Dark matter: theoretical perspectives.

Abstract

I both review and make the case for the current theoretical prejudice: a flat Universe whose dominant constituent is nonbaryonic dark matter, emphasizing that this is still a prejudice and not yetfact. The theoretical motivation for nonbaryonic dark matter is discussed in the context of current elementary-particle theory, stressing that (i) there are no dark-matter candidates within the "standard model" of particle physics, (ii) there are several compelling candidates within attractive extensions of the standard model of particle physics, and (ii) the motivation for these compelling candidates comes first and foremost from particle physics. The dark-matter problem is now a pressing issue in both cosmology and particle physics, and the detection of particle dark matter would provide evidence for "new physics." The compelling candidates are a very light axion (10-6-10-4 eV), a light neutrino (20-90 eV), and a heavy neutralino (10 GeV-2 TeV). The production of these particles in the early Universe and the prospects for their detection are also discussed. I briefly mention more exotic possibilities for the dark matter, including a nonzero cosmological constant, superheavy magnetic monopoles, and decaying neutrinos. OverviewOne of the simplest yet most fundamental questions we can ask in cosmology concerns the quantity and composition of the matter in the Universe: What is mass density, QO, expressed as a fraction of the critical density, and what are the contributions of the various constituents-e.g., baryons, photons, and whatever else? [The critical density PCRIT = 3H0/8irG = 1.88h2 x 10-29 g-cm3 = 1.05 x 104 eV-cm3, where Ho = 100h km sec-1 Mpc-1; 1 eV = 1.602 x 10-19 J; 1 megaparsec (Mpc) = 3.09 x 1022 m.] The answer to this question bears upon almost every topic discussed at this colloquium: the expansion age and fate of the Universe; the origin of structure in the Universe and cosmic background radiation (CBR) anisotropies; galactic disks, rotation curves, and morphology; cluster dynamics; gravitational lensing; and the distribution of light and mass. The only thing we know with great precision is the contribution of photons, nZ = 2.49h-2 X 10-4 (assuming Tyo = 2.73 K), and neutrinos, Ql, = 1.70h-2 X 10-4 (assuming all three species are massless); and based on primordial nucleosynthesis, we know the contribution of baryons to within a factor of two, nBh2 = 0.01-0.02 (see, e.g., refs.

The Helium-Core Mass at the Helium Flash in Low-Mass Red Giant Stars: Observations and Theory

“…Several authors have also noted (e.g., Krauss 1990;Turner 1993) that the expected annihilation cross sections lead to a WIMP abundance that is simply too low (both within the sun and in the Universe as a whole) for the most \interesting" cosmion candidates, representing another extremely serious challenge to the cosmion hypothesis. In fact, recent developments do not even o er compelling evidence for the presence of signi cant amounts of non-baryonic dark matter in the scale of galactic halos, as reviewed by Turner (1993) and Schramm (1994, particularly his Fig. 5).…”

Big-Bang Nucleosynthesis and the Baryon Density of the Universe

“…Further, there is now indirect evidence for the existence of dark stars, known as MACHOs for Massive Astrophysical Compact Halo Objects, through their gravitational microlensing of distant stars [18]. Second, there is strong|though not yet conclusive|evidence that the average mass density of the Universe is signicantly greater than 14% of the critical density [19]; if this is indeed the case, most of the mass density of the Universe must be \nonbaryonic," with the most promising possibility being elementary particles left over from the earliest moments of the Universe [20]. Large-scale experiments are underway in laboratories all over the world to directly detect the nonbaryonic dark matter associated with the halo of our own galaxy [21].…”

The physical basis of subtle bodies and near-death experiences.

“…The first scientific evidence for the existence of "dark matter" came when Fritz Zwicky (1933) showed that there wasn't nearly enough mass associated with visible light for gravity to hold the galaxies together in a region of space called the Virgo Cluster. This discovery was overlooked until the 1980s, when scientists, probing the universe with dedicated ground observatories and new instruments in space, discovered evidence of "dark matter" wherever they looked in the uni verse (Turner, 1993).…”

“…Exploration of this phenomenon has proceeded as a multidiscipli nary effort involving physical cosmologists, particle physicists, astro physicists, plasma physicists, and others, pursuing the detection of particle "dark matter" with the expectation that their search will re veal which model of particle physics is the most accurate: the Stand ard Model or some of its extensions, such as the Peccei-Quinn Symmetry, Supersymmetry, Technicolor, or Superstrings model. Each of these models proposes a different candidate for the identity of the "dark matter" particle, such as a very light axion, a light neutrino, and a heavy neutralino (Turner, 1993); however, it appears likely we need to introduce totally new candidates.…”