The evidence for Dark Matter
The evidence for dark matter is gravitational.
In Astronomy, as in all sciences, one can detect an object in one of two ways: either by observing it directly, or observing the effect that it has on other, more easily observed, objects.
It's always been known that there was matter in the night sky that we couldn't really directly see. When astronomers use telescopes, or even radio telescopes, they can only see objects which emit light or radio waves. Not all of the matter in the Universe does this - for instance, we wouldn't be able to see planets like our own, because they would be too dim to see.
All the mass of of all the planets in our solar system, though, is significantly less than one percent of the Sun's mass. So worrying about matter that didn't shine - non-luminous matter - wasn't of great concern.
The first evidence: Clusters of Galaxies
The first evidence that there was a significant amount of matter that we couldn't readily see was in investigating clusters of galaxies, which are simply aggregates of a few hundred to a few thousand clusters otherwise isolated in space.
In the thirties, chaps named Zwicky and Smith both examined closely two relatively nearby clusters, the Coma cluster and the Virgo cluster. They looked at the individual galaxies making up the clusters individually, and the velocities of the clusters. What they found was that the velocities of the galaxies were about a factor of ten to one hundred larger than they expected.
What did that mean? Well, in a group of galaxies like a cluster, the only important force acting between the galaxies is gravitation; it is the pulling of the galaxies on each other that gives rise to their velocities.
The velocities can indicate the total mass inside the cluster in two ways. The first way is simple; the more mass in the cluster, the greater the forces acting on each galaxy, which accelerates the galaxies to higher velocities.
Experiment 1
Try this experiment. It allows you to vary the mass inside a galaxy cluster, and watch the individual galaxies.
The second way that velocities indicate the mass in the cluster is almost as simple; if the velocity of a given galaxy is too large, the galaxy will be able to break free of the gravitational pull of the cluster; that is, if the galaxy velocities are larger than the escape velocity, the galaxy will simply leave the cluster! So by knowing that all of the galaxies have velocities of less than the escape velocity, you can estimate the total mass.
Although in retrospect this is fairly strong evidence, it wasn't treated as such at the time; the problem is, there are many ways that observations such as these can go wrong. Mainly, the problem is in `contamination'.
When you are looking at something as vast as a cluster of galaxies, even though the velocities may be quite large, they are nothing compared to the vast expanses of the cluster. So even observing a cluster over several years just gives a still-life picture of the cluster; we can't actually watch the galaxies jostle around like we could in Experiment 1. So maybe a galaxy with a particularly high velocity is leaving the cluster; or maybe it was never part of the cluster, but was just `sailing through'. And maybe some other galaxies were just `foreground galaxies' - galaxies in front of the cluster, along the line of sight from here to there; in that case, the velocity data for that galaxy would just be misleading.
The evidence for dark matter is gravitational.
In Astronomy, as in all sciences, one can detect an object in one of two ways: either by observing it directly, or observing the effect that it has on other, more easily observed, objects.
It's always been known that there was matter in the night sky that we couldn't really directly see. When astronomers use telescopes, or even radio telescopes, they can only see objects which emit light or radio waves. Not all of the matter in the Universe does this - for instance, we wouldn't be able to see planets like our own, because they would be too dim to see.
All the mass of of all the planets in our solar system, though, is significantly less than one percent of the Sun's mass. So worrying about matter that didn't shine - non-luminous matter - wasn't of great concern.
The first evidence: Clusters of Galaxies
The first evidence that there was a significant amount of matter that we couldn't readily see was in investigating clusters of galaxies, which are simply aggregates of a few hundred to a few thousand clusters otherwise isolated in space.
In the thirties, chaps named Zwicky and Smith both examined closely two relatively nearby clusters, the Coma cluster and the Virgo cluster. They looked at the individual galaxies making up the clusters individually, and the velocities of the clusters. What they found was that the velocities of the galaxies were about a factor of ten to one hundred larger than they expected.
What did that mean? Well, in a group of galaxies like a cluster, the only important force acting between the galaxies is gravitation; it is the pulling of the galaxies on each other that gives rise to their velocities.
The velocities can indicate the total mass inside the cluster in two ways. The first way is simple; the more mass in the cluster, the greater the forces acting on each galaxy, which accelerates the galaxies to higher velocities.
Experiment 1
Try this experiment. It allows you to vary the mass inside a galaxy cluster, and watch the individual galaxies.
The second way that velocities indicate the mass in the cluster is almost as simple; if the velocity of a given galaxy is too large, the galaxy will be able to break free of the gravitational pull of the cluster; that is, if the galaxy velocities are larger than the escape velocity, the galaxy will simply leave the cluster! So by knowing that all of the galaxies have velocities of less than the escape velocity, you can estimate the total mass.
Although in retrospect this is fairly strong evidence, it wasn't treated as such at the time; the problem is, there are many ways that observations such as these can go wrong. Mainly, the problem is in `contamination'.
When you are looking at something as vast as a cluster of galaxies, even though the velocities may be quite large, they are nothing compared to the vast expanses of the cluster. So even observing a cluster over several years just gives a still-life picture of the cluster; we can't actually watch the galaxies jostle around like we could in Experiment 1. So maybe a galaxy with a particularly high velocity is leaving the cluster; or maybe it was never part of the cluster, but was just `sailing through'. And maybe some other galaxies were just `foreground galaxies' - galaxies in front of the cluster, along the line of sight from here to there; in that case, the velocity data for that galaxy would just be misleading.
