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Two expanding universe questions.....
Can anybody give me the answers to these two questions, please
1. Red shifts are used to decide how far away a distant object is. It's difficult to imagine relative to what but these measurements refer to how fast the object is moving away from us, the observer. But, surely our galaxy is also affected by the expansion of the universe so the red shift of a distant galaxy is due to it's own relative motion PLUS our Milky Way motion. So, how do we know what proportion of shift to use to calculate how far away the distant object is? 2. As I understand it, the more distant an object is the faster the expansion of the the universe that it is in. Thus, some galaxies must be approaching the speed of light. When that velocity is close to 99% of light speed then Einstein's relativity ideas come into play. Time slows down and mass increases towards infinity. So, what becomes of red shift measurements then? Keep it simple please - I am but a simple man! |
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Two expanding universe questions.....
Pete L wrote:
Can anybody give me the answers to these two questions, please 1. Red shifts are used to decide how far away a distant object is. It's difficult to imagine relative to what but these measurements refer to how fast the object is moving away from us, the observer. So far so good. But, surely our galaxy is also affected by the expansion of the universe so the red shift of a distant galaxy is due to it's own relative motion PLUS our Milky Way motion. So, how do we know what proportion of shift to use to calculate how far away the distant object is? Actually we can determine the local peculiar motion of our galaxy by reference to the 4K microwave background surface of last scattering. There is a detectable dipole moment that is due to Doppler shift from our motion relative to the mean isotropic expansion. The Andromeda galaxy is one rare example of a blue shifted galaxy - it is headed towards us on a roughly collision course. Don't worry though it will take a very long time to get here. 2. As I understand it, the more distant an object is the faster the expansion of the the universe that it is in. Thus, some galaxies must be approaching the speed of light. When that velocity is close to 99% of light speed then Einstein's relativity ideas come into play. Time slows down and mass increases towards infinity. So, what becomes of red shift measurements then? Keep it simple please - I am but a simple man! The second is a much harder question to give a simple answer to. On cosmological distance scales the Einstein GR field equations come into play and the definition of the horizon needs careful consideration. There is a zone of the (presumed infinite) universe that is already moving away from us so fast due to expansion of the intervening space that we will never see any photons from it. In practice we can only see back to the point at which the universe became transparent to electromagnetic radiation. It is probably a bit heavier going than you asked for but the following isn't a bad introduction and has most of the right pictures in: http://www.astro.columbia.edu/~zolta...es/c3602.3.pdf Regards, Martin Brown |
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