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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.04. What do people mean by an "open," "flat," or "closed" Universe? Next Document: I.06. What is inflation? See reader questions & answers on this topic! - Help others by sharing your knowledge You, the reader, are not expanding, even though the Universe in which you live is. There are two ways to understand this. The simple way to understand the reason you're not expanding is that you are held together by electromagnetic forces. These electromagnetic forces are strong enough to overpower the expansion of the Universe. So you do not expand. Similarly, the Earth is held together by a combination of electromagnetic and gravitational forces, which again are strong enough to overpower the Universe's expansion. On even larger scales---those of the Solar System, the Milky Way, even the Local Supercluster of galaxies (also known as the Virgo Supercluster)---gravity alone is still strong enough hold these objects together and prevent the expansion. Only on the very largest scales does gravity become weak enough that the expansion can win (though, if there's enough gravity in the Universe, the expansion will eventually be halted). A second way to understand this is to appreciate the assumption of homogeneity. A key assumption of the Big Bang is that the Universe is homogeneous or relatively uniform. Only on large enough scales will the Universe be sufficiently uniform that the expansion occurs. You, the reader, are clearly not uniform---inside your body the density is about that of water, outside is air. Similarly, the Earth and its surroundings are not of uniform density, nor for the Solar System or the Milky Way. This latter way of looking at the expansion of the Universe is similar to common assumptions in modelling air or water (or other fluids). In order to describe air flowing over an airplane wing or water flowing through a pipe, it is generally not necessary to consider air or water to consist of molecules. Of course, on very small scales, this assumption breaks down, and one must consider air or water to consist of molecules. In a similar manner, galaxies are often described as the "atoms" of the Universe---on small scales, they are important, but to describe the Universe as a whole, it is not necessary to consider it as being composed of galaxies. Also note that the definitions of length and time are not changing in the standard model. The second is still 9192631770 cycles of a Cesium atomic clock and the meter is still the distance light travels in 9192631770/299792458 cycles of a Cesium atomic clock. User Contributions:Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.04. What do people mean by an "open," "flat," or "closed" Universe? Next Document: I.06. What is inflation? 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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