The present expansion of the universe

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Hi, my question just came to me the other day, and I can't appease myself as to the answer.
Basically, I am unsure as to how we know the universe is expanding right now at this present moment in time.
If our observations of the red shift are all from galaxies and stars that are extremely far away (~millions of light years) then the light has taken millions of years to reach Earth. Therefore we know nothing of what those stars are doing right NOW.
Similarly, we are told that objects are moving (the universe is expanding) quicker the further objects are from us, but mightn't this just be an illusion? The further an object is from us, the "older" the light we observe is, therefore, may we not just be looking at the light from a period when the universe was expanding quicker and seeing it now only because those objects are further from us in the first place?
Thanks in advance
 

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  • #2
Vanadium 50
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We don't. We don't even know that the distant galaxies are still there - every star outside our own galaxy could have completely disappeared and we wouldn't know in our lifetimes.
 
  • #3
Chalnoth
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Hi, my question just came to me the other day, and I can't appease myself as to the answer.
Basically, I am unsure as to how we know the universe is expanding right now at this present moment in time.
If our observations of the red shift are all from galaxies and stars that are extremely far away (~millions of light years) then the light has taken millions of years to reach Earth. Therefore we know nothing of what those stars are doing right NOW.
Similarly, we are told that objects are moving (the universe is expanding) quicker the further objects are from us, but mightn't this just be an illusion? The further an object is from us, the "older" the light we observe is, therefore, may we not just be looking at the light from a period when the universe was expanding quicker and seeing it now only because those objects are further from us in the first place?
Thanks in advance
As long as we accept that the laws of physics are unlikely to have changed radically since the light left those objects, there is no problem. And since all tests of the constancy of the laws of physics have shown that they are highly constant within our universe, I'd say that's a pretty good bet.
 
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As long as we accept that the laws of physics are unlikely to have changed radically since the light left those objects, there is no problem. And since all tests of the constancy of the laws of physics have shown that they are highly constant within our universe, I'd say that's a pretty good bet.
I wasn't questioning whether the laws of physics are changing, but only the state of the stars. For instance, the furthest visible stars from us could actually be changing direction and contracting back again, but we could not know since the light we see from them was sent millions of years ago. Therefore, how can anyone say that we know with any certainty that the universe is still expanding?
 
  • #5
Chalnoth
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I wasn't questioning whether the laws of physics are changing, but only the state of the stars. For instance, the furthest visible stars from us could actually be changing direction and contracting back again, but we could not know since the light we see from them was sent millions of years ago. Therefore, how can anyone say that we know with any certainty that the universe is still expanding?
They can't simply change direction without some sort of external force acting on them. Hence, there would have to be some sort of sudden change in the laws of physics for this to happen.

Now, there do exist somewhat plausible physical models that have the universe recollapsing sometimes in the distant future, but there really isn't any reasonable way for it to have happened since the light left those objects.
 
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They can't simply change direction without some sort of external force acting on them. Hence, there would have to be some sort of sudden change in the laws of physics for this to happen.

Now, there do exist somewhat plausible physical models that have the universe recollapsing sometimes in the distant future, but there really isn't any reasonable way for it to have happened since the light left those objects.
Gravity?
 
  • #7
Chalnoth
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Gravity?
Gravity, however, depends upon the contents of the universe. To have the expansion suddenly change would require a dramatic change in the contents of the universe.
 
  • #8
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Gravity, however, depends upon the contents of the universe. To have the expansion suddenly change would require a dramatic change in the contents of the universe.
So our conclusion that the universe is expanding to this day is based on the observation that at some point in the past we know it was expanding, and we do not know of any reason in our laws of physics that would cause it to stop expanding (roughly speaking)?
 
  • #9
Chalnoth
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So our conclusion that the universe is expanding to this day is based on the observation that at some point in the past we know it was expanding, and we do not know of any reason in our laws of physics that would cause it to stop expanding (roughly speaking)?
It's quite a bit more than that. Basically, when we look out in space, we are seeing a slice of the past universe through time. To this slice of the universe we fit a model that both describes gravity and the contents of the universe. That model is supported by a wide body of diverse evidence, so we have a fair amount of confidence that it is at least approximately accurate.

If you'd like to get some idea as to the general classes of evidence we have available, see here:
http://www.talkorigins.org/faqs/astronomy/bigbang.html
 
  • #10
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It's quite a bit more than that. Basically, when we look out in space, we are seeing a slice of the past universe through time. To this slice of the universe we fit a model that both describes gravity and the contents of the universe. That model is supported by a wide body of diverse evidence, so we have a fair amount of confidence that it is at least approximately accurate.

If you'd like to get some idea as to the general classes of evidence we have available, see here:
http://www.talkorigins.org/faqs/astronomy/bigbang.html
Thank you for your patience and advice.
 

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