Dark matter has been observed to have density spikes near black hole event horizons
What does that mean for alternatives to dark matter?
It is fairly common in the physics community to assume that dark matter exists. The group of physicists preoccupied with alternatives to dark matter, meanwhile, is small. So when people do studies of how dark matter behaves, they rarely consider alternative explanations. This study of dark matter density spikes near black hole event horizons is an example.
The study is an examination of actual data from known black holes that have companion star.
We know that binary systems like these tend to slowly draw closer together, called inspiral, eventually collapsing into one large black hole. In the mean time, the black hole draws matter away from the companion star causing enormous amounts of X-ray radiation to be released. These X-rays help us to measure the behavior of the systems more accurately.
Einstein’s general relativity predicts that gravitational energy will radiate away from the two bodies in the form of gravitational waves. This causes them to lose energy and slowly run down.
Even in the most pristine cases of classical physics there are no perpetual motion machines. Entropy must increase. Engines must run down. Nothing is 100% efficient even a two body system made of singularities.
It has been observed, however, that the rate of inspiral is higher than expected. And the reason for this is hypothesized to be because of gravitational friction with dark matter.
Any gravitationally massive object must interact with dark matter and so naturally, if these bodies are orbiting in a cloud of dark matter, they must crash together that much faster.
If there are clouds of dark matter around event horizons they are the most dense closest to the horizon because the dark matter falls in all the time.
Theoretically we would expect to see some kind of evidence based on the properties of that dark matter. For example, if dark matter particles can annihilate one another like ordinary matter, we would expect to see gamma rays from the annihilations. We don’t observe any, but that could be because dark matter doesn’t self-annihilate or the annihilation is too small to see.
Two black hole binaries have been measured precisely enought that we can find in their orbital periods indirect evidence of the dark matter theory. (One of these, A0620-00, has the closest known black hole to Earth at 3300 lightyears.) In these systems we have a black hole with a companion star and we observe anomalously fast orbital decays, almost 50 times larger than predicted. (The actual decay is quite small, on the order of milliseconds per year which just goes to show how accurately we can measure these things.)
There are two main theories for why the orbits are decaying. One is called magnetic braking where the companion star has a strong magnetic field that interacts with the surrounding solar winds to slow its orbital decay. The other is that the matter around the binary is somehow leaching angular momentum away from the star.
Both of these explanations require some amount of mass transfer that hasn’t been observed.
The dark matter explanation suggests that the star is pulling dark matter away from the black hole and that is causing the orbital decay.
The paper makes quite a few assumptions to get its results. While it starts with some pretty well established concepts for friction causing orbital decay, Kepler’s laws for binary systems, and so on, its assumptions about dark matter density profiles are based on convention and theoretical arguments rather than observation. The paper is certainly interesting but hardly evidence of dark matter. It would be more convincing if some of the other theories could be ruled out better.
Alternatives to dark matter such as MOdified Newtonian Dynamics (MOND), don’t have much to say about these particular binaries because they are too close to one another. MOND only modifies how gravity behaves when two bodies get very, very far apart, on the order of 2000 AU or about 50 times further apart than Pluto is from the Sun. For smaller distances, gravity is Newtonian and so MOND can’t explain the rapid orbital decay and wouldn’t even if they were far apart.
Relativistic version of MOND such as Aether-Scalar-Tensor (AeST) theory might have a better chance because they have additional degrees of freedom. AeST has both the ordinary gravitional field of general relativity as well as a vector field, similar to electromagnetism, and a scalar field. All of these fields would produce gravitational radiation and so all of them would contribute to orbital decay. It is hard to say that that would be enough.
Other theories of gravity could also be responsible such as the Randall-Sundrum model that invokes extra dimensions and branes or the Gauss-Bonnet theory. These are just a few examples of potential modified gravity theories that could explain rapid orbital decay.
In a sense, this anomalous orbital decay could have a very prosaic explanation in terms of known physics (magnetic braking) or it could lead to some new theory of gravity as the correct prediction of the anomalous perhelion precession of Mercury helped motivate general relativity. While the dark matter friction theory is intriguing it is only one of many potential explanations.
Chan, Man Ho, and Chak Man Lee. "Indirect evidence for dark matter density spikes around stellar-mass black holes." The Astrophysical Journal Letters 943.2 (2023): L11.
González Hernández, J. I., R. Rebolo, and J. Casares. "Fast orbital decays of black hole X-ray binaries: XTE J1118+ 480 and A0620–00." Monthly Notices of the Royal Astronomical Society: Letters 438.1 (2013): L21-L25.