Expanding Universe

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Expanding Universe

Postby Trevor.Watson.139829 » Mon Jan 29, 2018 10:24 am

Astrophysicists have indicated that the universe is expanding faster (stretching) and that the further Galaxies are away from our own Galaxy the faster they appear to be moving away. I have also assumed that when we look at Galaxies further away we are in effect looking back in time. So when we look at those distant Galaxies, are we not seeing their speed as it was in the past not as it is now?
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Re: Expanding Universe

Postby dave.b » Tue Jan 30, 2018 10:26 pm

It's the red shift of the light from these galaxies that is being used to determine the rate of expansion of the universe. This light has been stretched along its entire journey to us.
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Re: Expanding Universe

Postby Gfamily2 » Wed Jan 31, 2018 1:28 am

dave.b wrote:It's the red shift of the light from these galaxies that is being used to determine the rate of expansion of the universe. This light has been stretched along its entire journey to us.

That's right. The 'speed we measure' is entirely due to expansion of the universe in the period between the light being emitted and the light falling onto our detectors.

If it would be possible to have a 'God's Eye' view of the entire universe at once when the light set off, you would say that that those galaxies were (more or less) at rest with regard to us when the light was being emitted.
However, because of relativity, no such 'God's Eye, all of the universe at once' view isn't possible.
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Re: Expanding Universe

Postby spartan45 » Thu Feb 01, 2018 9:02 pm

Looking at a distant Galaxy about 13.3 billion (1.33 x10e10) light years from Earth the red shift of its light might give it a moving away speed of say 0.85c. However, that light took 13.3 billion years to get here and during that time the Galaxy has increased speed moving away to approximately 2.0c and now well over twice the 13.3 billion light year distance from Earth. So yes, we are seeing distant Galaxy speed as it was in the past and not as it is now.
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Re: Expanding Universe

Postby Trevor.Watson.139829 » Sat Feb 03, 2018 1:18 pm

Thanks for the three replies, I need to consider these responses further.
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