equation of state of dark energy\
    and the Hubble constant from stellar ages and the CMB
---

Introduction
============

There is now solid observational evidence that the universe is accelerating. The absolute ages of high-redshift galaxies [@D+96; @Spinrad+97] and the value of the Hubble constant $H_0$ [@Freedman+01], the Hubble diagram of Type Ia supernovae [@Riess+98; @Perlmutter+99], the angular power spectrum of the cosmic microwave background [CMB; @Benoit+02; @Ruhl+02; @Goldstein+02] in combination with galaxy surveys [@Efstathiou+2dF02; @Verde+2dF02] and lensing [@WTJZ02], all indicate that the universe is flat and dominated at present by some form of dark energy with negative pressure. Strong confirmation for this comes from the newly released results from the [*Wilkinson Microwave Anisotropy Probe*]{} ([*WMAP*]{}) first year data [@SpergelWMAP03].

The equation of state of the dark energy, $p=w \rho$, expresses the ratio between the pressure, $p$, and the mass density, $\rho$, of the dark energy in terms of the parameter $w$ (in units of $c=1$). The value of $w$ could either be constant, as in the case of a cosmological constant ($w=-1$), or be time-dependent, as in the case of a rolling scalar field or “quintessence” [@PeeblesRatra88; @CDS98]. Any such behavior would have far-reaching implications for particle physics.

In 