rates of star formation to calculate how much dark energy would be produced on a universe-wide scale. They found that it would account for the universe’s accelerating expansion.

However, this could be incorrect if our estimates of the star formation rate are wrong. Furthermore, the researchers estimated the black hole growth rate by comparing very distant, young galaxies with nearer, old galaxies and assuming this is how galaxies evolve — but it is possible they don’t grow like this.

The explanation could be strengthened with more galaxy observations, says Pearson, or by looking at signatures in the cosmic microwave background (CMB), a cloud of radiation produced shortly after the universe first formed. If their idea is correct, the black holes would affect the universe’s expansion and the CMB in a distinct way that could be detected. More evidence could come from measuring the rate at which black holes merge, which would also be affected by their dark energy nature.

It is a solid and reasonable explanation of the observations, says Andy Taylor at the University of Edinburgh, UK, but the interpretation of Einstein’s theory of relativity it uses to explain the black hole growth hasn’t been widely studied. “There’s some nice discussion there, but we have to be cautious that it’s not built on well-established theoretical principles, it’s built on more speculative models.”

Collecting more observational data is necessary before we can conclusively say that black holes are the source of dark energy, he says.

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