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question about the red shift and the velosity of the galaxies



 
 
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  #1  
Old September 25th 04, 09:23 AM
andreas
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Default 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  
Old September 25th 04, 10:55 AM
John Carruthers
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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/



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  #3  
Old September 25th 04, 05:01 PM
Mike Williams
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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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