LY-ALPHA CLOUDS AT LOW REDSHIFT AND THE COSMOLOGICAL CONSTANT

We point out that the distribution of Ly-alpha-clouds at low redshift (z < 2) is very sensitive to the cosmological constant. Recent spectroscopic observations with the Hubble Space Telescope (HST) towards 3C273 show that the number of Ly-alpha absorption systems found is higher than expected from the numbers at higher redshifts. If evolution is positive, in that it acts to decrease the number density of clouds at lower redshifts, these observations suggest that a universe highly dominated by the cosmological constant is not favoured, although the possibility of a negative evolution cannot be ruled out. We demonstrate these aspects in the framework of the pressure-confined evolutionary model, and emphasize that any discussion of evolution at low redshift should take into account the cosmological model.