Astronomers have been debating this issue for decades. We are very confident that the Big Bang theory is the most accurate model we have for the evolution of the universe. To determine just how many years have elapsed since the Big Bang is a difficult observational problem. Astronomers can get a handle on this number by estimating the ages of the oldest stars in the Milky Way galaxy, and by measuring just how rapidly the universe is expanding today.
The oldest stars seem to be found in globular star clusters. The Milky Way has about 130 of these clusters orbiting it just outside its flattened disk. Astronomers who determine the ages of stars and star clusters have repeatedly found that globular cluster stars are between 10 and 16 billion years old. This means that the universe must be at least 16 billion years old. Some very recent models have begun to show, however, that an upper limit closer to 14 billion years for these very old stars may be more in line with the uncertainties in the data.
Other astronomers who carefully measure the expansion rate of the universe are divided into two camps depending on which distance indicators they use. The first camp uses variable stars called Cepheid variables. They often arrive at estimates for the Hubble Constant that are close to 75 kilometers per second per megaparsec. The second camp uses, among other things, distant supernovae. They often get numbers suggesting a much slower expansion rate near 50 kilometers per second per megaparsecs. If you believe the first camp, the age of the universe is not much more than 10 billion years. The second camp gets ages closer to 20 billion years.
At the present time, using Hubble Space Telescope observations of Cepheid variables in Messier-100, the first camp seems to be in the ascendancy which means that the universe is about 10 billion years old. The problem is that this badly conflicts with the estimates given by dating the stars in globular clusters, and such a detailed study of distance has only been carried out by the Hubble Space Telescope for a handful of nearby galaxies for which the cosmological expansion effect is more difficult to observe. Meanwhile, Alan Sandage and his team using distant supernovae are beginning to propose values for the Hubble Constant closer to 60 and 65 kilometers/sec/mpc, which suggests that these two camps may soon converge on a mutually agreeable number, possibly near 65-70 kilometers/sec/mpc, for an age that would be slightly higher than 11 billion years. The gulf between 11 billion years and 14 billion years may just be in the 'error bars'
By 2000, astronomers have discovered from studies of distant supernovae and from studies of the microwave background radiation (see above figure) that there may indeed be a cosmological constant 'force' causing the universe to accelerate its expansion. Also, there must be a significant amount of dark matter (Omega-m) present as well. This changes the age estimates for the universe according to the big bang model. With the addition of the cosmological constant, the age of the universe is comfortably longer than the ages of the oldest stars and near 15 billion years. This estimate will be revised considerably as astronomers continue to get better confirmation of the cosmological constant and its magnitude.
Copyright (C) 1997 Dr. Sten Odenwald
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