A phenomenon known as Gravitomagnetism could do away with the need for dark matter. That is the idea presented in two papers which were published in late 2020 and early 2021.
In those papers, astrophysicists show how to explain the high orbital velocities of stars in spiral galaxies. The answer lies in a familiar and successful theory of gravity that we have been overlooking all this time.
Up to now, we had always believed that objects which made up galaxies obey Newton’s laws. However, this appears not to be true. This is surprising as most of these objects only experience weak gravitational forces.
Instead of Newton’s laws, the authors of both papers applied Einstein’s General Relativity. What they found was surprising. General Relativity predicts a phenomenon know as Gravitomagnetism. The authors found that Gravitomagnetism could explain why stars orbit their galaxies as if there was more matter present than can be visibly detected.
The Flat Rotation Curves of Spiral Galaxies
To understand the situation, we must review what we know about spiral galaxies. More specifically, how they rotate.
When we mention the rotation of galaxies, we are describing it’s stars orbital speed verses their distance from the galactic center. We can plot these two properties against each other to get the galaxy’s rotation curve. This is very useful for finding how mass is distributed throughout the galaxy.
Something like our solar system has nearly all it’s mass in one object at the center. In this scenario, the rotation curve would decrease with distance by the inverse square-root law. We see this with the planets in our own solar system were 99.86% of the mass is in the Sun.
Objects that has mass spread evenly will have it’s rotation curve increase with distance inside it. However, once outside the object, the curve would decrease the same way it does for the solar system.
Galaxies are somewhat more complicated than the two scenarios mentioned. Their mass is unevenly spread out. Spiral galaxies have much of their mass concentrated in their bulges. At the same time a smaller but still significant amount of their mass is in their spiral arms.
Because of this, astronomers originally predict that the rotation curve of spiral galaxies would first increase in the bulge, slowly decrease in the spiral arms, and then drop off like the solar system’s rotation curve outside the galaxy.
What we find is that the rotation curve does not drop outside of the galaxy’s bulge. Instead, it remains at a constant speed even for stars and gas orbiting far outside the galaxy’s disk.
One way of explaining this constant rotation curve is that there is more matter than we can see. This matter makes up a very large halo that extends far beyond the galaxy’s visible borders. Hence the reason for the shape of the rotation curve that extends far outside the galaxy.
This matter would not emit or absorb light which is the reason why we could not detect it earlier. Hence the name dark matter.
It is worth noting that elliptical galaxies appear to not have this large halo of dark matter. We can measure their masses by looking at how much they bend light through gravitational lensing. When we do this, we see that their mass is nearly in agreement with the amount of light coming from them.
Recently however, there has been an idea that could explain the rotation curve of galaxies without the need for dark matter. This idea was first proposed by Crosta et. al. 2020 and then elaborated on by Ludwig 2021.
What is Gravitomagnetism?
Crosta et. al. 2020 studied the speed and position of various stars in the Milky Way from data obtained by the Gaia spacecraft. They showed that by replacing Newton’s law of gravity with Einstein’s General Theory of Relativity, they can explain the rotation curve of the Milky Way without the need for dark matter.
Ludwig 2021 elaborated on this even further by showing that one could use Crosta’s team’s idea to explain the rotation curves of other spiral galaxies. To explain why this is the case, Ludwig focused on a feature of the equations for General Relativity.
As it turns out, the equations of General Relativity make some predictions that are analogous to Maxwell’s equations for electromagnetism. This is known as Gravitomagnetism.
The reason for the name is because one could draw analogies from magnetism from Gravitomagnetism. One such example is the phenomenon of frame-dragging.
Frame-dragging occurs when the spin of a massive object twists the space-time around it. This pulls objects in orbit around the mass in the direction of the spin causing it’s orbit to precess. It can also cause the rotational axis of a gyroscope to precess as well. This was detected in 2011 by Gravity Probe B.
Gravitomagnetism and spiral structures
Gravitomagnetism could explain other things we see in spiral galaxies. One example that comes to mind is the spiral structure of galaxies such as the Milky Way. This was the focus of a paper by G. G. Nyambuya in early 2021.
Nyambuya’s model is based off a theory of gravitation proposed by physicist Gunnar Nordstrom. This theory is the predecessor to Einstein’s General Relativity.
Nordstorm’s theory appears outdated. However, some parts of it is similar to General Relativity. It is possible to derived results from this theory that can also be derived from General Relativity.
Nyambuya showed that the gravitational pull on stars in the disk of spiral galaxies is not entirely towards the bulge or the center. There is a component that pull on the stars side ways. Nyambuya called this extra component the Gravitational Dark Force.
According to Nyambuya, this Gravitational Dark Force results in galaxies either forming spiral or barred-spiral structures.
Based on the papers reviewed in this article, it appears that we may have made assumptions about the dynamics of galaxies that were wrong all this time. Dark matter may not be necessary. However, as of now, this does not make those models in those articles the best we have in explaining the rotation curves of spiral galaxies.
There are many aspects of the motions of galaxies that these models have to be tested on. A new model will have to explain all the things that the old model successfully explains as well as things that it cannot explain.
This has nonetheless shown us that the answer to difficult problems could sometimes be hidden in well established theories.
In the end, it could be that the introduction of General Relativity may be the answer to the rotation curves of galaxies while dark matter could be responsible for the missing mass of galaxy clusters.
It could also be that General Relativity and Gravitomagnetism only explains part of the galaxy rotation curve problem. We may still need dark matter to explain a smaller fraction of missing mass.
Either way, Ludwig 2021 concludes that we have to review all calculations regarding the dynamics of galaxies in light of General Relativity.