> Cosmology isn't my field, so sometimes I'm a little out-of-date. After
> the recent Hubble results were announced, I asked my wife about this.
> (Cosmology *is* her field.) She tells me that in the last few years,
> stellar physicists have improved their models and measurements. The
> current best number for the oldest stars is now 12 billion years.
Ah yes! I was out-of-date thinking they were 15 billion years old.
This abstract has come to my attention:
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THE AGE OF GLOBULAR-CLUSTERS IN LIGHT OF HIPPARCOS - RESOLVING
THE AGE PROBLEM
Chaboyer, B., Demarque, P., Kernan, P.J. and Krauss, L.M.
ASTROPHYSICAL JOURNAL, 494(1), 96-110, 1998.
We review five independent techniques that are used to set
the distance scale to globular clusters, including subdwarf
main-sequence fitting utilizing the recent Hipparcos parallax
catalog. These data together all indicate that globular
clusters are farther away than previously believed, implying a
reduction in age estimates. We now adopt a best-fit value
M-epsilon (RR Lyrae stars) = 0.39 +/- 0.08 (statistical) at
[Fe/H] = - 1.9 with an additional uniform systematic
uncertainty of (+ 0.13)(- 0.18). This new distance scale
estimate is combined with a detailed numerical Monte Carlo
study (previously reported by Chaboyer et al.) designed to
assess the uncertainty associated with the theoretical age-
turnoff luminosity relationship in order to estimate both the
absolute age and uncertainty in age of the oldest globular
clusters.
Our best estimate for the mean age of the oldest globular
clusters is now 11.5 +/- 1.3 Gyr, with a one-sided 95 %
confidence level lower limit of 9.5 Gyr. This represents a
systematic shift of over 2 sigma compared to our earlier
estimate, owing completely to the new distance scale - a
shift which we emphasize results not only from the Hipparcos
data. This now provides a lower limit on the age of the
universe that is consistent with either an open universe or
with a flat matter-dominated universe (the latter requiring
H-0 less than or equal to 67 km s(-1) Mpc(-1)). Our new study
also explicitly quantifies how remaining uncertainties in the
distance scale and stellar evolution models translate into
uncertainties in the derived globular cluster ages. Simple
formulae are provided that can be used to update our age
estimate as improved determinations for various quantities
become available. Formulae are also provided that can be used
to derive the age and its uncertainty for a globular cluster,
given the absolute magnitude of the turnoff or the point on
the subgiant branch 0.05 mag redder than the turnoff.
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> The recent measurement of Hubble's constant gives an age of the universe
> of 12 billion if you assume a cosmological constant of zero. But recent
> best measurements give a non-zero value. Plug in the best measured value
> for the cosmological constant, combined with the latest Hubble's constant
> measurement, you get an age of the universe of 13.5 billion years.
If the most authoritative age (without the cosmological constant) is
12 billion years, the figures are still too close for comfort.
Cosmologists do need a non-zero figure for this constant to have a
coherent story.
> Cosmological constant just a "fudge factor" you say? Methinks it should
> sue you for libel. :-)
I'll take that risk!
> I agree with you that we have gaps in our knowledge regarding calibrating
> "molecular clocks" over billions of years and regarding microscopic
> pre-Cambrian fossils. But then, we already knew that before the Wang et.
> al. paper was published.
True. Their work has helped focus the issues.
> Paradigm shifts are fine to watch for, but I'm not holding my breath
> anticipating rockets bumping into a solid firmament anytime soon.
No - but I wonder what form it will take when it does come!
Thanks for your feedback,
Best regards,
David J. Tyler.