If Ω tot = 1 and structure formed from adiabatic initial conditions, then the age of the universe,
as constrained by measurements of the cosmic microwave background (CMB), is t 0 = 14.0 ± 0.5 Gyr.
The uncertainty is surprisingly small given that CMB data alone do not significantly constrain
either h or Ω Λ . This small uncertainty is due to the tight (and accidental) correlation in flat
adiabatic models of the age with the angle subtended by the sound horizon on the last-scattering
surface and, thus, with the well-determined acoustic peak locations. If we assume either the Hubble
Space Telescope Key Project result h = 0.72 ± 0.08 or simply that h > 0.55, we find Ω Λ > 0.4 at 95%
confidence—another argument for dark energy, independent of supernovae observations. Our analysis is
greatly simplified by the Monte Carlo Markov chain approach to Bayesian inference combined with a
fast method for calculating angular power spectra.
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