Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.03. Where is the center of the Universe? Next Document: I.05. If the Universe is expanding, what about me? or the Earth? or the Solar System? See reader questions & answers on this topic!  Help others by sharing your knowledge These different descriptions concern the future of the Universe, particularly whether it will continue to expand forever. The future of the Universe hinges upon its densitythe denser the Universe is, the more powerful gravity is. If the Universe is sufficiently dense, at some point in the (distant) future, the Universe will cease to expand and begin to contract. This is termed a "closed" Universe. In this case the Universe is also finite in size, though unbounded. (Its geometry is, in fact, similar to the *surface* of a sphere. One can walk an infinite distance on a sphere's surface, yet the surface of a sphere clearly has a finite area.) If the Universe is not sufficiently dense, then the expansion will continue forever. This is termed an "open" Universe. One often hears that such a Universe is also infinite in spatial extent. This is possibly true; recent research suggests that it may be possible for the Universe to have a finite volume, yet expand forever. One can also imagine a Universe in which gravity and the expansion are exactly equal. The Universe stops expanding only after an infinite amount of time. This Universe is also (possibly) infinite in spatial extent and is termed a "flat" Universe, because the sum of the interior angles of a triangle sum to 180 degreesjust like in the plane or "flat" geometry one learns in (US) high school. For an open Universe, the geometry is negatively curved so that the sum of the interior angles of a triangle is less than 180 degrees; in a closed Universe, the geometry is positively curved and the sum of the interior angles of a triangle is more than 180 degrees. The critical density that separates an open Universe from a closed Universe is 1.0E29 g/cm^3. (This is an average density; there are clearly places in the Universe more dense than this, e.g., you, the reader with a density of about 1 g/cm^3, but this density is to be interpreted as the density if all matter were spread uniformly throughout the Universe.) Current observational data are able to account for about 1030% of this value, suggesting that the Universe is open. However, motivated by inflationary theory, many theorists predict that the actual density in the Universe is essentially equal to the critical density and that observers have not yet found all of the matter in the Universe. User Contributions:Comment about this article, ask questions, or add new information about this topic:Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.03. Where is the center of the Universe? Next Document: I.05. If the Universe is expanding, what about me? or the Earth? or the Solar System? 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
