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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 (Omegam = 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 Omegam = 1, but rather Omegatot = Omegam + OmegaLambda = 1 with Omegam approx 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 sigma8, which measures the amplitude of the (linear) power spectrum on the scale of 8 h-1 Mpc. However, sigma8 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 sigma8 between about 0.7 and 1.1. But unless sigma8 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 alpha forest observed in the SDSS quasar spectra along with the first year WMAP data and the SDSS galaxy clustering data, finds sigma8 = 0.90 ± 0.03 and Omegalambda = 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 ident P / rho = - 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 Sigma mnu < 0.42 eV. However, the tiny quoted errors do not include systematic uncertainties in the interpretation of the Lyman alpha forest data, which require further analysis.

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