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2.3. Growth mechanism

Primordial fluctuations grow by gravitational instability: overdense fluctuations expand linearly, at a retarded rate relative to the Universe as a whole, until eventually they reach a maximum size and collapse non-linearly to form an equilibrium (or `virialized') object whose radius is approximately half the physical size of the perturbation at maximum expansion. The theory of fluctuation growth is lucidly explained by Peebles (1980).

Although gravitational instability is now widely accepted as the primary growth mechanism responsible for the formation of structure, it is only very recently that firm empirical evidence for this process was found. Gravitational instability causes inflow of material around overdense regions. From the perspective of a distant observer, this flow gives rise to a characteristic infall pattern which is, in principle, measurable in a galaxy redshift survey by comparing the two-point galaxy correlation function along and perpendicular to the line-of-sight. In this space, the infall pattern resembles a butterfly (Kaiser 1987). This pattern has been clearly seen for the first time in the 2dFGRS (Peacock et al. 2001) (3)

2.4. Cosmological parameters

After decades of debate, the values of the fundamental cosmological parameters are finally being measured with some degree of precision. The main reason for this is the accurate measurement of the acoustic peaks in the CMB temperature anisotropy spectrum whose location, height and shape depend on the values of the cosmological parameters. Some parameter degeneracies exist but some of these can be broken using other data, for example, the distant Type Ia supernovae or the 2dFGRS (eg. Efstathiou et al. 2002). The CMB data alone do not constrain the Hubble constant, but there is a growing consensus from the HST key project (Freedman et al. 2001), and other methods, that its value, in units of 100 km s-1 Mpc,-1 is h = 0.70±0.07. In addition to h and the other parameters listed in Table 1, the other important number in studies of large-scale structure is the amplitude of primordial density fluctuations which is usually parametrized by the quantity sigma8 (the linearly extrapolated value of the top-hat filtered fluctuation amplitude on the fiducial scale of 8 h-1 Mpc). The best estimate of this quantity comes from the observed abundance of rich galaxy clusters which gives sigma8 Omega0.6 = 0.5, with an uncertainty of about 10% (Eke, Cole & Frenk 1996, Viana & Liddle 1996, Pierpaoli et al. 2001).

3 Strictly speaking the `butterfly' pattern does not prove the existence of infall since the continuity equation would ensure a similar pattern even if velocities were induced by non-gravitational processes. However, it can be shown that such velocities, if present, would rapidly decay. Back.

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