We discuss briefly the relevance of Lyman \alpha forest observations for measuring cosmological parameters , comparing the properties of high z Keck QSO spectra with those of artifical spectra from hydrodynamic simulations , based on hierarchical cosmologies .
In particular , we describe a measurement of the baryon content of the universe obtained by matching the average opacity of the Lyman \alpha forest from simulations to the observed one from a new dataset observed with the Keck telescope .
For conservative assumptions about the intensity of the UV background we obtain a lower limit \Omega _ { b } h ^ { 2 } > 0.017 .
Searching for column density gradients in absorption systems common to adjacent gravitationally lensed quasar images we test for the presence of sub-kpc clumpiness which could invalidate the results of simulations with limited resolution .
We find that in those Ly \alpha forest clouds dominating the mean absorption , such structure , if present , can not exceed the 4 percent level ( over 100 – 200 pc ) .
Extending the study of lensed absorption to higher column densities we begin to sample the velocity field internal to high redshift galaxies .
Remarkable differences in column density ( of order 50 % ) and projected velocity ( tens of km/s ) between lines of sight separated by only a few hundred parsecs are found , and we may be observing structure in the early interstellar medium .