The adiabatic theory of galaxy formation

The predictions of the theory of galaxy formation due to the gravitational condensation of adiabatic density perturbations in an otherwise Friedmann universe are examined in relation to those of standard nonlinear theories. An expression for the density contrast is derived in terms of adiabatic theory, which predicts the appearance of pancakes of anisotropically collapsing matter at a certain stage in the growth of density perturbations, and compared with those derived from the Newtonian nonlinear theory of gravitational instability, and from the general relativistic nonlinear theory of the gravitational instability of an isotropic system. It is shown that the anisotropic collapse of condensations of matter is by no means a general consequence of Newtonian cosmology. Predictions of relativistic and adiabatic theories are found to coincide only at redshifts greater than or equal to 20, at which pancake formation occurs in adiabatic theory.