Kerr-type black holes are vacuum solutions of Einstein’s equations that have a singularity inside and are asymptotically flat; all the black holes observed in Nature are compatible with this mathematical solution. Black hole imposters are singularityless solutions of Einstein’s equations, with exotic matter inside (for example, dark energy with equation of state p = −ρ). There are black hole imposters that are not asymptotically flat and tend towards a Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological solution; in this case its mass grows with the size of the observable universe. It is published in Astrophysical Journal Letters that these black hole imposters could explain the contribution of dark energy in the ΛCDM model. The reason is that an article in Astrophysical Journal has estimated that the ratio between the mass of supermassive black holes and the stellar mass of their galaxy appears to have grown by a factor of between 8 and 20 from z ∼ 2.7 to z = 0. The idea has made headlines in many media, but we must Be very cautious, because it has neither head nor tail. As always, correlation does not imply causation.
The singularity of black holes bothers some astrophysicists, who prefer regular mathematical solutions, without singularity. They don’t care that these solutions require the existence of exotic forms of matter with equations of state as exotic as p/ρ < 0 (a negative pressure/density ratio). They argue in their favor that the Kerr metric is incompatible with the FLRW metric at infinity; they (intentionally) forget that black holes are the smallest astrophysical objects that exist, for a given mass, and that for them the size of the universe is virtually infinite (and already was since the cosmic dawn az ∼ 25). I find it inconceivable that for z ∼ 2.7 tiny supermassive black holes have a mass that depends on the size of the observable universe; still, the new paper proposes the existence of impostor black holes with a mass M(a) = M(a∗) (a/a∗)kcon k ∼ 3, where a is the size (or scale factor) of the observable universe and a∗ it is the size from which the impostors are coupled to the dynamics of the universe. The proposal does not make any sense and is based on the fact that certain observations point to a ratio MBH/M∗ = (1+z)3.5±1.4 at 90% CL (statistical confidence limit), where MBH is the mass of supermassive black holes and M∗ is the stellar mass of your galaxy. In fact, using these data it is estimated k = 3.11+1.19−1.33 at 90% CL Quite a leap into the void that has generated a certain media echo.
In my opinion, the general public should not be misled by calling black hole impostors black holes, despite the fact that this is done in scientific articles in journals such as ApJ and ApJL. Furthermore, it should be clarified that there is a vicious circle when explaining cosmological dark energy using objects that contain dark energy inside; explaining the unknown (the origin of dark energy) by sweeping it under the rug (slamming it in a place as unfathomable as the inside horizon of a black hole imposter) doesn’t seem like significant scientific progress to me. Articles are Duncan Farrah, Kevin S. Croker, …, Chris Pearson, “Observational Evidence for Cosmological Coupling of Black Holes and its Implications for an Astrophysical Source of Dark Energy,” The Astrophysical Journal Letters 944: L31 (15 Feb 2023), doi: https://doi.org/10.3847/2041-8213/acb704, and Duncan Farrah, Sara Petty, …, Andreas Efstathiou, “A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z ≲ 2,” The Astrophysical Journal 943: 133 (02 Feb 2023), doi: https://doi.org/10.3847/1538-4357/acac2e. There are hundreds of articles about black hole imposters and dozens with dark energy (exotic matter with p = −ρ) inside; below I have taken a figure from Irina Dymnikova, Evgeny Galaktionov, “Regular rotating de Sitter–Kerr black holes and solitons,” Classical and Quantum Gravity 33: 145010 (21 Jun 2016), doi: https://doi.org/10.1088/ 0264-9381/33/14/145010.
If you prefer an optimistic opinion about these articles, I recommend the press releases: Univ. Hawaii at Manoa, “First observational evidence linking black holes to dark energy,” EurekAlert!, 15 Feb 2023; Hayley Dunning, «Scientists find first evidence that black holes are the source of dark energy,» Imperial College London, 15 Feb 2023. In Spanish you can read Guillermo Carvajal, «Scientists find the first proof that black holes are the source of dark energy”, The Green Compass, Feb 15, 2023.
[PS 19 Feb 2023] I recommend the popular article (which includes critical opinions from some experts) by Adrian Cho, “Is dark matter made of black holes? Observations of compact little galaxies challenge fanciful idea,” Science, 09 Feb 2023. [/PS]
One of the great advantages of Einstein’s equations for gravitation in the general theory of relativity is that you can cut and paste solutions to build new solutions. The only problem is that the energy-momentum tensor compatible with this cut and paste is usually incompatible with known physics (in relativity it is said that it does not comply with the energy conditions). If we like a rotating Kerr-type black hole to have an interior singularity, it suffices to put a regular solution inside it (for example, a de Sitter-type cosmological metric); Obviously, the resulting energy-momentum tensor will tell us that unknown exotic matter must exist in that region and almost certainly does not. But for the rest, if said solution is robust to small perturbations, from a mathematical point of view, it is a mathematical solution as valid as any other physically realistic solution. These types of solutions are often called black hole imposters, because they are not vacuum solutions of Einstein’s equations; Remember, black hole-like solutions are soliton-like solutions of Einstein’s equations for empty spacetime, with a zero energy-momentum tensor. All black hole impostor-type solutions (usually called regular black holes) are solutions with a non-zero energy-momentum tensor (which is also exotic, violating the energy conditions imposed by cosmic censorship).
The article in ApJL is based on observational results studied in the article in ApJ, specifically, in a new estimate of the ratio MBH/M∗ for galaxies with z < 2 from two catalogs (WISE and COSMOS). As you can see in this figure, the observations in the M planeBH versus M∗ they are very scattered; the usual fit using a straight line (dashed line) is quite poor. But the new adjustment by means of a power law (the black line with the gray band) is also quite debatable (especially when the result has also been biased by selecting a subset of the two catalogs studied). In my opinion, it cannot be affirmed that these data support that MBH/M∗ = (1+z)3.5±1.4 at 90% CL And, therefore, not only this article falls, but also the one that relies on this to explain dark energy.
The article in ApJL proposes that imposters to supermassive black holes with mass dependent on the size of the universe following a power law with k ∼ 3 allow us to explain the contribution of dark energy (ΩL = 0.68) to the total density of the universe (Ω=1). They rely on some very simple simulations (scratching napkin calculations) that require there to be a sufficient density of black hole imposters. For this, the star formation rate (SFRD) must be sufficient; This figure shows that for z < 5, la tasa observada es compatible con la necesaria; para z > 5, the region supported by the new speculation is so wide (the large green region on the right) that it is not a problem. It might seem that, thanks to this, this speculation is well supported by observations. But remember that there is no known physical mechanism that would allow the collapse of a very massive star to lead to the appearance of a black hole impostor with a dark energy interior (p/ρ < 0).
In summary, it is proposed to explain dark energy with dark energy black holes; To do this, analyzes of astrophysical observations are combined with wild scientific speculation, so that it appears that said speculation appears to be a reasonable hypothesis. But no one should be fooled by this meaningless combination of theory and observations; In my opinion, this is a clear example that correlation does not imply causation in the field of theoretical physics. In fact, we are dealing with a supposed correlation, since the evidence in its favor is quite poor. For me, supporting a nonsensical theoretical hypothesis is the most serious thing. And, I repeat, all this to explain all dark energy using dark energy inside black holes (if they at least proposed to explain dark energy without dark energy…). I am sorry to be pessimistic about this type of speculation, very much to the liking of the general media; but I don’t think it will have much of a scientific journey in the coming years.