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There may be about 90 times more dark matter mass than visible matter. This could be enough to make the universe's mass density = critical density. Some evidence for the presence of dark matter has already been presented in the previous chapters. The list below summarizes the evidence for dark matter's existence.
Orbital speeds of stars in galaxies
- Flat rotation curves
of spirals even though the amount of the light-producing matter falls off
as the distance from the galaxy center increases.
Remember the enclosed mass = (orbital speed)2 × (orbit size)/G.
Below is the rotation curve for our Milky Way Galaxy (a typical spiral galaxy).
Also, the orbital speeds of stars in elliptical galaxies are too high to be explained by the gravitational force of just the luminous matter in the galaxies. The extra gravitational force is supplied by the dark matter in the ellipticals.
Faint gas shells around ellipticals
- Ellipticals have faint gas shells that need massive ``dark'' haloes to
contain them. The gas particles are moving too quickly (they are too hot) for
the gravity of the visible matter to hang onto it. However, the number of
ellipticals with these faint gas shells is too large to be only a temporary
feature of ellipticals. The dark haloes must extend out to 300,000 light years
around each galaxy. The extent of this dark matter pushes W
up to around 0.2. If the haloes are larger than originally thought,
W could approach 1.
Motion of galaxies in a cluster
- Galaxy cluster members are moving too fast to be gravitationally bound
unless there is unseen mass. The reasonable assumption is that we do not live at a
special time,
so the galaxies in the cluster must have always been close to each other. The large
velocities of the galaxies in the clusters are produced by more gravity force than
can be explained with the gravity of the visible matter in the galaxies.
Hot gas in clusters
- The existence of HOT (i.e., fast moving) gas in galaxy clusters. To keep
the gas bound to the cluster, there needs to be extra unseen mass.
Quasar spectra
- Absorption lines from hydrogen in quasar spectra tells us that there is
a lot of material between us and the quasars.
Gravitational Lensing
- Gravitational lensing of the light from distant galaxies and quasars by closer galaxies or galaxy clusters enables us to calculate the amount of mass in the closer galaxy or galaxy cluster from the amount of bending of the light. The derived mass is greater than the amount of mass in the visible matter.
Current tallies of the total mass of the universe (visible and dark matter) indicate that there is too little matter to halt the expansion---we live in an open universe. Astronomers and physicists are exploring the possibility that perhaps there is an additional form of energy not associated with ordinary matter that would greatly affect the fate of the universe. This is discussed in the last section of this chapter.
A good book on the history of dark matter is The Dark Matter: Contemporary Science's Quest for the Mass Hidden in Our Universe by Wallace and Karen Tucker (New York: Morrow, 1988).