Evolution and classification of cosmological perturbations

The evolution of perturbations in a Friedmann universe is investigated by using a new method. In a first step fluctuations of the density, the pressure, and the four-velocity are removed by performing a gauge transformation. Subsequently the problem can be solved and that solution is finally transformed into some important gauges. With the help of this method the evolutions of all possible perturbations are classified; this leads to a physical interpretation of the obtained solutions. The complete solution of the density fluctuation in the de Donder gauge is given here for the first time; the result obtained by Rose, Rahmstorf, and Dehnen is not valid in the case of a really perturbed universe.