Model-independent insights into the nature of the Lyα forest and the distribution of matter in the universe

Straightforward physical arguments are used to derive the properties of Ly alpha forest absorbers. It is shown that many aspects of the current physical picture of the forest, in particular the fact that the absorption arises in extended structures of moderate overdensities that contain a large fraction of the baryons in the universe, can be derived directly from the observations without making any specific assumptions about the presence and distribution of dark matter, the values of the cosmological parameters, or the mechanism for structure formation. The key argument is that along any line of sight intersecting a gravitationally confined gas cloud, the size of the region over which the density is of order the maximum density, is typically of order the local Jeans length. This is true for overdense absorbers, regardless of the shape of the cloud and regardless of whether the cloud as a whole is in dynamical equilibrium. The simple analytic model is used to derive the mass distribution of the photoionized gas directly from the observed column density distribution. It is demonstrated that the shape of the column density distribution, in particular the observed deviations from a single power law, and its evolution with redshift, reflect the shape of the matter distribution and can be understood in terms of the growth of structure via gravitational instability in an expanding universe.