Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.01. What do we know about the properties of the Universe? Next Document: I.03. Where is the center of the Universe? See reader questions & answers on this topic! - Help others by sharing your knowledge The fundamental properties of the Universe, summarized above, one can develop a simple model for the evolution of the Universe. This model is called the Big Bang. The essential description of the Big Bang model is that it predicts the Universe was hotter and denser in the past. For most of the 20th century, astronomers argued about the best description of the Universe. Was the BB right? or was another model better? Today, most astronomers think that the BB is essentially correct, the Universe was hotter and denser in the past. Why? When Einstein was working on his theory of gravity, around 1915, he was horrified to discover that it predicted the Universe should either be expanding or collapsing. The prevailing scientific view at the time was that the Universe was static, it always had been and always would be. He ended up modifying his theory, introducing a long-range force that cancelled gravity so that his theory would describe a static Universe. When Hubble announced that galaxies were receding from us, astronomers realized quickly that this was consistent with the notion that the Universe is expanding. If you could imagine "running the clock backwards" and looking into the past, you would see galaxies getting closer together. In effect, the Universe would be getting denser. If the Universe was denser in the past, then it was also hotter. At some point in the past, the conditions in the Universe would have resembled the interior of a star. If so, we should expect that nuclear fusion would occur. Detailed predictions of how much nuclear fusion would have occurred in the early Universe were first undertaken by George Gamow and his collaborators. Since then, the calculations have been refined, but the essential result is still the same. After nuclear fusion stopped, about 1000 seconds into the Universe's history, there should be about one Helium-4 atom for every 10 Hydrogen atoms, one Deuterium atom (heavy hydrogen) for every 10,000 H atoms, one Helium-3 atom for every 50,000 H atoms, and one Lithium-7 atom for every 10 billion H atoms. These predicted abundances are in very good agreement with the observed abundances. As the Universe expanded and cooled, the radiation in it should have also lost energy. In 1965 Arlo Penzias and Robert Wilson were annoyed to discover that no matter what direction they pointed a telescope, they kept picking up faint glow. Some physicists at Princeton recognized that this faint glow was exactly what was expected from a cooling Universe. Since then, the COBE satellite has measured the temperature of this radiation to be 2.728 +/- 0.002 K. It is the combination of these excellent agreements between prediction and observation that lead most astronomers to conclude that the Big Bang is a good model for describing the Universe. User Contributions:Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.01. What do we know about the properties of the Universe? Next Document: I.03. Where is the center of the Universe? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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