Globular Cluster Ages and Cosmology

Abstract

The age of a Λ = 0, Friedmann universe is given in Table 1, for various combinations of the Hubble constant H _o and the density parameter Ω_o. In turn, it is usual to assume the present age of the universe as given by its age at the epoch of galaxy formation, plus the age of globular clusters, i.e.:

$$ {t_o} = t({z_{GF}}) + {t_{GC}},$$

(1)

where z _GF is the redshift of the epoch of galaxy formation. Preasumably t(z _GF)≪ t _GC [say, t(Z_GF) ≃ 1– 2 Gyr], so t _o ≃ t _GC. Unlike for the case of H _o (for which well known discrepancies exist in current estimates), virtually every practicioner in the field of cluster dating would agree that

$${t_{GC}} = (13 - 15) \pm 3Gyr$$

(2)

represents a fair estimate of the age of the oldest clusters in the Galaxy. The agreement follows from everybody using basically the same input physics in stellar model calculations and the same observational data, and as such it does not really ensure that GC ages have been correctly estimated. In any event, this determination is marginally consistent with the standard inflationary scenario (Ω_o = 1, Λ = 0) if H _o = 50, while it would clearly require Λ ≠ 0 if H _o = 100. Although the possibility of living in a Λ ≠ 0 universe is now more widely entertained than a few years ago (see Weinberg 1989, for a recent extensive discussion), as other lines of evidence may now suggest Λ ≠ 0, still the obvious questions to answer are “How good are GC age determinations?”, and also “Do GC ages set tight limits on the other cosmological parameters?”