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1. Dark Matter on Different Distance Scales

1.1. Cosmological Density Parameters

As we have heard at this meeting, there is abundant evidence for the dominance of dark matter and dark energy on the largest distance scales. The cosmological microwave background (CMB) radiation [1] tells us that the total energy density of the Universe, Omegatot, is very close to the critical value marking the boundary between open and closed universes [2]. This information is provided, in particular, by the value of the multipole ell ~ 210 at which the first acoustic peak appears in the CMB, as seen in Fig. 1. This tells us, in effect, the relative sizes of the Universe today and when the nuclei and free electrons in the primordial plasma combined to form neutral atoms. There are now indications for a second and even a third acoustic peak in the CMB at higher ell [1], but these are not yet securely established. However, the magnitudes of the fluctuations deltaT / T at these larger values of ell already tell us that the overall baryon density Omegab << 1, agreeing to within ~ 50 % with the value estimated on the basis of Big Bang nucleosynthesis calculations. Other information about the large-scale geometry of the Universe for redshifts z ltapprox 1 is provided by data on high-z supernovae [3], which constrain a combination of the matter density Omegam and the vacuum energy density OmegaLambda. Combining the CMB and high-z supernova data, one finds fairly accurate values for the cosmological density parameters [2]:

Equation 1 (1)

where h is the present-day Hubble expansion rate in units of 100 km/s/Mpc, consistent with the `concordance model': Omegatot ~ 1, OmegaLambda / Omegam ~ 2 rightarrow 3 with Omegab small, as seen in Fig. 2. These data may also be used to constrain neutrino degeneracy [5].

Figure 1

Figure 1. Recent compilation of data on the cosmic microwave background (CMB), exhibiting clearly the first acoustic peak, whose location fixes Omegatot, and the possible second and third peaks at larger ell [1].

Figure 2

Figure 2. The combination of cosmological data favours the `concordance' model with Omegatot ~ 1, OmegaLambda / Omegam ~ 2 rightarrow 3 and Omegab small, in agreement with cosmological nucleosynthesis [4].

There are excellent prospects for significant progress in improving the CMB constraints using data from the MAP and Planck satellites [6]. One of the open issues concerns the amount of information likely to be obtained at large ell [7], particularly from polarization measurements [6]. These must contend with weak lensing effects that must be subtracted in order to extract useful cosmological information.

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