Dark Matter Versus MOND

First, you may want to read my earlier post that describes the basic problem of dark matter.

Second, 'dark matter' (if it actually exists) should not really be called dark, it should actually be called transparent matter. It's not dark, it just doesn't interact interact with light. Neil deGrasse Tyson thinks it should instead be called "Dark Gravity".

Okay, now that I have gotten that out of the way I am going to try to explain one of the proposed solutions to the dark matter problem which is called MOdified Newtonian Mechanics or MOND (the capitalized 'MO' in MOdified is not a typo BTW :) ).

Dark matter at first blush feels a lot like the somewhat embarrassing concept called the aether (or ether depending on your preference). This is a hypothesis that explained how light could pass through a vacuum and it was debunked quite thoroughly over 100 years ago.

Dark matter just feels like this all over again: inventing something that no one can see or detect to explain away an observation.

Some physicists feel like this (count me in that group) and so they have proposed modifications to the known laws of physics for both gravity as well as motion.

MOND is an hypothesis that proposes a modification of Newton's laws to account for the observed rotation curves of galaxies.

It was proposed in 1982 by physicist Mordehai Milgrom.

The idea is that as the acceleration due to gravity becomes very very small the normal behaviour for acceleration changes from the familiar:

to an equation that may look something like this:

where F is force,
m is the mass,
f(a/a₀) is just some, currently unknown, modifying function that defines a constant,
a₀ is some very small acceleration (a threshold accerelation below which you start seeing the effects), and
a is acceleration.

All this equation means in words is that the expected motion of objects may behave differently at very very small accelerations.

Again the idea of MOND is proposed because humans have never tested Newton's law for acceleration at very very small accelerations (the kind of accelerations that you would find at the edges of galaxies).

The Successes

MOND is successful for explaining a lot of the odd behaviour exhibited by stars as you move towards the edges of galaxies. Two good examples are:

1. The rotation curve of galaxies predicted using MOND matches the observations made by astronomers (reference).
2. In extremely massive galaxies the acceleration does not drop down below the critical threshold of a₀ until large distances out from the centre. MOND seems to predict the rotation curves of these types of galaxies better than dark matter (reference, reference).

 

The Challenge To MOND

All is not done though as there seems to be astronomical evidence that MOND (or other proposals of its kind) cannot explain.

The best challenging evidence is the so-called 'Bullet Cluster'. This is a cluster of galaxies that are slamming into each other at high speed.

Astronomers have measured the distribution of stars and gas in this cluster using visible light, X-ray light and the effects of weak gravitational lensing generated by this cluster.

If MOND were true the gravitational acceleration effects of the cluster as a whole should be located on the centre of the visible mass.

In the bullet cluster the gravitational acceleration effects appear to be well offset from the the centre of the visible mass.

That is, dark matter is unaffected by the gas clouds of the galaxies slamming into each other. It just keeps on going as if nothing were there (except for the overall and ever evolving gravity field of course).

This implies that most of the mass in this cluster is outside of the region where the visible matter exists. Something that MOND seems unable to model.

Closing Words

The proponents of MOND and the other modifications of the laws of physics have made rebuttals to the above and other evidence and the final decision, although leaning towards dark matter, does not seem to be made.

In following posts I will be covering the proposals for what Dark Matter could be made of and how it could be detected.

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