PDF《18. DARK MATTER》

18.

Dark matter

1 18. DARK MATTER Revised Oct.

1997 by M. Srednicki (University of California, Santa Barbara). There is strong evidence from a variety of di?erent observations for a large amount of dark matter in the universe [1]. The phrase dark matter means matter whose existence has been inferred only through its gravitational e?ects. There is also extensive circumstantial evidence that at least some of this dark matter is nonbaryonic: that is, composed of elementary particles other than protons, neutrons, and electrons. These particles must have survived from the Big Bang, and therefore must either be stable or have lifetimes in excess of the current age of the universe. The abundance of dark matter is usually quoted in terms of its mass density ρdm in units of the critical density, ?dm = ρdm/ρc;

the critical density ρc is de?ned in Eq. (15.5) (in Section

15 on Big-Bang Cosmology in this Review). The total amount of visible matter (that is, matter whose existence is inferred from its emission or absorption of photons) is roughly ?vis 0.005, with an uncertainty of at least a factor of two. The strongest evidence for dark matter is from the rotation curves of spiral galaxies [1,2]. In these observations, the circular velocity vc of hydrogen clouds surrounding the galaxy is measured (via Doppler shift) as a function of radius r. If there were no dark matter, at large r we would ?nd v2 c GN Mvis/r, since the visible mass Mvis of a spiral galaxy is concentrated at its center. However, observations of many spiral galaxies instead ?nd a velocity vc which is independent of r at large r, with a typical value vc ?

200 km s?1. Such a ?at rotation curve implies that the total mass within radius r grows linearly with r, Mtot(r) G?1 N v2 c r. A self-gravitating ball of ideal gas at a uniform temperature of kT =

1 2 mdmv2 c would have this mass pro?le;

here mdm is the mass of one dark matter particle. The rotation curves are measured out to some tens of kiloparsecs, implying a total mass within this radius which is typically about ten times the visible mass. This would imply ?dm &

10 ?vis 0.05. In our own galaxy, estimates of the local density of dark matter typically give ρdm 0.3 GeV cm?3, but this result depends sensitively on how the halo of dark matter is modeled. Other indications of the presence of dark matter come from observations of the motion of galaxies and hot gas in clusters of galaxies [3]. The overall result is that ?dm ? 0.2. Studies of large-scale velocity ?elds result in ?dm &

0.3 [4]. However, these methods of determining ?dm require some astrophysical assumptions about how galaxies form. None of these observations give us any direct indication of the nature of the dark matter. If it is baryonic, the forms it can take are severely restricted, since most forms of ordinary matter readily emit and absorb photons in at least one observable frequency band [5]. Possible exceptions include remnants (white dwarfs, neutron stars, black holes) of an early generation of massive stars, or smaller objects which never initiated nuclear burning (and would therefore have masses less than about 0.1 M ). These massive compact halo objects are collectively called machos. Results from one of the ongoing searches for machos via gravitational lensing e?ects [6] indicate that a signi?cant fraction (roughly 20% to 60%, depending on the details of the model of the galaxy which is assumed) of the mass of our galaxy'

s halo is composed of machos. CITATION: C. Caso et al., European Physical Journal C3,