©Fernando Caracena 2015
Introduction to Dark Matter
I wrote about dark matter in a previous post called "Divine Intervention and Dark Matters". I also wrote about dark energy. These two kinds of ghosts are haunting physicists these days. Dark matter and energy are monkey wrenches tossed into the well oiled theoretical machinery of physics. It would be better if they were not there. Or are they really there? Anyway, at present, they are a thorn in the side of physics.
The post, "Divine Intervention and Dark Matters", introduces the problem of dark matter. Some physicists think that it exists, and others disagree. They think that it is just a fudge factor for correcting Newton's formula for gravity. Physicists also talk about dark energy. These dark areas of physics are causing a lot of problems for physicists. We can no longer say that we know very much. There is a huge piece of nature missing from what we can see and study. We are looking at ghost matter that is very heavy. A quote from the post referenced above (by hyperlink), defines the magnitude of the problem as follows:
As it stands today, astrophysicist propose that only 4.6 % of the universe is visible, and that there is a necessary 95.4% invisible stuff to be able to account for how all that visible stuff moves.
The problem surfaced when astrophysicists tried to understand the observed rotation of galaxies. They estimated the mass distributions of luminous matter in galaxies by using the relationship between stellar absolute brightness and mass, and the statistical distribution of stars and star types. The Doppler effect, gave them a means of estimating the orbital velocities of stars in galaxies. From knowing the distribution of luminous mass in a galaxy and how fast the stars are orbiting, scientists hoped to apply the known laws of gravity to explain orbital motion of stars the way that, those of planets are explained for our solar system. They found out that not only was the mass of luminous matter insufficient to hold galaxies together, but that it was way out of the ball park range. It was much much too small. A whole lot more mass was needed to boost the gravitational force back into the ball park range. To make the dynamics of galaxies consistent with gravitational forcing, scientists suggested that galaxies formed around vast...[lumps] of dark matter, which interacted with ordinary matter predominantly through gravity, and either very weakly or not at all through any other mechanism.
The nature of what dark matter is, was left hanging in the air by astrophysicists, but particle physicists wanted to know. If it is matter, it must consist of "particles". Will our understanding of elementary particles be able to account for dark matter? Will we have to develop new particle theories? Do dark matter particles collide with each other? Physicists attempt to answer such questions through experiments on the Large Hadron Collider (LHC). So far, the LHC has given us no infromation about dark matter.
The Biggest Colliders in the Universe
Nature has provided us with the largest particle colliders. Occasionally galaxies have head-on collisions at fantastic speeds. The little kid in us wants to see that. What fun! In a galactic train wreck different parts of it crash together in their own ways. Stars actually have a lot of space between them. Their chance of colliding in a galactic train wreck is very small. They just go by each other. They barely bow to each other's gravity. Masses of dark matter go right through each other as if they were not there. Gas clouds, however, are almost stopped in their tracks as their molecules collide head-on. Colliding gas clouds get very hot and radiate x-rays.
Perhaps an analogy with a train wreck will make it clear what is going on. Say that the train engines are made of dark matter. Ordinary passenger cars are like stars, and coal cars are like gas clouds. Imagine that two such trains suffer a head-on collision. The engines go right through each other. The passenger cars are pulled along with them. But, the coal cars are pulverized. They are stopped dead in their tracks and go up as large fire-balls.
Dark Matter is Weird
A group of astrophysicists have examined seventy two colliding galaxies in a new study. They concluded that dark matter is more mysterious and weirder than previously thought. An article called, "Dark Matter is Weirder than Previously Thought", explains how.
They looked at the galaxies in collisions through two types of telescopes. Through the Hubble Space Telescope they saw the visible matter [stars]. Through the Chandra Telescope, sensitive to x-ray radiation, they imaged the hot gas clouds.
In head-on collisions, the dark centers of gravity of the galaxies, keep moving through each other without any evidence of collisions of their constituent parts. Because they are the nine hundred pound gorillas of the galactic universe they gravitationally drag the rest of the matter involved in the collisions along with them. The gas clouds are left behind, glowing very hot.
The Bullet Cluster is an example of an observed, galactic head-on collision that provided some compelling evidence that dark matter exists. But there are still some die-hards who dispute whether the evidence is really conclusive.
So what is Dark Matter?
Dark matter has some pretty strange properties. Unfortunately, it does not interact with anything or itself in ways that we can understand in particle physics. "The Dark Matter Poltergeist is real, but what is it? "
"If we can identify dark matter as a new fundamental particle, say "ghostium", we will have the first major departure from the standard model of particle physics—a paradigm that has held up to nearly every experimental test to an astounding degree of accuracy."
Dark matter and energy are two things that have been hypothesized to save the equations of physics. If their effects were not so indelibly written on the motions of galaxies and the universe itself, it would not be a big issue.
In the past, physicists have been sceptical of explanations that are needed to save the equations. Physicists would feel much better about the two dark entities, if there was corroborating evidence for their existence.
Physicists do not have a lot of trouble with the idea of dark energy. There is good evidence that links it to the zero point energy of the electromagnetic field. The link does cause some problems, but these problems may be gradually eliminated. Unfortunately, dark matter is a real stinker. It is the tar baby of physics.