Old Galaxies at High Redshift and the Cosmological Constant
  In a recent striking discovery, Dunlop {\bf \it et al} observed a galaxy at redshift z=1.55 with an estimated age of 3.5 Gyr. This is incompatible with age estimates for a flat matter dominated universe unless the Hubble constant is less than $ 45 kms^{-1}Mpc^{-1}$. While both an open universe, and a universe with a cosmological constant alleviate this problem, I argue here that this result favors a non-zero cosmological constant, especially when considered in light of other cosmological constraints. In the first place, for the favored range of matter densities, this constraint is more stringent than the globular cluster age constraint, which already favors a non-zero cosmological constant. Moreover, the age-redshift relation for redshifts of order unity implies that the ratio between the age associated with redshift 1.55 and the present age is also generally larger for a cosmological constant dominated universe than for an open universe. In addition, structure formation is generally suppressed in low density cosmologies, arguing against early galaxy formation. The additional constraints imposed by the new observation on the parameter space of $h$ vs $\Omega_{matter}$ (where $H= 100 h kms^{-1}Mpc^{-1}$) are derived for both cosmologies. For a cosmological constant dominated universe this constraint is consistent with the range allowed by other cosmological constraints, which also favor a non-zero value. 
Lawrence M. Krauss (Case Western Reserve University)