How does the expansion of the universe stretch light?

  • #51
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Yes, I noticed that. I was trying to say that that reasoning doesn't work. Hopefully the car analogy I used was a sufficiently good engineering example for you to see it. If not, here's another, different analogy that is no less relevant: saying the expansion is faster than the speed of light is like saying that the height of a building is faster than 30m/s. The statement doesn't even make sense.


Sort of. The issue here is that it is not only the redshift that matters, but also how our universe has expanded over time. The redshift only tells us the total amount of expansion since the light was emitted. How fast that expansion occurred over time determines both how far away the object was and how long ago it emitted that light.

One rough analogy I might use is that of a person driving between two locations. Imagine, if you will, that knowing the redshift is sorta kinda like knowing the origin and destination, e.g. knowing that the person drove from Denver to Chicago. But knowing the origin and destination is not sufficient to know how far the driver went or how long it took to do it: we also need to know how fast the driver was moving, and what specific route the driver took. With a light ray, the speed is, of course, given by the speed of light. But the path the light ray takes is given by how space expands in the intervening time.

To go back to the example of the furthest light we see, that from the Cosmic Microwave Background, that light was emitted a mere 42 million light years away, but because of the expansion of the universe it was forced to take a path towards us that made it take 13.7 billion years. That that stuff that originally emitted that light is now some 46 billion light years away is interesting but incidental.
Thanks Chalnoth. I believe I understand. What I forget is that me, the observer on earth, has also moved in space, due to expansion since the big bang. This would take what would of been a 42 milliion year journey (for CMBR to reach earth (observer) from the origin), if both objects were fixed and static, take 13.7 billion years due to the change in locations as a result of expansion (of observer and origin) resutling in a much more circuitous and longer path for light to travel. The redshift of the light would then indicate the object to be 46 billion light years away since the CMBR was first emitted thus giving us a rough size of the visible Universe.
 
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  • #52
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Basic Problem Statement: If the Universe is expanding at roughly 3 times the speed of light, and the visible Universe is approximately 46 billion light years in any direction. Why can we see 46 billion light years away? Remember the actual Universe is about 14 billion years old in absolute time. So in theory we should only be able see 14 billion light years in any direction, not 46 billion light years.

Explanation: I have no explanation because it would all be empirical and would be considered philosophy. Maybe somebody else has a better explanation.
When we look out into deep space we are looking back into time. We can only see that part of the event that occurred ~14 billion years ago with light reaching us now. The particles that produced this light are now 46 billion light years away, way out of sight. Every moment we are receiving light from this event from a larger sphere of more distant time away from us. What amount of the universe that exists 'now' can we see. Clearly the light hasn't reached us yet so we can't see anything it it's 'present' state.

mathal
 
  • #54
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Regarding post 46 and the subsequent posts:

from MISCONCEPTIONS ABOUT THE BIG BANG by Lineweaver and Davis in the March 2005 issue Scientific American.

"If space were not expanding, the most distant object we could see would be about 14B light years away from us, the distance light could have travelled in the 14 billion (B) years since the big bang."... but because the universe is expanding as light travels towards us and light therefore takes longer to get here, we can see out about 46B light years. In other words, the oldest light is 46B years old.

Age of the Universe:
About 380,000 years after inflation concluded, what we now see as cosmic microwave background radiation (CMBR) could begin to get through early matter after all the initial high energy ionization subsided...

We only get the CMBR light from some particular batch of early matter once and it passes by. Tomorrow we will get light from matter that is farther away than that batch whose light we got yesterday because the universe continues to expand. The source of the CMBR is NOW about 46b light yrs away but the light we get from it was emitted 41 million light years ago. It has taken so long to get here because the universe has expanded....by a factor of about 1080 times..."

and a prior discussion in these forums explaining redshift:

https://www.physicsforums.com/showthread.php?t=368958
 
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  • #55
Chalnoth
Science Advisor
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but because the universe is expanding as light travels towards us and light therefore takes longer to get here, we can see out about 46B light years. In other words, the oldest light is 46B years old.
This part is just wrong, no matter how you slice it. The furthest we can see is the cosmic microwave background, the light from which was emitted about 43 million light years away, but due to the expansion took 13 billion years to get here. The matter that once emitted that light is currently about 46 billion light years away.
 
  • #56
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Chalnoth: thanks....
I'll have to search exactly where I got that quote...if I were younger with better memory, I'd remember but alas!!!!!.....anyway, the article I thought I referenced now seems to cost $7.95 from Scientific American...I know I found it free of charge previously....

When I posted the quotes I noticed that "46B years old.." and briefly wondered myself.....but did not think about it

....the light from which was emitted about 43 million light years away, but due to the expansion took 13 billion years to get here
yes, of course the universe is about 13B years old.
 
  • #57
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I reread the article by Lineweaver and Davis and sure enough they got it right here:

If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.
Somehow I got an excerpted quote wrong, but it was copied not rewritten (not retyped) , so I'm stumped....anyway, I updated my notes with the correct excerpt.
 
  • #58
Chalnoth
Science Advisor
6,195
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I reread the article by Lineweaver and Davis and sure enough they got it right here:



Somehow I got an excerpted quote wrong, but it was copied not rewritten (not retyped) , so I'm stumped....anyway, I updated my notes with the correct excerpt.
Well, if I'm reading your use of quotes in your post correctly, you quoted the first part of that statement, and paraphrased the second part.
 

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