How on Earth This is possible? (distance to a far galaxy)

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Discussion Overview

The discussion revolves around the implications of observing a galaxy (ESO 325-G004) that is currently 17 billion light years away, despite the universe being approximately 13.7 billion years old. Participants explore the concepts of cosmic alignment, light travel time, and the effects of an expanding universe on distance measurements.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants express confusion about how a galaxy can be 17 billion light years away when the universe is only 13.7 billion years old.
  • One participant clarifies that the 17 billion light years refers to the current distance of the galaxy, not the distance at the time the light was emitted.
  • Another participant elaborates on the implications of an expanding universe, suggesting that as light travels, the distance to the source galaxy increases, resulting in a longer travel time than would be expected in a non-expanding universe.
  • There is a challenge to the initial understanding, with one participant urging others to seek additional information online.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are differing interpretations of the implications of light travel time and the expanding universe on the observed distance of galaxies.

Contextual Notes

There are limitations in the discussion regarding assumptions about the nature of light travel in an expanding universe and the definitions of distance in cosmology, which remain unresolved.

Prof Sabi
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https://amp.scroll.in/article/88372...ivity-holds-true-even-beyond-the-solar-system

Read the Section in this link:
Cosmic alignment
The galaxy we investigated has the catchy name ESO 325-G004 – let’s call it E325. Located some 450m light years away, it is one of the closest examples of a rare cosmic alignment – sitting directly between us and a second, more distant, galaxy. The background galaxy in this case is some 17 billion light years further behind. The centres of these two galaxies are aligned to better than one ten-thousandth of a degree.

17 billion light years? Isn't the universe 13.7 billion years old?
 
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At 17 Gly is where it is now, not at the time when it emitted the light that is captured in the picture.
 
BvU said:
At 17 Gly is where it is now, not at the time when it emitted the light that is captured in the picture.
Need a lil elaboration :eek: ... as I am imagining in this way that the light from that Galaxy literally took 17 billion years to travel to us in with velocity 'c'.
 
Prof Sabi said:
Need a lil elaboration :eek: ... as I am imagining in this way that the light from that Galaxy literally took 17 billion years to travel to us in with velocity 'c'.
Let's say the light is emitted at a distance D from the observer. In a non-expanding universe, that initial distance would remain constant in time, and it'd take t=D/c to cover the distance.
In an expanding universe, as the light travels, the distance D grows - both in the part that the light still has to cover, and the part already covered.
This means that the time needed to reach the observer is higher than t=D/c, and the distance at which the source galaxy ends up being by the time of reception is higher than the initial distance D.

edit: typo
 
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