the old envelope to determine $H_0$. The prospects of using this method to derive a measurement of $w(z)$ are also discussed. Section 4 summarizes and discusses the results.

Throughout the paper we adopt the following notation. The Hubble constant is $H_0=100\,h\,$ km s$^{-1}$ Mpc$^{-1}$ and $c$ is the speed of light. The density of baryons, matter, radiation, photons, neutrinos, and dark energy, at present, in critical units, are indicated as $\Omega_b$, $\Omega_m$, $\Omega_{\rm rad}$, $\Omega_{\gamma}$, $\Omega_{\nu}$, and $\Omega_{\Lambda}$ respectively ($\Omega_{\rm rad}\equiv\Omega_{\gamma}+\Omega_{\nu}$). The physical quantities, independent of $h$, are indicated with $\omega$; e.g., $\omega_m=\Omega_mh^2$. When needed, a standard, flat, reference $\Lambda$CDM model with $\Omega_m=0.27$, $h=0.71$, $\Omega_b h^2=0.023$, and $w=-1$ is assumed [@SpergelWMAP03].

The value of $w$ from stellar ages
==================================

In this section we use the ages of the oldest globular clusters in combination with the location of the first acoustic peak of the CMB power spectrum [@PageWMAP03] to constrain the value of $w$. The method is briefly described in Section 2.1 and the results are given in Section 2.2.

Method
------

If the universe is assumed to be flat, the position of the first acoustic peak ($\ell_1$ in the standard spherical harmonics notation) depends primarily on the age of the 