Stars as Laboratories for Fundamental Physics - MPP Theory Group

Miscellaneous Exotica 553
Fig. 15.1. Required present-day ĠN/G N in order to achieve a true globularcluster
age τ, given that the apparent age is τ ∗ . A linear G N (t) variation as
in Eq. (15.4) was assumed.
can relate a desired true age τ to a required value for Γ 0 (Fig. 15.1).
Conversely, it is probably safe to assume that the true ages of globular
clusters do not exceed 20 Gyr. Then Fig. 15.1 implies that today
Ġ N /G N ∼ < 7×10 −12 yr −1 . (15.6)
A lower age limit is less certain. Taking 8 Gyr one finds Ġ N /G N ∼ >
−35×10 −12 yr −1 which is less certain, and also less interesting relative
to the limits discussed in the previous sections.
Fig. 15.2. Summary of limits on the present-day ĠN/G N . The big-bang
nucleosynthesis limit is not shown as it depends sensitively on the assumed
G N (t) variation at early cosmic times.