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Astronomy/The "look-back time" concept


Hello,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.

Hello Law...and thank you for using AllExperts

With regard to so-called "look-back-time," your thinking is correct. Actually we never see anything as it is right now. Even when you hold your hand up in front of your eyes you never see it as it is right now ; it takes light time to get from your hand to your eyes,hence the delay.

In your imagined galaxy at 1-billion light years moving to 10-billion light years, we could never see any changes to it as they occur but would have to wait 1-billion years plus n-billion years for the light from that event to reach us. Light-travel-time would be 1-billion years plus the additional light-travel-time introduced by the galaxy's movement.

Looking at all this another way --- your assumed galaxy is initially at a distance of 1-billion light years and rapidly receding from us. While at 1-billion light years a supernova occurs in the galaxy. We would see the supernova 1-billion years after it happened. Now assume that another supernova explodes when the galaxy has moved to 5-billion light years away. This time the light from this second explosion takes 6-billion years to reach us.


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Patrick Weiler


I`d be pleased to answer questions about any aspect of astronomy, particularly those related to cosmology, astrophysics, and planetary sciences. I can also provide reliable information on unique topics like dark energy, dark matter, black holes, etc.,.


Teacher, adult after-hours education at local community college, including frequent "star parties." I have my own telescope system, and continue to stay apace of recent developments and emerging theories in the field.

BA, liberal arts with emphasis on sciences. BS, computer technology.

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