Why is it so difficult for current observations to determine the initial state of galaxy clustering and even earlier of galaxy formation? The answer, in aword, is dissipation. Much energy changed as its entropy gained, first as galaxies formed and then as they clustered.

To see the magnitude of this transformation, imagine a cube now a hundred megaparsecs across in a universe with Ω0 = 1 as an example. If the matter in these million cubic megaparsecs had not condensed at all as the universe expanded, if its temperature had decayed adiabatically α R–2 since decoupling so that now T ≈ 3 × 10–3 K, then the total random kinetic energy in this volume would be about 3 × 1056 erg. Gravitational condensation produces dissipation. In gaseous condensation, much of the energy exits as radiation, and some leaves as hot particles. If the dissipation is mostly particulate, as in many-body clustering, escaping orbits carry energy away. The remaining part of the system condenses into a deepening gravitational well and acquires the increased random kinetic energy it needs for quasi-stability. The magnitude of this kinetic energy, K ≈ |W|/2 ≈ –Etotal, provides an estimate of dissipation.