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Is the distance covered by light from an extremely distant source in light years equal to the time taken in years? Surely it would have to be, for the speed of light is absolute, after all.

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The tricky aspect of cosmology is that in an expanding universe, distances are not absolute. If the light takes a sufficiently long time to get to us, the intervening space may have expanded significantly and the number of light years traversed by the light is not equal to the number of years it had been moving.

For example, JADES-GS-z13-0 has a redshift of 13.2. That means light from it has traveled for 13.4 billion years. But its present proper distance is 33.6 billion light-years. But when the light emitted it was just 2.4 billion light-years away from us.

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  • $\begingroup$ 33.6 - 2.4 = 31.2 / 2 = 15.6 + 2.4 = 18 - 13.4 = 4.6 $\endgroup$ Feb 10 at 15:36
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    $\begingroup$ @CuriousCat - Remember that the expansion has changed nonlinearly, you need to use integrals dependent on the scale factor. $\endgroup$ Feb 10 at 16:43

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