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#1
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question about the red shift and the velosity of the galaxies
Hi,
For understanding the red shift of galaxies on great distances one is always comparing this with the doppler effect but we know that this shift is made during the time the light travels in space by the expanding space ( I think so ). With the red shift one knows the velocity and with that the Hubble constant. One says that the Hubble constant is the value for now and could be ( is ) different for the past and for the future. But the light comes from several gyr and has to go in expanding space in the past where the expanding velocity may be different. How can be the red shift and thus the Hubble constant and thus the expanding velocity the value for today ? It is all confusing to me. Thanks for any response. |
#2
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How can be the red shift and thus the Hubble constant and thus the
expanding velocity the value for today ? It is all confusing to me. You need the OU S282 course where cosmological parameters like the Hubble parameter and the acceleration of expansion are discussed. jc -- http://mysite.wanadoo-members.co.uk/jc_atm/ --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.769 / Virus Database: 516 - Release Date: 24/09/2004 |
#3
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Wasn't it andreas who wrote:
Hi, For understanding the red shift of galaxies on great distances one is always comparing this with the doppler effect but we know that this shift is made during the time the light travels in space by the expanding space ( I think so ). Not really. All that matters is the difference in velocity between the object that emitted the light (at the time that it was emitted) and the velocity of the Earth (at the time the light arrives). With the red shift one knows the velocity and with that the Hubble constant. Only if you know the distance. Distances can be tricky to measure accurately for remote objects. Errors in distance estimates give errors in the estimate for the Hubble Constant. One says that the Hubble constant is the value for now and could be ( is ) different for the past and for the future. But the light comes from several gyr and has to go in expanding space in the past where the expanding velocity may be different. How can be the red shift and thus the Hubble constant and thus the expanding velocity the value for today ? The changes in the Hubble Constant are quite small. Until quite recently the errors in our estimates for the Hubble Constant were quite a bit larger than the changes in its value, and even now it's quite rare for there to be a situation where applying the complicated corrections for the variation in the Hubble Constant to be worth the bother. Astronomers are usually quite happy to work directly with redshift values for very remote objects, and not really care very much whether the a galaxies at redshift 5.9 is at 12.5 or 13.0 Giga Light Years. -- Mike Williams Gentleman of Leisure |
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