In the mid-1990s there was a crisis in cosmology, because the age of
the old globular cluster stars in the Milky Way, then estimated to be
16 ± 3 Gyr, was higher than the expansion age of the universe,
which for a critical density
(
m = 1)
universe is 9 ± 2 Gyr (using Hubble parameter
h = 0.72 ± 0.08). But when the data from the
Hipparcos astrometric satellite became available in 1997, it showed
that the distances to the globular clusters had been underestimated,
which (combined with improved stellar evolution models) implied that
their ages are 12 ± 3 Gyr.
The successful Hubble telescope key project on the extragalactic
distance scale determined that the Hubble parameter H0
= 100h km s-1 Mpc-1 is h = 0.72
± 0.08
[8].
Several lines of evidence - including CBR, supernovae, and clusters -
now show that the universe does not have
m = 1,
but rather
tot =
m +
= 1
with m
0.3, which gives an
expansion age of about 14 Gyr. The WMAP cosmic background data alone
give an expansion age of 13.4 ± 0.3 Gyr, which becomes
13.7 ± 0.2 with the WMAP running power spectrum index model
[4].
A new type of age measurement based on radioactive decay of Thorium-232 (half-life 14.1 Gyr) measured in a number of stars gave a completely independent age of 14 ± 3 Gyr. A similar measurement, based on the first detection in a star of Uranium-238 (half-life 4.47 Gyr), gave 12.5 ± 3 Gyr; a second such star gave an age of 14.1 ± 2.5 Gyr [9]. These stellar lifetimes are of course lower limits on the age of the universe.
All the recent measurements of the age of the universe are thus in excellent agreement. It is reassuring that three completely different clocks - stellar evolution, expansion of the universe, and radioactive decay - agree so well.
Ever since the cosmological crisis regarding the age of the universe
was thus resolved, all the data has been consistent with the cosmology
described above, with the main cosmological parameters now all
determined to about 10% or better
[4,
11]
with the sole exception of
8, which
measures the amplitude of the (linear) power
spectrum on the scale of 8 h-1 Mpc. However,
8 is a
crucial cosmological parameter which has a big influence over the
growth of fluctuations in the early universe. The current analyses
lead to values of
8 between
about 0.7 and 1.1. But unless
8 is at least
0.85 or so, it is very hard to see how the
universe could have formed stars and quasars early enough to have
become ionized at z ~ 17
[12]
as indicated by the WMAP detection of large-angle polarization
[13].
The latest analysis of the
cosmological parameters, for the first time including the Lyman
forest observed in the
SDSS quasar spectra along with the
first year WMAP data and the SDSS galaxy clustering data, finds
8 = 0.90
± 0.03 and
= 0.72
± 0.02
[11].
This study finds the primordial spectral index of scalar fluctuations
ns = 0.98 ± 0.02 with no evidence for running of
the spectral index, equation of state parameter
w 
P /
= -
0.98+0.10-0.12
at redshift z = 0.3 with no evidence for variation with redshift,
and a stringent upper limit on neutrino mass
m
< 0.42 eV.
However, the tiny quoted errors do not include systematic
uncertainties in the interpretation of the Lyman
forest data,
which require further analysis.