Dark Matter

We have now seen many reasons to believe that the Universe is full of "dark matter", matter that influences the evolution of the Universe gravitationally but is not seen directly in our present observations. For example, evidence such as the confined hot X-ray gas seen in clusters (adjacent image) indicates that 90 percent or more of the matter in the Universe does not give a signal detectable at any wavelength in our telescopes.

Evidence for Dark Matter
The nature of this "dark matter", and the associated "missing mass problem", is one of the fundamental issues of modern astrophysics. Among the independent pieces of evidence for large quantities of nonluminous matter in the Universe we may list:

  • Rotation curves for spiral galaxies like our own show consistently that such galaxies have a halo of nonluminous matter much more massive than the luminous matter. Likewise, analysis of the motion of stars in elliptical galaxies suggests the presence of more mass than can be seen.
  • Faint halos of gas observed around elliptical galaxies can only be held in place by larger gravitational fields than those produced by the visible matter.
  • Hot X-ray gas in clusters of galaxies would have dispersed long ago unless the gravitational field in the cluster is much larger than that expected from the visible matter.
  • As we shall discuss in the next chapter, the images of distant galaxies and quasars are distorted by the gravitational lensing effect of mass between us and the objects. Analysis of the amount of lensing indicates that there must be much more mass than can be seen between us and the lensed objects.
  • The light coming from distant quasars shows evidence from hydrogen absorption lines that it has passed through more matter than we can see between us and the quasar.
  • The motion of galaxies in clusters is only consistent with persistence of the cluster over long times if there is much more mass present than we can see.

    • Notice that the popular labeling of this as the "missing mass problem" is somewhat misleading. It is not that the mass is missing, because we see it gravitationally. It is that the mass does not give detectable evidence of its presence except through gravitation.

    Black Holes and Red Dwarfs

    There is some observational evidence against either black holes or red dwarfs as a major source of dark matter. Black holes are formed when massive stars (which are rare) explode, and are more likely to form nearer the center of galaxies where more ordinary mass is available to form massive stars. But the evidence suggests that dark matter is found mostly in halos and between galaxies, not in their cores.

    Recent Hubble Space Telescope searches for faint red dwarfs suggest that they may be much less abundant than previously thought. Further, as indicated in the right panel, there is evidence from the abundance of light elements that the bulk of dark matter is not baryonic. This would rule out red, white, or brown dwarfs, since they are composed of ordinary baryonic matter. Finally, gravitational lensing experiments to be discussed in the next chapter do not find evidence for abnormally large numbers of unseen compact objects in our galaxy.

    Exotic and Non-Exotic Dark Matter
    As we have already noted, dark matter could either be ordinary matter but in a form very difficult to detect, or a completely new form of matter never before seen. The primary candidates in the former category are objects like black holes and brown dwarfs and low-mass red dwarfs--that is, objects made of normal matter that are not very luminous.

    The more exotic possibilities usually center around conjectured new subatomic particles that have been suggested by some theories in elementary particle physics, but never detected experimentally. Some candidates include species of neutrinos having a mass and a class of conjectured elementary particles called supersymmetric particles.

    Hot and Cold Dark Matter
    We can also classify dark matter into two theoretical categories according to the mass of the particles making up the dark matter:

  • Hot dark matter
  • Cold dark matter

    • Hot dark matter is composed of particles that have near-zero mass (massive neutrinos, for example--see the box below). The special theory of relativity requires that massless particles move at the speed of light and that nearly massless particles move at nearly the speed of light. Thus, low mass particles must move at high velocities and form (by the kinetic theory of gases) very hot gases. Such low-mass, high-velocity matter is called relativistic matter.

    On the other hand, cold dark matter is composed of objects sufficiently massive that they move at low velocities relative to the speed of light. Thus they form much colder gases. Such high-mass, low-velocity matter is called nonrelativistic matter. The difference between cold dark matter and hot dark matter is significant in the formation of large-scale structure like clustering of galaxies, because the high velocities of hot dark matter cause it to wipe out structure that forms on all but the largest scales. We shall discuss this further when we consider the role of dark matter in the formation of structure in the Universe.

    Massive Neutrinos and the Missing Mass

    An attractive candidate for at least part of the dark matter is neutrinos with a small mass. As we saw in the discussion of the solar neutrino problem in Chapter 18, there are three "families" of neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. Each family consists of a neutrino and its antiparticle, so this is a total of six kinds of neutrinos or antineutrinos (sometimes called flavors of neutrinos).

    The discovery of neutrino oscillations suggests that at least some of the neutrinos (probably all) have small masses. Because there are so many neutrinos in the Universe, if some of them are massive, they could contribute a large total mass. Neutrinos are relativistic and so would be classified as hot dark matter if they contribute to the missing mass. However, the most recent evidence suggests that neutrino masses are not large enough to contribute to the mass of the Universe at more than the few percent level, so the bulk of the dark matter must be something other than the known neutrinos.