The classical cosmological tests applied to world models with pressure

We apply the three classical tests of observational cosmology to world models with pressure and demonstrate that they are degenerate with respect to the pressure parameter α0 in the range 0-1. This degeneracy is due to the fact that for all redshifts, z, less than ≅ 2 the results of the test for a given value of the deceleration parameter, q0, are almost identical for models with different values of α0, the differences being very small as compared to observational uncertainties. Only at very high redshifts (z ≥ 5) are there appreciable differences between models with different α0 and observationally relevant values of q0. These tests can therefore not be used at present to determine α0. (author)