8.5 Masses of galaxies and clusters
The main way of determining the mass of a galaxy is to do more or less what we did with our own galaxy, namely orbits of stars, as in our galaxy. Galaxies rotate too slowly for us to see any actual motions. What we measure are the Doppler shifts of stars or interstellar gas at different distances from the center of the galaxy.
This actually tells us how the mass is distributed in a galaxy.
The total masses of galaxies are up to about 1013 times the mass of the Sun, but there are many smaller ones, down to 108 solar masses.
When we study the rotation curves of many spiral galaxies we get a strange result. There seems to be extra mass that is not producing as much light as we would expect if the matter were in the form of normal stars.
We call this extra mass dark matter. This is a major mystery in astronomy right now. There are signs that there is 3-10 times more dark matter than regular mass in galaxies. The dark matter in galaxies seems to be spread more widely than the visible matter. It is not concentrated into stars, or even into the disks of spiral galaxies.
Clusters of galaxies
The galaxies in a cluster are held to each other by gravity. We can estimate the average gravitational force in a cluster by looking at how the galaxies move with respect to each other. We cannot see them move sideways, but we can can measure their Doppler shifts. When we do this we find that there is much more mass in the clusters of galaxies than we see in the form of starlight from the galaxies themselves. This is the second piece of evidence for dark matter.
Dark matter is deduced to exist from the studies of galaxies and galaxy clusters described above.
One of the most powerful ways of measuring the amount of dark matter in the Universe is to use gravitational lensing.
Light from a distant galaxy gets bent by passing near the enormous mass that is in the galaxy cluster. As a result, one may see more than one image of a distant galaxy. By studying the locations and shapes of these images one can work out how much dark matter is in a cluster, and even map it.
We now believe that there is at least 10 times more dark matter than matter in the Universe. In other words we know just about nothing about 90% of the Universe.
Start lecture 42
What is dark matter?
Ordinary matter is atoms, and the more fundamental particles that make up atoms, like protons. Ordinary matter is "visible" in the sense that it can absorb light, scatter light and emit light.
Dark matter seems to be different: it appears to respond to gravity, but to nothing else.
Let us examine the possibilities for what it is:
Hydrogen and helium. If dark matter were hydrogen or helium gas we would see absorption lines. Some dark matter is hydrogen gas, but certainly not all of it. Hydrogen and helium cannot be solid in space.
Rocks or terrestrial-type planets. The problem here is that these are made almost entirely out of "heavy elements". If dark matter were rocks then the ratio of heavy element to hydrogen would have to be more than 10, instead of only 1%. All our theories of the origin of the elements (which are theories that are solidly backed up by observational data) would have to be discarded.
Black holes. The only way we know of making black holes is when big stars die. If there had been thousands of times more big stars in the past than we now estimate, we would see plenty of evidence for this in the Galaxy, such as a higher abundance of heavy elements.
Machos Acronym for Massive Compact Halo Objects. The idea here is that there are zillions of small stars or large planets. They either do not have nuclear reactions (like Jupiter) or have so few that the stars hardly shine at all. They are so faint that they hide from our telescopes. They are called Halo, because there is evidence that the dark matter in our Galaxy is spread out in the galactic halo, where the globular clusters are, rather than just in the disk of the Galaxy. The problem is that there needs to be an awful lot of them; more than you get by extrapolating the numbers of normal stars. One way of looking for them is to watch for them to move in front of a distant star and amplify the light by gravitational lensing. This has been tried, and the answer seems to be that there are not enough Machos to explain dark matter
Wimps Acronym for Weakly Interacting Massive Particle. The idea is that there is a new kind of particle, separate from protons, electrons, quarks etc. To verify this idea scientists want to find actual traces of these particles, or create examples of them in a lab. This is an enormous challenge to physicists.
Does Dark Matter matter? ..Although there is a
lot of dark matter in the Universe, it is very spread out, and not
concentrated at the Earth or even in the Solar System, so there
is a negligible amount of it around here. The equivalent of
a few atoms per cubic meter! You don't have to worry about walking
But the future of the Universe depends overwhelmingly on the amount of dark matter in the Universe.