| From: matt | 24/07/99
16:28:23 |
| Subject: The expanding universe | post id:
26014 |
| Dear Dr Karl, I have often heard the expanding balloon analogy to describe the expanding universe. I have also heard that if you stick pieces of paper on the balloon, then they can represent the galaxies. In my mind, this model poses three questions: 1. Does this suggest that the galaxies themselves don't expand, only the balloon. If this is true, then what is so special about galaxies that stops them expanding. 2. If this is not true, then presumably my living room is expanding as well. While I am comfortable that this can occur at any rate unnoticed, does the wavelength of light grow in proportion. If it doesn't, then shouldn't it appear to be reducing? 3. If the wavelength of light does increase with the expansion of the universe, then how can a red shift occur? Wouldn't the wavelength of star light simply expand into the more stretched space then contract as it reaches us? I hope you can answer this question more simply than I have posed it!! Thanks Dr Karl | |
| From: James Richmond (Avatar) | 25/07/99
13:07:01 |
| Subject: re: The expanding universe | post id:
26083 |
| The expansion of the universe
happens on every scale. However, on scales smaller than extragalactic
distances, the general expansion effect is swamped by local effects, such
as the individual gravitational attractions between stars in any
particular galaxy. Therefore, although galaxies (or more often groups of
galaxies) are observed to move apart from each other over time, the stars
and planets in those galaxies do not. Thus, we do not see a net recession
of the stars in our galaxy; nor do we see the Magellanic Clouds receding
from the Milky Way. On a microscopic level, the space inside every atom should also be participating in the general expansion of the universe. However, electromagnetic and nuclear forces act against the spatial expansion, holding atoms together. In other words, when an electron moves a little farther from an associated atomic nucleus due to the expansion of space, the electromagnetic force between the electron and the nucleus pulls it back to its equilibrium position. Obviously, if everything expanded at the same rate, including the spaces within atoms, we would not be able to observe the expansion. Your living room resists the expansion of the universe mostly via electromagnetic forces. The wavelength of light emitted from distant, receding galaxies is red shifted as a result of the expansion of the universe. One way to think of this is that the wavelength gets "stretched out" as the space the light is travelling through expands. There is nothing to "contract" the wavelength again before it reaches us, so we see a net red shift. JR | |