The discussion revolves around understanding the implications of observing an object 1 billion light years away from Earth, particularly in the context of cosmic expansion. Participants explore how to determine the current distance between Earth and the source, as well as the distance at the time the light was emitted, while considering the effects of the expanding universe.
Participants do not reach a consensus on the exact calculations needed to determine the distances involved. There are multiple viewpoints on how to approach the problem, particularly regarding the role of redshift and cosmic expansion.
Limitations include the lack of clarity on how to incorporate the universe's expansion into the distance calculations and the assumptions made about relative motion.
You wouldn't do the calculation like this, though. You'd get a redshift of the observed photon, and then relate this to the distance between the observer and the emitter when the photon was emitted.mathman said:This is probably an elementary exercise, but my head spins trying to think about it. Assume that on Earth we see something 1 billion light years away (i.e. light took 1 billion years to reach us). To simplify, assume no relative proper motion, although the distance will change due to universe expanding. How far apart are the Earth and the source now and how far apart were they when the light was emitted ( 1 billion years ago)?
He means that the distance is increasing due to the "universe expanding."Redbelly98 said:Assuming no relative motion, the object was and still is 1 billion light-years away. Or am I missing something in your question?
cristo said:He means that the distance is increasing due to the "universe expanding."
mathman said:Does anyone know the answer to my original question?
cristo said:I answered your question, didn't I? The point is, how would you know that the light had taken a billion years to travel to you? It's not like a 100m race or something like that, where you can measure it; you need to use the redshift.