The Square Kilometer Array: new challenges for cosmology

The cosmological case for a next generation radio observatory, the Square Kilometer Array, is discussed and reviewed. An instrument like the SKA would be able to measure galaxy redshifts of normal late-type galaxies, via the 21 cm line of HI, out to redshifts of $\sim 3$. Not only would such very deep redshift surveys enable us to map the large scale galaxy distribution and probe the large scale structure of the universe out to previously unexplored scales, it would also allow for the first time to obtain direct observational data on the evolution of this structure. Other promising applications concern the mapping of the local velocity field of the universe, study of the formation and evolution of galaxies, and determining the global cosmological parameters $H_0$, $q_0$ and $\Lambda$ through the application of classical cosmological tests like source counts. Particularly emphasized is the redshift survey capability of the SKA. A review is given of the current knowledge of the galaxy distribution, starting from an inventarisation of nearby cosmic structures, through a discussion of how it all fits together in a coherent ``foamlike pattern''. After providing a short overview of the basics of theories of structure formation, a description is provided of different observational strategies to probe the structure of the universe out to larger depths, ranging from pencil-beam surveys and cluster surveys out to the new and ambitious complete and deep galaxy redshift surveys like the 2dF and the Sloan survey. It is argued that a survey with the SKA would be a natural and complementary follow-up. We finally conclude with a specification of the technical requirements for the SKA to make it into an instrument ideally suited for these purposes.