Towards an accurate determination of the age of the Universe

In the past 40 years a considerable effort has been focused in determining the age of the Universe at zero redshift using several stellar clocks. In this review I will describe the best theoretical methods to determine the age of the oldest Galactic Globular Clusters (GC). I will also argue that a more accurate age determination may come from passively evolving high-redshift ellipticals. In particular, I will review two new methods to determine the age of GC. These two methods are more accurate than the classical isochrone fitting technique. The first method is based on the morphology of the horizontal branch and is independent of the distance modulus of the globular cluster. The second method uses a careful binning of the stellar luminosity function which determines simultaneously the distance and age of the GC. It is found that the oldest GCs have an age of $13.5 \pm 2$ Gyr. The absolute minimum age for the oldest GCs is 10.5 Gyr and the maximum is 16.0 Gyr (with 99% confidence). Therefore, an Einstein-De Sitter Universe ($\Omega=1$) is not totally ruled out if the Hubble constant is about $65 \pm 10$ km s$^{-1}$ Mpc$^{-1}$. On the other hand, the newly discovered red elliptical 53W069 ($z=1.43$) provides an stronger constraint since its minimun age is 3.2 Gyr, thus ruling out an Einstein-De Sitter Universe unless the Hubble constant is $\leq 45 \pm 10$ km s$^{-1}$. Using 53W069 we find an age at $z=0$ of $13 \pm 2$ Gyr, in excellent agreement with the GC determination.