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PostPosted: Sat Apr 03, 2010 11:29 am 
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By this I mean the distribution galaxies as they are now.

The position of galaxies are recorded in light years - that is where they were when their light left them. In the time it takes the light to reach earth the galaxies have receded even further.

Is there a map of the universe that shows their distances now rather than then. If not how can we apply the Hubble law to calculate the now distances?

Martin


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PostPosted: Sat Apr 03, 2010 6:55 pm 
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But how can we know "where they are" now. We know they are moving away from us and have been for quite a long time but we don't know if the Hubble constant has actually been constant or is changing. We have estimates of its current value but no knowledge as to wether it has been that same or has been changing over time.

And then there are the galaxies outside the Hubble Sphere ...

I had the impression that, whilst you can measure/estimate the distances to distant galaxies, those estimates are based on the estimate of the Hubble Constant. I thought that more distant objects tend to have their distances quoted in terms of their red shift (z) - being the more direct measurement.

Ian


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PostPosted: Mon Apr 05, 2010 5:39 am 
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Ok for the sake of the ecercise lets assume the Hubble Constant (HC) is constant at 72 kps/Mpc.

Here is my logic:

Galaxy at 100 Mpc (326 LY)

Recession velocity is 100 * 72 = 7,200 kps

in 360 years it has receded

360 * 31.5e6 (sec in year) * 7200 = 8.165e13 kms

or 8.165e13 / 9.45e12km (1LY) = 8.6 LY

Now I assume this extra distance is in space-time not absolute space.

I would need to reiterate for a slightly larger HC at the new distance?

Would that be right?

Martin


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PostPosted: Mon Apr 05, 2010 7:07 am 
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Would that be right?

"Galaxy at 100 Mpc (326 LY)".

100 Mpc is 326,000,000 light years.


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PostPosted: Mon Apr 05, 2010 11:10 am 
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Perhaps there would be too much guesswork involved, especially with very distant galaxies. The more distant, the longer the lookback and therefore we see them early in their evolution. We wouldn't be able to predict collisions and mergers. I might be wrong but I seem to remember seeing some diagrams that show future positions of the Local Group. But even then there is still uncertainty about the transverse motion.

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