When astronomers discovered that the universe is expanding at an accelerating rate, they theorized that some force must be pushing things farther apart and overcoming gravity, which should be slowing things down. That force was suggested to be dark energy, but no one has ever figured out from where it comes.
But a team of 17 international researchers led by the University of Hawaii has discovered the first evidence for the origin point of dark energy: Black holes.
Black holes acquire mass in two ways: accretion of gas and mergers with other black holes. But in studying nine billion years of black hole evolution in dormant giant elliptical galaxies, the researchers discovered that the older black holes are much larger than they should be based on those two methods of growth. That means there must be another way these black holes are acquiring mass. Researchers suggest the answer is dark energy in the form of vacuum energy, "a kind of energy included in spacetime itself ... [that] pushes the universe further apart, accelerating the expansion," according to a statement (opens in new tab).
Related: Black holes: Everything you need to know
"If the theory holds, then this is going to revolutionize the whole of cosmology, because at last we've got a solution for the origin of dark energy that's been perplexing cosmologists and theoretical physicists for more than 20 years," Dr. Chris Pearson of STFC RAL Space, a co-author of a study on the discovery, said in a statement.
The idea that black holes are a source of dark energy isn't new. In fact, it's part of Einstein's theory of general relativity. But this is the first time astronomers have obtained observational evidence to support the theory.
"We're really saying two things at once: That there's evidence the typical black hole solutions don't work for you on a long, long timescale, and we have the first proposed astrophysical source for dark energy," said study author Duncan Farrah, University of Hawaii astronomer, in the statement.
A paper on the team's research was published in The Astrophysical Journal Letters (opens in new tab) on Wednesday (Feb 15).
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Sabine Hossenfelder: "The solution of general relativity that describes the expanding universe is a solution on average; it is good only on very large distances. But the solutions that describe galaxies are different - and just don't expand. It's not that galaxies expand unnoticeably, they just don't. The full solution, then, is both stitched together: Expanding space between non-expanding galaxies...It is only somewhere beyond the scales of galaxy clusters that expansion takes over." http://backreaction.blogspot.bg/2017/08/you-dont-expand-just-because-universe.html
So cosmologists apply the expansion solutions only to voids deprived of galaxies; to galaxies and galactic clusters they apply nonexpansion solutions. Why do cosmologists resort to this trick? Because, if they applied expansion solutions to galaxies and galactic clusters, observations would immediately disprove the expansion theory. Here is why:
If expansion is actual inside galaxies and galactic clusters, the competition between expansion and gravitational attraction would distort those cosmic structures - e.g. fringes only weakly bound by gravity would succumb to expansion and fly away. And the theory, if it takes into account the intragalactic expansion, will have to predict the distortions.
But no distortions are observed - there is really no expansion inside galaxies and galactic clusters. And cosmologists, without much publicity, have simply made the theory consistent with this fact.
Since there is no expansion inside galaxies and galactic clusters, there is no expansion anywhere else. The so called cosmological (Hubble) redshift is due to the speed of light slowing down as photons travel through vacuum, in a non-expanding universe. This is not a totally unacceptable idea:
"Some physicists, however, suggest that there might be one other cosmic factor that could influence the speed of light: quantum vacuum fluctuation. This theory holds that so-called empty spaces in the Universe aren't actually empty - they're teeming with particles that are just constantly changing from existent to non-existent states. Quantum fluctuations, therefore, could slow down the speed of light." https://www.sciencealert.com/how-much-do-we-really-know-about-the-speed-of-light
I would like to bring to your attention my paper on ResearchGate in 2018 about my idea that Dark Matter is actually caused by Black Holes.
The idea is that Black Holes consume the vacuum energy, which has the effect of drawing inwards all embedded matter (stars, planets etc) in the space surrounding the Black Holes. Thus an apparent extra inward acceleration which is constant with distance is produced and can explain Galaxy rotation rates.
It also explains other phenomena such as the apparent lopsided halos of dark matter that seem to exist. You can read about why this occurs and other details in my paper here:
An Explanation for Galaxy Rotation Rates without Requiring Dark Matter
Declan Traill BSc (Melb)
"the first time astronomers have obtained observational evidence to support the theory "
It really isn't. All they have is that old BHs seem to be more massive than they can account for.
Anything else at this point is pure speculation.
"The theory rests on observations of galaxies which have insufficient visible mass to account for the calculatied mass to hold them together. "
That's Dark Matter, not Dark Energy. They speculate about Dark Energy because the expansion of the universe is accelerating, and they can't find a reason, so they make up something called "Dark Energy".
It's supposedly "Dark Matter" that is holding galaxies together.