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The Hubble Constant
Jeremy Mould, PASA, 17 (1), 45.

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The age of the Universe

The age of the first globular clusters to form in the Milky Way has been reviewed by Mould (1998). The error budget, like that for the Hubble Constant, is a list of daunting length. The distances of the clusters, their chemical composition, the photometry of their main sequence turnoff stars, their reddening, and the effect of undetected binary stars all contribute to the observational uncertainty in determining where the stars leave the main sequence. Stellar structure models transform this measurement into an estimate of how long such stars take to reach this evolutionary state. The treatment of convective energy transport and the sedimentation of helium in such old stars are unconstrained by observations and must be included in the uncertainties too.

The most recent results are by Carretta etal.(1999)

$\begin{displaymath}t_0 = (13.2 \pm 2.9) \times 10^9 ~{\rm years} \qquad {\rm 95\% ~confidence~ limits} \qquad \eqno (2)\end{displaymath}$

This is consistent with all other work stemming from post-Hipparcos definition of the subdwarf main sequence and its fitting to the main sequences of globular clusters to measure their distances (Reid 1997; Chaboyer etal.1998; Gratton etal.1998; Pont etal.1998). Carretta etal.take the extra step of checking that the RR Lyrae distance of the Large Magellanic Cloud which follows from this, is consistent with the LMC distance assumed in the Key Project. In all this work the first globular clusters are taken to be those formed in the chemically most pristine epoch of the Galaxy.

Globular cluster ages are a lower bound on the age of the Universe. How close a bound is this ? How early did globular cluster formation begin ? The earliest conceivable time is 10⁶ years after the Big Bang, when the Jeans mass had fallen to 10⁶ M $_\odot$ . By 10⁷ years the intergalactic medium had reached a temperate 300K. Quite possibly the Universe waited 10⁸ years, which is a dynamical time for galaxies. The first observational constraint one can cite is the work of Pahre (1997) who found z_f > 5 for the major star formation epoch in elliptical galaxies in his study of the evolution of the Fundamental Plane. This corresponds to a time of formation, t_f < 1 x 10⁹ years in typical cosmologies.

Next Section: Constraints on Decelerating Cosmologies
Title/Abstract Page: The Hubble Constant
Previous Section: The results of the
Contents Page: Volume 17, Number 1

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