By the 1920s, astronomers learned that the Universe was expanding as Einstein’s Theory of General Relativity predicted. This led to a debate among astrophysicists between those who believed the Universe began with a Big Bang and those who believed the Universe existed in a Steady State. By the 1960s, the first measurements of the Cosmic Microwave Background (CMB) indicated that the former was the most likely scenario. And by the 1990s, the Hubble Deep Fields provided the deepest images of the Universe ever taken, revealing galaxies as they appeared just a few hundred million years after the Big Bang.
Over time, these discoveries led to an astounding realization: the rate at which the Universe is expanding (aka. the Hubble Constant) has not been constant over time! This led to the theory of Dark Energy, an invisible force that counteracts gravity and causes this expansion to accelerate. In a series of papers, an international team of researchers led by the University of Hawaii reported that black holes in ancient and dormant galaxies were growing more than expected. This constitutes (they claim) the first evidence that black holes could be the source of Dark Energy.
The research was made up of astronomers and astrophysicists from the University of Hawai’i, the Kavli Institute for Cosmological Physics, the Enrico Fermi Institute, the European Southern Observatory (ESO), the Netherlands Institute for Space Research (SRON), the National Radio Astronomy Observatory (NRAO), the Instituto de Astrofísica e Ciências do Espaço (IA), the Mitchell Institute for Fundamental Physics and Astronomy, and multiple universities. Their findings appeared in two papers published in The Astronomical Journal and The Astronomical Journal Letters.
Cosmological Crisis
According to the most widely accepted model of the Universe, Dark Energy accounts for 68% of the mass-energy content in the Universe. This theory resurrected an idea Einstein had proposed but later rejected – that there was a “Cosmological Constant” (represented by the scientific symbol delta) that “held back” gravity and prevented the Universe from collapsing in on itself. The force and Dark Matter (which accounts for 26.8% of the mass-energy content) are integral to the most widely held cosmological model today, known as the Lambda-Cold Dark Matter (LCDM) model.
The main argument behind Dark Energy is that there is a special type of energy within spacetime (called vacuum energy) pushing the Universe apart. There are a few problems with this theory, though, not the least of which has to do with the fact that no direct evidence exists for this mysterious energy. Moreover, while this vacuum energy is consistent with quantum mechanics, all attempts to calculate it using quantum field theory have come up dry. On top of that, there is the question of how this energy coincides with supermassive black holes (SMBHs) in our Universe.
By the 1970s, astronomers had determined that the persistent radio source at the center of our galaxy (Sagittarius A*) was a black hole with a mass of 40 million Suns. Further observations demonstrated that most massive galaxies had SMBHs in their core region, which was the reason for Active Galactic Nuclei (AGNs), or quasars. The extremely powerful gravity of SMBHs causes surrounding matter to infall around them, forming accretion disks and powerful relativistic jets where matter is sped up to close to the speed of light (and releases tremendous amounts of radiation in the process).
The presence of these mammoths at the center of most massive galaxies would require an extremely strong force to counteract them. This is particularly true when it comes to the singularities theorized to exist at their cores, where the very laws of physics break down and become indistinguishable. This gave rise to an exotic theory known as “cosmological coupling,” which states that SMBHs might have tremendous vacuum energy and that they are the reason the Universe is expanding.
Extraordinary Claims…
In their papers, the team led by Duncan Farrah (an astronomer with the University of Hawai’i at Manoa and a former Ph.D. with Imperial College report the first observational evidence that black holes gain mass in a way consistent with them containing vacuum energy. Whereas astrophysicists have been looking for a theoretical resolution to the problem of Dark Energy and Black Holes, the team’s findings allegedly constitute the first observational evidence that black holes are the source of Dark Energy.
If true, the finding removes the need for singularities to form at the center of black holes, resolving a long-standing debate. It also means that nothing more is needed (no new forces or modified theories of gravity) for our cosmological models to make sense. Dr. Chris Pearson, a researcher from RAL Space, a research council overseen by the UK’s Science and Technology Facilities Council (STFC), and Dr. Dave Clements of the Department of Physics at Imperial College were co-authors on the studies.
“If the theory holds, then this will revolutionize the field 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,” said Pearson in a RAL Space press release. “This is a really surprising result. We started off looking at how black holes grow over time, and may have found the answer to one of the biggest problems in cosmology,” added Clements.
…Require Extraordinary Evidence
The team reached this conclusion by examining the evolutionary history of SMBHs at the center of giant elliptical galaxies. This refers to a type of “early galaxy” that formed early in the Universe and has since ceased forming stars (aka. “dormant galaxies”). Decades of observations have shown that black holes can increase their mass in two ways: by accreting matter or by merging with black holes. As they indicated in their first paper, the team examined giant elliptical galaxies at redshifts of less than z<2 (as they appeared nine billion years ago).
These dormant galaxies have little material left for their SMBHs to accrete, meaning that further growth cannot be explained by the two mechanisms mentioned above. The team then compared observations of these elliptical galaxies – which still appear young – to local galaxies dated to ca. 6.6 billion years ago, which have since become dormant. These observations revealed that the SMBHs were 7 to 20 times larger than they were nine billion years ago, much greater than what is predicted by accretion or mergers.
In their second paper, they further state how measurements of related populations of galaxies at different points in their evolution (ca. 7.2 billion years ago) showed a similar correlation between the mass of SMBHs and the size of the Universe. This constitutes the first evidence of “cosmological coupling” by showing that the expansion of the Universe and the growth of SMBHs are related. If this is born out by further observations, it could effectively redefine our understanding of the Universe and the nature of black holes. As Farrah concluded:
“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. What that means, though, is not that other people haven’t proposed sources for dark energy, but this is the first observational paper where we’re not adding anything new to the Universe as a source for dark energy: black holes in Einstein’s theory of gravity are the dark energy.”
Correlation, not Coupling?
Naturally, these claims have been met with some skepticism by the astronomical/astrophysical community. In particular, the authors’ claim that their observations constitute proof of coupling has been challenged for conflating correlation with causation. Astrophysicist, author, science communicator, and Forbes senior contributor Ethan Siegel addressed this in a recent installment of Ask Ethan – a special series in his column Starts with a Bang!, where he addresses audiences’ questions. Upon examining their research, Siegel notes how the authors’ conclusions rest upon a major assumption.
This assumption is that there is “a universal relationship between the mass of the central black hole and the mass of the stars within a galaxy, which can evolve over cosmic time but should be universal at any particular time.” From this, they compared the SMBHs they chose for their sample data to determine whether there is a “coupling parameter” (represented as k) that has the same value across cosmic time. In the end, the team determined with a confidence of 99.8% that k has a non-zero value. While ostensibly compelling, this conclusion comes down to an assumed relationship. As Ethan concluded:
“The authors are assuming the existence of a coupling that isn’t there and ascribing the perceived evolution of the black hole-to-stellar mass ratios to a coupling, when what’s happening is these galaxies and their black holes are evolving. Since we’re only measuring each galaxy at a ‘snapshot’ in time, we have no way of knowing how any individual object is evolving, and this particular method is precisely how the article authors are fooling themselves, and by extension, anyone who believes them.”
At the risk of repeating the overused adage, “extraordinary claims require extraordinary evidence.” The ability to verify results repeatedly is one of the most important qualifiers for evidence to be considered sound. In other words, results must be demonstratable again and again and (preferably) using varying methods. The authors acknowledge this and hope that repeated observations will bear them out. But for the time being, the claim they’ve made remains extraordinary and (given the implications) demands further investigation.
Further Reading:
I’m thinking another problem with this idea is that dark energy appears to very evenly-distributed, hence the common assumption that it is an intrinsic property of spacetime itself. If SMBHs were the source of DE, you’d think there would be regions of high and low DE concentration correlating with the distribution of SMBHs across the Universe?
Understand that this is opinion and I do not claim to be able to prove the following – or even show that it is consistent with current observations.
I’ve thought for a long time that black holes are related to Dark Energy. But I think that it is actually a bit too simplistic.
Think of it sort of like the Heisenberg Uncertainty Principle. Not the same thing but it gives the flavor. It’s also sort of Entropy.
Space-time and matter aren’t really totally different and are pretty tightly linked. If you concentrate matter then space-time expands so that the amount of localization overall remains the same. Presumably if space-time contracts you will get less concentration of matter.
So it would not be limited to black holes. Black holes would just be an extreme example but even matter coalescing into a normal star like our Sun would result in space-time expansion. The effect on space-time needn’t be local – it’s a fundamental structural change to the universe so you could get some expansion throughout the totality of the universe.
A somewhat related concept which is not at all original to me is that the common cosmological concept that the universe started from a quantum smear or point is actually quite inconsistent with reality. Heck, even our best measurement suggests a flat topology (to the limits of our measurement) and that tends to imply an infinite universe.
If one assumes an infinite universe then a far more rational (although certainly not proven) concept is the one I saw from a cosmologist or astrophysicist (can’t remember which) where the “Big Bang” was actually a transition of the universe from one state to another. When combined with what I’m going to call the “Localization Principle” this might actually explain a number of things which have been puzzling.
We appear to have gotten black holes and such much sooner than modeling has suggested should be the case. . .
So perhaps what actually happened is that some items (and perhaps particularly black holes) persisted through the transitions. Perhaps in the previous phase of the universe they were something else, but when the transition completed those items were black holes in our phase. This might mean that what drove the period of supraluminal expansion of space-time was that the transition required incredible expansion of space-time to accommodate the mass which persisted (or arose) from the transition.
An idea which I entertain but think might be silly is that the reason Dark Matter is not explained by our experimentation is that it is something which did not transition with the phase-change of the universe. So you can’t explain it in terms of the Standard Model because the Standard Model (and similar) are based on a conventional understanding of the laws of this phase of the universe.
And again, I’m not telling you that the above is the reality. But aside from the Dark Matter thing I don’t think it is silly, either.
Have fun with it.
The obvious problem is that they have a correlation as Ethan Siegel points out, but the more damning problem which he first covers is that black holes have too little binding energy. No matter if black holes can be coupled, and that proposed coupling is problematic as apparently their black hole mass theory is position dependent. They have to average that too, similarly to how Siegel notes they are fooling themselves on individual black hole growth.
To keep this to the problem of the paper they rely on a particular singularity free solution as their “flat spacetime” null hypothesis to fail, not the basic Kerr solution. There are many ways to make singularity free black hole models, a review of the 15 popular models had 5 which were such. [Unfortunately I don’t have it handy, but if asked I hope to find it again.] Further, by introducing a superfluous black hole “dark energy” they add to the LCDM model and can’t explain the dark vacuum energy density from the quantum fields as e.g. the late Weinberg did in the 80s. The implicit assumption is that they add to zero energy density, but they would have to show that too. When the black holes evaporate their decreased influence deviate from the perfectly constant vacuum energy density we observe today [as Steve mentions in his comment], so their non-LCDM cosmology will have a different destiny.
In this separate comment I wanted to field some comments on the generally good article here – nice find of Ethan Siegel’s context – and by that route get to my weaker objections to the “coupling” backreaction idea. The article use a popular label of “direct” evidence but there is no testable definition of it, it appears empirically meaningless. We do have testable evidence of vacuum energy both in cosmology and in small scale physics. “The effects of vacuum energy can be experimentally observed in various phenomena such as spontaneous emission, the Casimir effect and the Lamb shift, and are thought to influence the behavior of the Universe on cosmological scales.”
“Moreover, while this vacuum energy is consistent with quantum mechanics, all attempts to calculate it using quantum field theory have come up dry.” The quantum field calculations are currently too complex for anything but simple problems, c.f. the problems of calculating the anomalous magnetic moment, but Weinberg predicted the vacuum energy density value with his anthropic multiverse theory in the early 80s before it was observed in the late 80s.
The passed predictive test implies that the current cosmology may be mistaken when they take the large time (dark energy dominated) de Sitter approximation for our Friedmann-Lemaître-Robertson-Walker as the real physics. Besides being an approximation current physics is manifestly effective and specifically general relativity has similar degeneracy problems at spatial infinity as the discussed paper position dependence. Merging general relativity with quantum field theory we get the average flat space universe we observe and the theory respective degeneracies go away, for cosmology at least. In a consistent manner inflation gives naively a sufficiently large (actually, infinite) multiverse for Weinberg’s theory.
Which roundabout takes me back to the backreaction paper. A flat space total zero energy density universe – which is currently an extraordinary claim, so unfortunately no better than the paper – has no backreaction effect and it has simple non-singularity solutions to black holes according to reviews of such Kerr-like solutions.
Matt, this is the best coverage I’ve seen on these papers. Keep up the well-researched and factual reporting.
Despite the impressive correlation uncovered, ‘new think DE core BHs’ is imo magic thinking, because of the following checklist:
1: Starting with that the measured SMBH masses are positive. Otherwise runaway dynamic instability would not permit them to remain within a galaxy at all.
2: IF DE is real it must have a net negative gravitational source density – negative pressure contribution overpowering 3:1 that of positive energy density. Well that is at least the standard understanding.
3: From 2:, DE must be a minor fraction of net BH mass. Otherwise there could not be a net gravitationally attractive BH!
4: Apart from 1: above, how could a negative mass DE core not be inherently dynamically unstable – runaway self acceleration, tearing itself apart, etc.?
5: Even ignoring 1 – 4 above, where does a fantastic leverage ratio arise from, whereby a small fraction of a small fraction of galaxy mass ‘explain’ 2/3 of the entire universe’s energy content? How is that not the ultimate magician pulling a cosmic rabbit out of the cosmic hat trick?
6: Even assuming 5: makes any sense, what is the nature of the mystical coupling driving accelerated Hubble expansion? It can’t be gravity owing to 1:
7: Finally, may I suggest a serious look at a theory sidelined by the GR community, but making a lot of self-consistent sense:
1507.07809 (type it into the search box at arxiv dot org).
Following on from my own above listed reasons to reject the claimed BH-DE linkage, two recent arXiv articles support that, but from very different perspectives:
Observational evidence against the supposed BH-DE ‘connection’:
https://arxiv.org/abs/2302.12386
[Submitted on 24 Feb 2023]
Constraints on the Cosmological Coupling of Black Holes from the Globular Cluster NGC 3201
Carl L. Rodriguez
Globular clusters are among the oldest stellar populations in the Milky Way; consequently, they also host some of the oldest known stellar-mass black holes, providing insight into black hole formation and evolution in the early (z?2) Universe. Recent observations of supermassive black holes in elliptical galaxies have been invoked to suggest the possibility of a cosmological coupling between astrophysical black holes and the surrounding expanding Universe, offering a mechanism for black holes to grow over cosmic time, and potentially explaining the origin of dark energy. In this paper, I show that the mass functions of the two radial velocity black hole candidates in NGC 3201 place strong constraints on the cosmologically-coupled growth of black holes. In particular, the amount of coupling required to explain the origin of dark energy would either require both NGC 3201 black holes to be nearly face on (a configuration with probability of at most 10^?4) or one of the BHs would need to have formed with a mass below that of the most massive neutron stars (2.2M?). This emphasizes that these and other detached black hole-star binaries can serve not only as laboratories for compact object and binary astrophysics, but as constraints on the long-term evolution of astrophysical black holes.
Theoretical Rebuttal of a fundamental common sense nature:
https://arxiv.org/abs/2302.13333
[Submitted on 26 Feb 2023]
Can black holes be a source of dark energy?
S L Parnovsky
The hypothesis that the mass of BHs increases with time according to the same law as the volume of the part of the Universe containing it and therefore the population of BHs is similar to dark energy in its action was recently proposed. We demonstrate the reasons why it cannot be accepted, even if all the assumptions on which this hypothesis is based are considered true……
Can black holes be a source of dark energy? 4 Both MBH and the volume of this part increase proportionally to a3. Therefore, the MBH density does not change with time. But this is not enough to ensure the properties of DE. It is also necessary to have a negative pressure with the equation of state (EoS) P = ?? (it corresponds to the cosmological constant) or close to it. Here P is the pressure and ? is the density of mass and energy (in units with c = 1). In the first case, the constancy of ? and P is provided automatically if we proceed from the fact that DE cannot be transformed into something else and vice versa.
Within GR, both gravitational attraction and repulsion are possible. Everything is determined by the sign of the combination ? + 3P. For ordinary matter, it is positive and attraction occurs. For the case of the cosmological constant or the more general case of DE, it is negative. This corresponds to gravitational repulsion or anti-gravity, which ensures the accelerated expansion of the Universe.
The black hole system does not have negative pressure. Therefore, it does not provide anti-gravity and accelerated expansion. It cannot be considered as something that works as an analogue of DE. Moreover, at present, the influence of DE prevails in the cosmological expansion, while the mass of black holes is a very small fraction of the mass of everything that fills our Universe…..
There is no mention in the article (Farrah et al, 2023a) of the reasons why the authors came to the conclusion that the BH population has a negative pressure, and it is huge in absolute value. Indeed, without the fulfillment of condition ? + 3P < 0 there will be no antigravity and, accordingly, no accelerated expansion. Standard concept of the properties of black holes rule out this possibility.
Even if we assume that our knowledge of the BH properties will change significantly in the future, they are unlikely to include negative pressure. The reason is simple. Black holes do not uniformly fill the entire space, but are concentrated into small objects. Even if they would have a negative pressure capable of providing anti-gravity, then the gravitational repulsion would be observed primarily in the region around the black hole. In this case, instead of accretion of matter from the surrounding space onto the BH, we would observe its expansion, dispersion, or flying apart, which contradicts the astronomical observations.