From Physics World, something unexpected. The “problem” is, er, the Universe. Problems don’t come much bigger! To put this in perspective, the major theory used to describe it is Einstein’s General Relativity. There is only one problem with that theory: everything should collapse into a singular “point” and it doesn’t. To get around that problem, Einstein introduced something he called “the Cosmological Constant”, which effectively was something pulled out of thin air in an ad hoc way to at least admit the obvious problem that the Universe remained large. Worse, this steady universe could not be extrapolated to the infinite past. Somewhere along the line it had to be different from now. It was then that Georges Lemaȋtre proposed a theory that space was expanding, to which Einstein replied that his maths were correct, but his physics abominable. However, Einstein had to backtrack because Hubble showed it was expanding. (Actually, Lemaȋtre had provided evidence, but Hubble’s was better.) The idea was that all matter started from a point, and space expanded to let the energy collapse into matter. Fred Hoyle jokingly referred to this as “The Big Bang”.
All was well. Space was expanding uniformly, so the galaxies were moving away from each other uniformly, on a very large scale, but with localized variation in between. Why space was expanding was left unanswered. Thus on a very large scale, the gravitational interactions between distant galaxies was diminishing, which violates the concept of the law of conservation of energy. (Energy does not have to be conserved, though. Energy is a tricky topic in general relativity.) Hoyle was keen on a steady state universe, and while he accepted the Universe was expanding, he took the idea that if everything was moving apart, that loss of gravitational energy was made up for by the creation of new matter. This was not generally accepted, and the question of this energy imbalance remained.
There is worse. To maintain a constant expansion rate, general relativity requires a constant energy density, and how can that arise when space is expanding? The only way would seem to be that this energy density is a property of vacuum. Vacuum is, therefore, not nothing. There is nothing like an opportunity in physics to get speculation going, and “not nothing” is such an opportunity. Quantum field theory announced the vacuum is full of extraordinarily tiny harmonic oscillators, which convey a zero point energy to space. We have our energy accounted for. All was good, until it wasn’t. It was obvious to take the quantum field theory and see if it fitted with observation on galaxies. It did not. In fact the error was a factor of ten multiplied by itself 120 times (Adler, R. J., Casey, B., Jacob, O. C. 1995. Vacuum catastrophe: an elementary exposition of the cosmological constant problem. Am J. Phys 63: 620 – 626.) This was the most horrendous disagreement between calculation and observation ever.
Then came a shock: not only were the galaxies moving apart, but such motion was accelerating. This was caused, in terms of labelling, by something called dark energy. It should be noted that at this point, dark energy is merely a term that essentially is a holding term to recognize that something must be causing the acceleration. So, what is it? The good news is that whatever it is can also account for the general expansion. All we need is something that is getting bigger as the universe expands.
Now, we have a proposition, which is called “cosmological coupling”. The concept starts with the observation that the mass of black holes at the heart of distant galaxies have been growing about ten times faster than simply accreting mass or merging with other black holes would allow. The coupling means that the growth of the black holes matched the accelerating expansion of the universe. The concept seems to be that the singularity of black holes is replaced by additional “vacuum energy”. Their coupling means that if the volume of the universe doubles, so does the mass of black holes, but the number of black holes remains constant. The logic is that “something” must give rise to the expansion of the universe, and since no other object exhibits similar behaviour, black holes must be the “something”. Is that valid? There is a problem for me with that explanation, apart from the fact that correlation does not mean causation. The evidence is the Universe took on massive expansion initially, then calmed down, and then the expansion accelerated again. During the initial expansion there would be few if any black holes. Then, suddenly, there was a massive growth of them, but that happened at a time when the universe expansion was at its slowest, then when the final acceleration started, the black holes had settled down to minimal growth. The required correlation for the hypothesis seems to be, if anything, an anti correlation.