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# Astronomy/The "look-back time" concept

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Question
I am a chemistry undergraduate student.
I have recently encountered the concept of
"Look-back time" in an elementary astronomy course. I am quite confused about one thing. From my understanding, the greater the distance we look into the space the further we look back in time. If something is at a distance,say 1 billion light year from us, we observe that thing in 1 billion years ago. Then if that thing move from
1 billion light year to 10 billion light year distance from us, assume that during the travel, the thing keep evolving. What will we observe? Will we see that thing evolve backward to 10 billion years ago?
I hope I get the question clear. Thank you.

Answer
I have to leave in a few minutes and won't be back until this evening. At that time I'll send a more detailed answer. In the meantime, the short answer is that we would always see the object moving forward in time, but since it is moving away from us, the time delay involved would be greater and greater. Right now we see the object as it was 1 billion years ago. At some time in the future (say 100 billion years from now) we will see it with a time delay of 10 billion years, but since that is so far in the future, we will still be seeing it as it will be at a much later date (if the numbers quoted were correct, as it would be 90 billion years from now, or 91 billion years later than the 1-billion year old image we currently see).

So as the Universe expands we only see things "evolve" in the forward direction of time, but if they are far enough away to have a significant change in their distance then we would see them change a little slower than they really do.

As I said I will send a more detailed answer later this evening, but hopefully that makes things a bit clearer for now.
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thank you for your answer. To check my understanding, I would like to make the following example. Assume at 1 billion light year distance a supernova occur, while it is occurs it moves to 5 billion light year distance. After 1 billion years we will observe the supernova going on and since it moves to a greater distance we will see the supernova occuring with a slower pace. Did I get that right?

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#### Courtney Seligman

##### Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

##### Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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