What is dark matter?
In astronomy, dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field.
Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be seen.
Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.
What is MOND?
There are no particles in the hugely successful Standard Model of particle physics that could be the dark matter — it must be something quite exotic.
This has led to the rival idea that the galactic discrepancies are caused instead by a breakdown of Newton’s laws.
The most successful such idea is known as Milgromian dynamics or Modified Newtonian dynamics (MOND), proposed by Israeli physicist Mordehai Milgrom in 1982.
MOND's Challenges
The main postulate of MOND is that gravity starts behaving differently to what Newton expected when it becomes very weak, as at the edges of galaxies.
MOND is quite successful at predicting galaxy rotation without any dark matter, and it has a few other successes.
But many of these can also be explained with dark matter, preserving Newton’s laws
MOND only changes the behaviour of gravity at low accelerations, not at a specific distance from an object.
You’ll feel lower acceleration on the outskirts of any celestial object — a planet, star or galaxy — than when you are close to it.
But it is the amount of acceleration, rather than the distance, that predicts where gravity should be stronger.
This means that, although MOND effects would typically kick in several thousand light years away from a galaxy, if we look at an individual star, the effects would become highly significant at a tenth of a light year.
That is only a few thousand times larger than an astronomical unit (AU) – the distance between the Earth and the Sun.
But weaker MOND effects should also be detectable at even smaller scales, such as in the outer Solar System
MOND fails to explain small bodies in the distant outer Solar System.
Comets coming in from out there have a much narrower distribution in energy than MOND predicts.
These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to.
MOND would cause the inclinations to be much larger.
MOND also fails on scales larger than galaxies: it cannot explain the motions within galaxy clusters.
Dark matter was first proposed by Fritz Zwicky in the 1930s to account for the random motions of galaxies within the Coma Cluster, which requires more gravity to hold it together than the visible mass can provide.
MOND cannot provide enough gravity either, at least in the central regions of galaxy clusters.
But in their outskirts, MOND provides too much gravity.
Assuming instead Newtonian gravity, with five times as much dark matter as normal matter, seems to provide a good fit to the data.
Future
The standard dark matter model of cosmology isn’t perfect, however.
There are things it struggles to explain, from the universe’s expansion rate to giant cosmic structures.
So we may not yet have the perfect model.
Ultimately though, MOND, as presently formulated, cannot be considered a viable alternative to dark matter any more
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