Jeans instability

Maybe the most annoying problem of the hot Big-Bang scenario is that it does not provide any explanation for the existence of structures in the Universe. It is well-known that structures can form through the Jeans instability only in a matter dominated era. However since the matter domination occurred quite recently in the history of the universe (around z 104, see Eq. (7.29)), one is forced to suppose that small density fluctuations already existed before that epoch. Since no efficient process is known to form density perturbations in a radiation-dominated universe, so one has to suppose that the seeds for the astrophysical objects we observe were part of the initial condition of the whole scenario. As we shall now see, the biggest success of inflation is to provide a simple explanation for the presence of such density perturbations, in addition to solving quite naturally the other problems. 

This redshift can be quite high if wq is close to 0 it varies between 0.33 and 3.1 when wq goes from —1 to —0.2. After the matter-to-quintessence transition, the Jeans instability is no longer efficient. Therefore, structure can no longer grow and the power spectrum is reduced by a factor... 

In the hypothesis of an exponential potential, the situation is even worse because Jeans instability is always reduced by the presence of quintessence (i.e., o ia is never equal to 1). More detailed calculation give the following value of the matter power spectrum normalization as as a function of Wq in Fig. 8.2. 

Jeans, who at first demonstrated the nature of this so called Jeans instability, considered perturbations in a uniform infinite gas. Considering the continuity equation, the equation of motion and the Poisson equation ... 

Jeans instability Instability in a cloud of gas in space due to fluctuations in the density of the gas, causing the matter in the cloud to clump together and lead to gravitational collapse. The conditions under which this occurs were worked out by Sir James Hopwood Jeans (1877-1946) in terms of Newtonian gravity. The analogous analysis of this problem using general relativity theory is the basis of the theory of structure formation. 

Liquid Interfacial Systems Oscillations and Instability, Rudoiph V. Birikh, Vladimir A. Briskman, Manuei G. Veiarde, and Jean-Claude Legros... 

Gravitational instability can occur in a cloud characterised by high density and low temperature (Jean s criterion). More precisely-, the mass of the cloud must be of the order of 20 M0 (where M0 is the solar mass) if the density (expressed as the number of dihydrogen molecules per cubic centimeter) is around 103 and the temperature around 10 K. Such a density is precisely what is observed in so-called dense clouds. These dense clouds are also regions of space where a lot of complex molecules are detected by spectroscopy, and where dust particles are observed. Readers interested in interstellar chemistry will find an excellent review of the subject in the recent book by Duley and Williams 13). 

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