Install Raid for Free ✅ IOS: ✅ ANDROID: ✅ PC: and get a special starter pack 💥 Available only for the next 30 days! Is it possible that what we have invented as dark matter to explain the motion of stars in galaxies and clusters is a figment of our imagination? Could it be that Einstein’s General Relativity has a flaw? Does dark matter exist?
Vera Rubin verified that stars in the outer edges galaxies are moving around the galactic center at about the same speed as those closer to the center. This cannot be explained by all the visible matter. This faster than predicted motion it was theorized is due to extra unseen matter, or Dark Matter that surrounds all galaxies.
But could this motion be equally well explained by a modification of Einstein’s General Relativity, just like Newton’s equations were modified by Einstein’s General Relativity?
GR has been shown to be highly accurate for massive objects up to several light years apart. Is it possible that gravity behaves very differently, at larger distances? In 1983, Israeli physicist Mordehai Milgrom published a hypothesis showing that a modification of Newtonian mechanics could almost perfectly explain our observations.
Milgrom found that if you calculate using just the laws of Newton, the acceleration due to gravity at the point where you need dark matter to explain the velocity of stars, this acceleration was the same for every galaxy. There was some critical value of acceleration, such that if the acceleration was larger than that value you didn’t need dark matter, but when the acceleration was smaller, you needed dark matter.
So what Milgrom said is that maybe we just need to modify the theory of gravity such that at high acceleration, it falls off by one over radius squared, as in Newton's equation But at smaller accelerations, gravity falls off at one over the radius, instead of radius squared.
If you invoke Milgrom’s hypothesized modification of gravity, it fits the data almost perfectly. It fits our observations of the motions of stars not just in our own galaxy, but dozens of other galaxies.
So the idea that you don’t need dark matter, and Newton’s laws work just fine inside a certain radius, but a modification of Newton’s laws works on the outside of this radius seems to make it work quite well in many galaxis. Milgrom’s hypothesis is called Modified Newtonian Dynamics, or MOND.
Both the idea of MOND, and the theory of a dark matter halo surrounding galaxies seem to work equally well. Why don't we adopt MOND, and get rid of dark matter theory? Because the problem is that while Milgroms theory explains the motion of stars within galaxies well, it does not explain the motion of CLUSTERS of galaxies.
This means that the bullet cluster is not explained by Milgrom’s theory. Gravitational lensing indicates that the majority of the mass of this cluster is located in the blue areas where less ordinary matter can be seen. So there must be unseen dark matter in these areas as indicated by the gravitational lensing. This is very strong evidence for the idea of dark matter being a particle. This cannot be explained or predicted by Milgrom’s modification of newton’s equations.
Maybe we need both - a modification of Newton’s laws as well as dark matter particles to fully explain our observations of both the motions of stars within galaxies. One benefit of adopting both ideas would be that it would allow the possibility that the dark matter you would need to explain the behavior of clusters of galaxies may be different than the popular candidates likeWIMPs and Axions, which are theoretical and have never been detected.
This dark matter could perhaps be ordinary matter such as Neutrinos. Scientists don’t think neutrinos can explain the motion of stars in galaxies because neutrinos move too fast to allow halo formations around galaxies. But they do move slowly enough that they could form halos around clusters of galaxies on the scales millions of light years across, such as the bullet cluster.
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But another nagging reason Dark matter needs to exist is the microwave background radiation. This is the way the universe looked at about 380 thousand years after the big bang. And its structure tells us a lot about the way ordinary matter came together in a process called nucleosynthesis. And based on matching our observations with prediction made by known equations, we can estimate that the amount of ordinary matter in the universe, and is 5%. But the critical amount of matter needed to make a flat universe, that is a universe that is not curved in 4 dimensions, is 30%. So this means that 25% of this 30% can not be ordinary matter. This number matches perfectly with our observations of galaxy rotations.
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