Dark Matter - "Houston, We Have A Problem"

There is a problem in physics and it has been a problem for a long time.

Either the laws of gravity as we know them are wrong or there is a whole lot of the Universe missing. Interesting times.

The Issue

Here is the issue: when astronomers measure the speed that stars should be moving around galaxies they find that they are moving far too fast for the amount of visible matter that is observed.

Galaxies should literally be flying apart based on the amount of matter in them.

Even weirder, astronomers first realized this problem almost 90 years ago.

Figure 1. The speed of stars as a function of the distance from the galaxy's centre. "Houston we have a problem."

The problem is simple. We have equations for the force of gravity as described by Newton. This really is usually good enough for predicting the orbits of most of the planets as well as for satellites that we send out into space.

There is a lot of data that shows that using this law is sufficient, however it is not good enough for some objects such as Mercury. Not a problem.

We also have the behaviour of the law of gravity as described by General Relativity. The laws of General Relativity combined with Special Relativity can be used to predict the precession of Mercury's orbit with a high degree of accuracy.

There is a lot of data that shows these calculations are good to a very high degree of accuracy. So one would naturally think that we have gravity nailed.

Good, time to go to the pub. Well, not so fast bucko.

A problem arises when the laws of gravity and motion are applied to galaxies as a whole based on their observed mass distribution. Not even the refinements of Special Relativity and General Relativity can fix things for us.

Looking at the figure above we see that stars are predicted to orbit more slowly the farther they get away based on mass distribution observations. When astronomer measure stars' velocities using data from their redshifts they find that the velocity data just doesn't agree with the observations.

Not only do they not agree, the do not agree by a LARGE AMOUNT.

This means that something is very, very wrong.

The short list of things that could be wrong are:

• We don't understand the laws of circular motion (both Newton's Laws of Motion and/or centripetal force).
• We don't understand the laws of gravity.
• We don't understand the amount of matter in the galaxies and thus the Universe.
• The observations and calculations for the amount of matter in galaxies is wrong.
• Or something else.

Much work has gone into exploring all of the above possible causes and that will be the subject of a later post. Stay tuned.

_{Thank you for reading my post. My goal is to write about interesting science and health topics and explain them in a simple, clear and intuitive manner for the learning enjoyment of all.}

_{I, Procrastilearner, donate the above image to the Public Domain.}