29:50 Modern Astronomy
Fall 1999
Lecture 31 ...November 8, 1999
Cosmology III

See notes from last time on the history from the Big Bang to the present. Note in particular the ``big events'' at times of 3 minutes after the BB and 700000 years after the BB.

Surely you don't believe such rot! (from Dr. Watson to Sherlock Holmes in ``The Hound of the Baskervilles'')

What observational evidence do we have to support this ``Standard Big Ban Cosmology''? There are four observational results that support this model, and provide the empirical basis for our belief in the Big Bang cosmology.

1. Hubble's Law. This says that the universe is expanding. When augmented with the theory of General Relativity, this says that there really was a time when the radius of the universe was zero. This is sort of a cheat, though, since Hubble's Law was the discovery that got cosmology started.
2. The abundances of the light elements. Big Bang cosmology says that for the first three minutes of the history of the universe, the densities and temperatures were like the core of the Sun everywhere in the universe. At such densities and temperatures, nuclear fusion reactions went on. Hydrogen, the primordial substance, fused to form Helium (both He and He, Deuterium, and Lithium. After three minutes the temperature dropped to the point where fusion reactions did not occur anymore. The total amount of Helium, Deuterium, and Lithium created in this phase depended on the density of the universe at that time. The higher the density, the more atomic nuclei there were to collide with. This effected how much, Lithium, Deuterium, etc, were produced. The density back in the good old days is directly related to the density now, since the amount of matter in the universe is the same, it just has expanded into a large volume. Figure 26-16 of your textbook shows the predictions of theory about the light element abundances versus the density of material in the universe at the present time. The theoretical values are in very good agreement with the measurements, if the density in the universe is a few times kilograms per cubic meter. This is well below the closure density of kg/m , and so confirms our result that the universe is open.
3. The Cosmic Background Radiation. At 700,000 years after the BB matter and radiation ``decoupled''. The blackbody radiation characteristic of that early hot phase was no longer absorbed and emitted by the matter. However, that radiation still exists, and should be a blackbody with a redshift appropriate to an age of 700,000 years after the BB, which corresponds to a redshift of . This radiation was discovered in 1965, and nowadays is extensively studied. At a wavelength of about 1 millimeter, the sky glows with the leftover radiation from the early days of the universe. It is a unique prediction of the Big Bang cosmology.
4. How Old Things Are. If the universe is in the range 12 - 14 billion years old (the age of the universe according to BB cosmology), we had better not see anything older than that. This seems to be the case. Although there have been and continue to be controversies about objects that might be a couple of billion years older than 14 billion years, there are no cases of a really well-established age on an astronomical object which is significantly older than 14 billion years. The fact that there are lots of objects with ages about 10 - 14 billion years is also consistent with the idea that things got started about that time.

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